Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

The Moon interacts with the incident solar wind plasma in various ways, and most of these interactions are accompanied by variations of the interplanetary magnetic field (IMF) around the Moon. Here we first report decreases of the IMF strength observed at 100 km altitude on the lunar dayside and over the polar region, comparing upstream solar wind data from ACE with Kaguya (SELENE) data. We note that the magnetic field decreases are observed above non-magnetized regions or very weakly magnetized regions. In one event the IMF is weakened in the dayside northern hemisphere when the IMF is roughly anti-parallel to the solar wind flow. We estimate that the decrease in the magnetic pressure can be partly compensated by the thermal pressure of the back-scattered solar wind protons, which suggests that the magnetic field decrease is interpreted as diamagnetic effect by the back-scattered protons. In another event an IMF decrease is continuously detected from the northern polar region to the dayside mid-latitude region, which is not fully explained by the thermal pressure of the back-scattered protons. We also discuss the diamagnetic current system in the upstream (fore-moon) solar wind region formed by the back-scattered protons.

DOI： 10.1016/j.icarus.2019.113392

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

DOI： 10.1093/gji/ggy374

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Narrowband electromagnetic ion cyclotron waves first discovered by the Apollo 15 and 16 Lunar Surface Magnetometers were surveyed in the magnetic field data obtained by the Kaguya satellite at an altitude of similar to 100 km above the Moon in the tail lobe and plasma sheet boundary layer of the Earth's magnetosphere. The frequencies of the waves were typically 0.7 times the local proton cyclotron frequency, and 75% of the waves were left hand polarized with respect to the background magnetic field. They had a significant compressional component and comprised several discrete packets. They were detected on the dayside, nightside, and above the terminator of the Moon, irrespective of the lunar magnetic anomaly, or the magnetic connection to the lunar surface. The waves with the same characteristics were detected by Geotail in the absence of the Moon in the magnetotail. The most likely energy source of the electromagnetic ion cyclotron waves is the ring beam ions in the plasma sheet boundary layer.Plain Language Summary The origin of 10 s waves that were found in the data from Apollo 15 and 16 Lunar Surface Magnetometers were studied by Japanese spacecraft Kaguya and Geotail. The 10 s waves were detected when the Moon was in the Earth's magnetotail, an extended region of the Earth's magnetic field on the antisolar side. They were thought to be generated by ions gyrating around the magnetic field, but the origin of the ions was not understood. Observations form Kaguya and Geotail revealed that the waves were found irrespective of the presence of the Moon, suggesting that the waves were generated by ions intrinsic in the Earth's magnetotail rather than the Moon.

DOI： 10.1002/2017ja024505

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

© 2017 Elsevier Inc. There forms a tenuous region called the wake behind the Moon in the solar wind, and plasma entry/refilling into the wake is a fundamental problem of the lunar plasma science. High-energy ions and electrons in the foreshock of the Earth's magnetosphere were detected at the lunar surface in the Apollo era, but their effects on the lunar night-side environment have never been studied. Here we show the first observation of bow-shock reflected protons by Kaguya (SELENE) spacecraft in orbit around the Moon, confirming that solar wind plasma reflected at the terrestrial bow shock can easily access the deepest lunar wake when the Moon stays in the foreshock (We name this mechanism ‘type-3 entry’). In a continuous type-3 event, low-energy electron beams from the lunar night-side surface are not obvious even though the spacecraft location is magnetically connected to the lunar surface. On the other hand, in an intermittent type-3 entry event, the kinetic energy of upward-going field-aligned electron beams decreases from ∼ 80 eV to ∼ 20 eV or electron beams disappear as the bow-shock reflected ions come accompanied by enhanced downward electrons. According to theoretical treatment based on electric current balance at the lunar surface including secondary electron emission by incident electron and ion impact, we deduce that incident ions would be accompanied by a few to several times higher flux of an incident electron flux, which well fits observed downward fluxes. We conclude that impact by the bow-shock reflected ions and electrons raises the electrostatic potential of the lunar night-side surface.

DOI： 10.1016/j.icarus.2017.04.005

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We conducted long-period magnetotelluric observations in northeastern Japan from 2010 to 2013 to investigate the three-dimensional electrical resistivity distribution of the subduction zone. Incorporating prior information of the subducting slab into the inversion scheme, we obtained a three-dimensional resistivity model in which a vertically continuous conductive zone is imaged from the subducting slab surface to the lower crust beneath the Ou Backbone Range. The conductive body indicates a saline fluid and/or melt pathway from the subducting slab surface to the lower crust. The lower crust conductor is less than 10 Omega m, and we estimate a saline fluid and/or melt fraction of at least 0.7 vol.%. Other resistivity profiles in the across-arc direction reveal that the conductive body segregates from the subducting slab surface at 80-100 km depth and takes an overturned form toward the back arc. The head of the conducting body reaches the lower crust just beneath Mt. Gassan, one of the prominent back-arc volcanoes in the system.

DOI： 10.1002/2015JB012028

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

DOI： 10.1093/gji/ggv233

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©2015. The Authors.We have provided preliminary global maps of three components of the lunar magnetic anomaly on the surface applying the surface vector mapping (SVM) method. The data used in the present study consist of about 5 million observations of the lunar magnetic field at 10-45 km altitudes by Kaguya and Lunar Prospector. The lunar magnetic anomalies were mapped at 0.2 equi-distance points on the surface by the SVM method, showing the highest intensity of 718 nT in the Crisium antipodal region. Overall features on the SVM maps indicate that elongating magnetic anomalies are likely to be dominant on the Moon except for the young large basins with the impact demagnetization. Remarkable demagnetization features suggested by previous studies are also recognized at Hertzsprung and Kolorev craters on the farside. These features indicate that demagnetized areas extend to about 1-2 radii of the basins/craters. There are well-isolated central magnetic anomalies at four craters: Leibnitz, Aitken, Jules Verne, and Grimaldi craters. Their magnetic poles through the dipole source approximation suggest occurrence of the polar wander prior to 3.3-3.5 Ga. When compared with high-albedo markings at several magnetic anomalies such as the Reiner Gamma anomalies, three-dimensional structures of the magnetic field on/near the surface are well correlated with high-albedo areas. These results indicate that the global SVM maps are useful for the study of the lunar magnetic anomalies in comparison with various geological and geophysical data.

DOI： 10.1002/2014JE004785

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© 2014 Elsevier Inc. Plasma signature around crustal magnetic fields is one of the most important topics of the lunar plasma sciences. Although recent spacecraft measurements are revealing solar-wind interaction with the lunar crustal fields on the dayside, plasma signatures around crustal fields on the night side have not been fully studied yet. Here we show evidence of plasma trapping on the closed field lines of the lunar crustal fields in the solar-wind wake, using SELENE (Kaguya) plasma and magnetic field data obtained at 14-15. km altitude from the lunar surface. In contrast to expectation on plasma cavity formation at the strong crustal fields, electron flux is enhanced above Crisium Antipode (CA) anomaly which is one of the strongest lunar crustal fields. The enhanced electron fluxes above CA are characterised by (1) occasional bi-directional field-aligned beams in the lower energy range (<150. eV) and (2) a medium energy component (150-300. eV) that has a double loss-cone distribution representing bounce motion between the two footprints of the crustal magnetic fields. The low-energy electrons on the closed field lines may come from the lunar night side surface, while supply mechanism of medium-energy electrons on the closed field line remains to be solved. We also report that a density cavity in the wake is observed not above the strongest magnetic field but in its vicinity.

DOI： 10.1016/j.icarus.2014.12.007

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

Magnetic fluctuations in the extremely low-frequency (ELF) range from 0.1 to 10 Hz were found by the Lunar Magnetometer (LMAG) of the magnetic field and plasma experiment (MAP) on board the spacecraft Kaguya in the deepest wake behind the moon, where the magnetic field is usually quiet. The fluctuations were compressional and non-monochromatic, showing no preferred polarization. They were often accompanied by "type-II entry" solar wind protons that were reflected by the dayside lunar surface or crustal magnetic field, gyrated around the solar wind magnetic field, then entered the deepest wake. The ELF waves persisted for 30 s to several minutes. The duration was often shorter than that of the type-II protons. Most of the waves were detected on the magnetic field lines disconnected from the lunar surface, along which the solar wind electrons were injected into the wake. Since a large cross-field velocity difference is expected between the type-II protons and the solar wind electrons injected along the magnetic field, some cross-field current-driven instability such as the lower hybrid two-stream instability is expected to be responsible for the generation of the waves.

DOI： 10.1186/s40623-015-0196-0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

Harmonic spectral features of electromagnetic waves in the frequencies of several Hz around the Moon have been identified by Kaguya. The waves have steepened waveforms peculiarly in the compressional component. The fundamental waves have almost the same properties as narrowband whistler-mode waves with the frequencies near 1 Hz, which have been observed around the Moon. The waves are observed around the terminator region in the solar wind near the lunar magnetic anomalies at the altitudes under 120 km. We suggest that the harmonic spectra are a result of the nonlinear steepening of narrowband whistler-mode waves. Although the narrowband whistler-mode waves have been observed in the upstream region of many planetary bow shocks, such harmonics have rarely been observed there. Since the harmonics are more frequently observed at lower altitudes of the Moon, they are possibly caused by lunar intrinsic environments including lunar dusts and local structures of lunar magnetic anomalies.

DOI： 10.1186/s40623-015-0203-5

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DOI： 10.1038/ngeo2150

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/2013JA019585

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/2013JE004529

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We have developed a new method for regional mapping of the lunar magnetic anomalies as the vector field at the surface using the satellite observation, that is the surface vector mapping (SVM). The SVM is based on the inverse boundary value problem with a spherical boundary surface. There are two main procedures for reducing effects of bias and noise on mapping: (1) preprocessing the data to provide first derivatives along the pass, and (2) the Bayesian statistical procedure in the inversion using Akaike's Bayesian Information Criterion. The SVM was applied to two regions: the northwest region of the South Pole-Aitken basin as a strong magnetic anomaly region, and the southeast region of the lunar near side as a weak magnetic anomaly region. Since the results from the different datasets of the Kaguya and Lunar Prospector observations show good consistency, characteristic features of the lunar magnetic anomalies at the surface are considered to be well estimated except for components of wavelength shorter than about 1°. From the results by the SVM, both of the regions show elongation patterns of the lunar magnetic anomalies, suggesting lineated structures of the magnetic anomaly sources. © 2013 Elsevier Inc.

DOI： 10.1016/j.icarus.2013.09.026

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© 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. In the Earth's magnetotail, Japanese Moon orbiter Kaguya repeatedly encountered the plasmoid or plasma sheet. The encounters were characterized by the low energy ion signatures including lobe cold ions, cold ion acceleration in the plasma sheet-lobe boundaries, and hot plasma sheet ions or fast flowing ions associated with plasmoids. Different from the previous observations made in the magnetotail by the GEOTAIL spacecraft, the ions were affected by the existence of the Moon. On the dayside of the Moon, tailward flowing cold ions and their acceleration were observed. However, on the night side, tailward flowing cold ions could not be observed since the Moon blocked them. In stead, ion acceleration by the spacecraft potential and the electron beam accelerated by the potential difference between lunar surface and spacecraft were simultaneously observed. These electron and ion data enabled us to determine the night side lunar surface potential and spacecraft potential only from the observed data for the first time.

DOI： 10.1016/j.asr.2013.05.011

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We present latitude and longitude distributions of Na+and K+fluxes from the Moon derived from Kaguya low-energy ion data. Although the latitude distribution agrees with previous ground-based telescope observations, dawn-dusk asymmetry has been determined in the longitude distribution. Our model of the lunar surface abundance and yield of Na and K demonstrates that the abundance decreases to approximately 50% at dusk compared with that at dawn due to the emission of the exospheric particles assuming the ion fluxes observed by Kaguya are proportional to the yield. It is also implied that the surface abundance of Na and K need to be supplied during the night to explain the observed lunar exosphere with dawn-dusk asymmetry. We argue that the interplanetary dust as well as grain diffusion and migration/recycling of the exospheric particles may be major suppliers. Key Points Kaguya data present structure of the ionized lunar alkali exospheres We found dawn-dusk asymmetry in the longitude distribution Our model shows that the surface abundance decreases to 50% ©2014. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/2013JE004529

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We investigate Kaguya observation of ion acceleration around a lunar crustal magnetic anomaly located in the South Pole-Aitken basin at an altitude of 100 km. The accelerated ions in the 230 eV to 1.5 keV energy range were identified by a characteristic dispersion signature in the energy-time spectrogram that appeared repeatedly upon Kaguya's approach to the magnetic anomaly. The interplanetary magnetic field was almost parallel to the solar wind velocity and thus the electric field was very small. The results of our analysis show that ions with energies below 230 eV were accelerated up to 1.5 keV by an electric field produced by the interaction between the solar wind and the magnetic anomaly. We argue that the low-energy ions mainly originated from the solar wind ions with energies of 450 eV that were backscattered on the lunar surface. © 2014 Elsevier Ltd.

DOI： 10.1016/j.pss.2014.02.007

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The origins of the lunar crustal magnetic fields remain unclear although dozens of magnetic field measurements have been conducted on and above the lunar surface. A major obstacle to resolving this problem is the extreme difficulty of determining a surface distribution of small-scale magnetization. We present a new technique to map small-scale magnetic fields using nonadiabatic scattering of high-energy electrons in the terrestrial plasma sheet. Particle tracing, utilizing three-dimensional lunar magnetic field data synthesized from magnetometer measurements, enables us to separate the contributions to electron motion of small- and large-scale magnetic fields. We map significant kilometer-scale magnetic fields on the southwestern side of the South Pole-Aitken basin that are correlated with larger-scale magnetization. This implies that kilometer-scale magnetization may be ubiquitous over the lunar surface and related to the large-scale magnetization. Key Points Lunar small-scale magnetization can be inferred from plasma sheet electrons Strong small-scale magnetization is related to large-scale magnetization Small-scale magnetization may be ubiquitous over the lunar surface ©2013. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/grl.50662

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Crustal fluids exist near fault zones, but their relation to the processes that generate earthquakes, including slow-slip events, is unclear. Fault-zone fluids are characterized by low electrical resistivity. Here we investigate the time-dependent crustal resistivity in the rupture area of the 1999 M-w 7.6 Izmit earthquake using electromagnetic data acquired at four sites before and after the earthquake. Most estimates of apparent resistivity in the frequency range of 0.05 to 2.0 Hz show abrupt co-seismic decreases on the order of tens of per cent. Data acquired at two sites 1 month after the Izmit earthquake indicate that the resistivity had already returned to pre-seismic levels. We interpret such changes as the pressure-induced transition between isolated and interconnected fluids. Some data show pre-seismic changes and this suggests that the transition is associated with foreshocks and slow-slip events before large earthquakes.

DOI： 10.1038/ncomms3116

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

Electric field variations coincident with the passage of seismic waves are commonly observed irrespective of whether the seismic events are natural or artificial. We present 10 examples of electric field variations obtained for artificial seismic waves whose typical frequency is a few times higher than that of natural seismic waves. In several cases, the electric fields showed left- and/or right-handed circular polarization, indicating the motion of ions with positive and/or negative electric charge, respectively, generated by ground motion in the Earth's magnetic field. In three cases, we have estimated transfer functions relating the electric field to the ground velocity. Furthermore, we have performed time-frequency analysis with the continuous wavelet transform and have constructed spectrograms of the electric field and ground velocity. In both results, we have found some peaks at the specific frequencies where the resonance of the motion of ions in groundwater with the Earth's magnetic field is expected, thereby supporting the proposed mechanism in terms of the seismic dynamo effect.

DOI： 10.1093/gji/ggt110

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

The first magnetotelluric study in the Marmara Sea, Turkey, was undertaken to resolve the structure of the crust and upper mantle in the region, and to determine the location of the westward extension of the North Anatolian Fault (NAF) in the Cinarcik area. Long-period ocean bottom magnetotelluric data were acquired at six sites along two profiles crossing the Cinarcik Basin, where a significant increase in microseismic activity was observed following the devastating 1999 Izmit and Duzce earthquakes. 2-D resistivity models indicate the existence of a conductor at a depth of similar to 10 km in the middle of both profiles along with a deeper extension into the upper mantle, implying the presence of fluid in the crust and partial melting in the upper mantle. The northern and southern boundaries of this conductor are interpreted to represent the northern and southern branches of the NAF in the Marmara Sea, respectively. These conductors have been previously identified farther to the east along the NAF, suggesting that the electrical characteristics of this fault are continuous from onland areas into the Marmara Sea. Microseismic activity in the Cinarcik area is located above the conductor documented here, and indicates a possible seismogenic role of crustal fluids present in the conductive zone. In comparison, resistive zones along the NAF may act as asperities that could eventually result in a large earthquake.

DOI： 10.1093/gji/ggt025

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Our recent observations around the Moon revealed that so-called type-II (T2) entry of the solar wind protons into the near-Moon wake occurs when the IMF is dominated by the non-radial components (i.e. BYand/or Bz). Under this condition a part of the solar wind protons scattered/reflected at the lunar dayside surface subsequently enters the central region of the near-Moon wake after a large-scale cycloid motion, which accelerates electrons along the filed line into the wake. The situation handled in the previous studies is that the relevant magnetic field line is detached from the lunar surface, leaving a possibility of the T2 entry under magnetic connection left open. Here we report that the protons can access the central wake region that is magnetically connected to the lunar nightsicle surface, which we categorize into the T2 entry with magnetic connection to the lunar surface (T2MC). Furthermore we show that the energy of the electron beams induced by the proton entry into the wake depends on the magnetic connectivity. Strong electron acceleration (up to several hundred eV to 1 keV) along the magnetic field associated with the T2 entry is prominent when the field line has its both ends in the solar wind, that is, when the magnetic field is detached from the lunar surface (i.e. the previously reported T2 entry that we rename toT2MD). On the other hand, no significant electron acceleration is found in the T2MC cases, although an enhancement of the electron flux associated with the T2 proton entry is evident. We also report that the T2 entry process takes place even under radial (Bx-dominated) IMF condition. Our results indicate that, while the T2 entry of solar wind protons into the wake itself does not require a special IMF condition but is a rather general phenomenon, the characteristic energy of associated electrons does show a strong dependence on the magnetic connectivity to the lunar surface. © 2013 Elsevier Ltd.

DOI： 10.1016/j.pss.2013.08.017

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DOI： 10.1016/j.icarus.2012.10.029

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

DOI： 10.5047/eps.2012.05.017

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We study an interaction between the solar wind and crustal magnetic fields on the lunar surface using SELENE (Kaguya) data. It has been known that magnetic enhancements are at times detected near the limb external to the lunar wake, which is thus called lunar external magnetic enhancement (LEME), as a result of direct interaction between the solar wind and lunar crustal fields. Although previous observational studies showed that LEMEs in the high solar zenith angle region favor stronger interplanetary magnetic field (IMF) and higher solar wind density, the relation between the IMF and the crustal field orientation has not been taken into account. We show evidence that the relation between the IMF and crustal field orientation is also one of the key factors that control the extent of LEME, focusing on one-day observations at 100 km altitude that include data above strong crustal fields around South Pole-Aitken (SPA) basin. Strong LEMEs are detected at 100 km altitude around SPA basin under the stronger and northward IMF condition, while they weaken under southward IMF. All LEME's peaks are located in the region where unperturbed crustal fields at 300 km altitude are directed northward while they are less related to unperturbed crustal fields at 100 km or lower, which suggests that lunar crustal fields are compressed by the solar wind dynamic pressure, and its large scale component parallel to the IMF is essential to the formation of the LEME. © 2012 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.pss.2012.09.011

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Broadband whistler-mode waves in the frequency range from 0.1 to 10 Hz are detected near the Moon by the Lunar Magnetometer (LMAG) on board Kaguya. The generation process and statistical properties of the waves have not been understood yet. We analyze the distributions of their occurrence and reveal that most of the waves are generated by the solar wind interaction with lunar crustal magnetic field. We also clarify that the waves are observed when Kaguya is connected by the ambient magnetic field with the lunar surface. The statistical study indicates that the broadband waves are observed in the vicinity of the region where narrowband whistler-mode waves in the frequency of near 1 Hz are observed, showing the close relationship between them. The analysis of the wave vector directions suggests that these two types of waves are different views of the same waves propagating in the solar wind frame. The narrowband waves are possibly explained by a part of the broadband waves largely Doppler shifted in the spacecraft frame. The present results suggest a possible scenario of the generation process of the two types of waves through the solar wind interaction with the crustal magnetic field. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012GL052818

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.5047/eps.2011.07.011

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Language：English   Publishing type：Research paper (scientific journal)  

At ∼25 km altitude over magnetic anomalies on the Moon, the deceleration of the solar wind ions, acceleration of the solar wind electrons parallel to the magnetic field, and heating of the ions reflected by magnetic anomalies were simultaneously observed by MAP-PACE on Kaguya. Deceleration of the solar wind ions was observed for two major solar wind ion compositions: protons and alpha particles. Deceleration of the solar wind had the same ΔE/q (ΔE: deceleration energy, q: charge) for both protons and alpha particles. In addition, the acceleration energy of the electrons was almost the same as the deceleration energy of the ions. This indicates the existence of an anti-moonward electric field over the magnetic anomaly above the altitude of Kaguya. The reflected ions were observed in a much larger area than the area where magnetic field enhancement was observed. These reflected ions had a higher temperature and lower bulk velocity than the incident solar wind ions. This suggests the existence of a non-adiabatic dissipative interaction between solar wind ions and lunar magnetic anomalies below Kaguya. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS).

DOI： 10.5047/eps.2011.07.011

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union  

We have analyzed nongyrotropic electron velocity distribution functions (VDFs) obtained near the lunar surface. Electron VDFs, measured at 10-100 km altitude by Kaguya in both the solar wind and the Earth's magnetosphere, exhibit nongyrotropic empty regions associated with the 'gyroloss' effect; i.e., electron absorption by the lunar surface combined with electron gyromotion. Particle-trace calculations allow us to derive theoretical forbidden regions in the electron VDFs, thereby taking into account the modifications due to nonuniform magnetic fields caused by diamagnetic-current systems, lunar-surface charging, and electric fields perpendicular to the magnetic field. Comparison between the observed empty regions with the theoretically derived forbidden regions suggests that various components modify the characteristics of the nongyrotropic electron VDFs depending on the ambient-plasma conditions. On the lunar nightside in the magnetotail lobes, negative surface potentials slightly reduce the size of the forbidden regions, but there are no distinct effects of either the diamagnetic current or perpendicular electric fields. On the dayside in the solar wind, the observations suggest the presence of either the diamagnetic-current or solar wind convection electric field effects, or both. In the terrestrial plasma sheet, all three mechanisms can substantially modify the characteristics of the forbidden regions. The observations imply the presence of a local electric field of at least 5 mV/m although the mechanism responsible for production of such a strong electric field is unknown. Analysis of nongyrotropic VDFs associated with the gyroloss effect near solid surfaces can promote a better understanding of the near-surface plasma environment and of plasma-solid-surface interactions. © 2012. American Geophysical Union.

DOI： 10.1029/2012JA017642

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Language：Japanese   Publishing type：Research paper (bulletin of university, research institution)  

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DOI： 10.5194/angeo-29-889-2011

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Because the solar wind (SW) flow is usually super-sonic, a fast-mode bow shock (BS) is formed in front of the Earth's magnetosphere, and the Moon crosses the BS at both dusk and dawn flanks. On the other hand, behind of the Moon along the SW flow forms a tenuous region called lunar wake, where the flow can be sub-Alfvnic (and thus sub-sonic) because of its low-density status. Here we report, with joint measurement by Chang'E-1 and SELENE, that the Earth's BS surface is drastically deformed in the lunar wake. Despite the quasi-perpendicular shock configuration encountered at dusk flank under the Parker-spiral magnetic field, no clear shock surface can be found in the lunar wake, while instead gradual transition of the magnetic field from the upstream to downstream value was observed for a several-minute interval. This finding suggests that the 'magnetic ramp' is highly broadened in the wake where a fast-mode shock is no longer maintained due to the highly reduced density. On the other hand, observations at the 100 km altitude on the dayside show that the fast-mode shock is maintained even when the width of the downstream region is smaller than a typical scale length of a perpendicular shock. Our results suggest that the Moon is not so large to eliminate the BS at 100 km altitude on the dayside, while the magnetic field associated with the shock structure is drastically affected in the lunar wake. © 2010 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.pss.2011.01.002

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Large amplitude, monochromatic ultra low frequency (ULF) waves were detected by MAP/LMAG magnetometer onboard Kaguya during the period from 1 January 2008 to 30 November 2008 on its orbit 100 km above the lunar surface. The dominant frequency was 8.3 × 10-3-1.0 × 10 -2 Hz, corresponding to the periods of 120 s-100 s. The amplitude was as large as 3 nT. They were observed in 10% of the time when the moon was in the solar wind far upstream of the Earth's bow shock. They were detected only by Kaguya on the orbit around the moon, but not by ACE in the upstream solar wind. The occurrence rate was high above the terminator and on the dayside surface. The direction of the propagation was not exactly parallel to the interplanetary magnetic field, but showed a preference to the direction of the magnetic field and the direction perpendicular to the surface of the moon below the spacecraft. The sense of rotation of the magnetic field was left-handed with respect to the magnetic field in 53% of the events, while 47% showed right-handed polarization. The possible generation mechanism is the cyclotron resonance of the magnetohydrodynamic waves with the solar wind protons reflected by the moon. The energy of the reflected protons can account for the energy of the ULF waves. The propagation direction which are not parallel to the incident solar wind flow can explain the observed frequency and the nearly equal percentages of the left-handed and right-handed polarizations. Copyright 2012 by the American Geophysical Union.

DOI： 10.1029/2011JA017249

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Other Link： http://onlinelibrary.wiley.com/doi/10.1029/2011JA017249/abstract

Language：English   Publishing type：Research paper (scientific journal)  

© 2011 The Society of Geomagnetism and Earth, Planetary and Space Sciences, The Seismological Society of Japan.Non-monochromatic fluctuations of the magnetic field over the frequency range of 0.03-10 Hz were detected by Kaguya at an altitude of 100 km above the lunar surface. The fluctuations were almost always observed on the solar side of the moon, irrespective of the local lunar crustal field. They were also detected just nightside of the terminator (SZA < 123°), but were absent around the center of the wake. The level of the fluctuation enhanced over the wide range from 0.03 to 10 Hz, with no clear peak frequency. The fluctuations had the compressional component, and the polarization was not clear. The fluctuations were supposed to be whistler waves generated by the protons reflected by the lunar surface. The reflected protons are scattered in various directions, resulting a wide range of distribution of the velocity component parallel to the magnetic field. It may account for the wide range of frequency as observed, through cyclotron resonance of the wave with the reflected ions, in which the resonant frequency depends on the velocity component parallel to the magnetic field. However, there is also the possibility that the waves were generated by some nonresonant process.

DOI： 10.5047/eps.2010.01.005

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Publishing type：Part of collection (book)   Publisher：Springer Netherlands  

DOI： 10.1007/978-94-007-0501-2_20

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

Clear electric field variations have been observed during the passage of seismic wave. The most notable feature is circular polarization of electric field and it has been interpreted as reflecting circular motion of ions in groundwater under the Earth's magnetic field. We have called such a mechanism 'seismic dynamo effect' but more convincing evidence is required to support this mechanism. Here we show strong evidence in terms of transfer function relating the electric field to the ground velocity due to seismic wave. We estimate transfer functions for electric field and ground velocity records for five aftershocks after the occurrence of M 6.9 earthquake. Then some peaks in the transfer function are found at specific frequencies where the resonance of ion motion in groundwater with the Earth's magnetic field is expected. This result clearly supports the mechanism, which provides a method for in-situ estimation of ion contents in groundwater.

DOI： 10.5047/eps.2011.03.010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Analysis of the data obtained by SELENE (Kaguya) revealed a partial loss in the electron velocity distribution function due to the "gyro-loss effect", namely gyrating electrons being absorbed by the lunar surface. The Moon enters the Earth's magnetosphere for a few days around full moon, where plasma conditions are significantly different from those in the solar wind. When the magnetic field is locally parallel to the lunar surface, relatively high-energy electrons in the terrestrial plasma sheet with Larmor radii greater than SELENE's orbital height strike the lunar surface and are absorbed before they can be detected. This phenomenon can be observed as an empty region in the electron distribution function, which is initially isotropic in the plasma sheet, resulting in a non-gyrotropic distribution. We observed the expected characteristic electron distributions, as well as an empty region that was consistent with the presence of a relatively strong electric field (∼10 mV/m) around the Moon when it is in the plasma sheet. © 2010 by the American Geophysical Union.

DOI： 10.1029/2010GL044574

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We present observations of electrostatic solitary waves (ESWs) near the Moon by SELENE (KAGUYA) in the solar wind and in the lunar wake. SELENE is a lunar orbiter with an altitude of 100 km and measured wave electric field, background magnetic field, and fluxes of ions and electrons, etc. ESWs are categorized into three types depending on different regions of observations: ESWs generated by electrons reflected and accelerated by an electric field in the wake boundary (Type A), strong ESWs generated by bi-streaming electrons mirror-reflected over the magnetic anomaly (Type B), and ESWs generated by reflected electrons when the local magnetic field is connected to the lunar surface (Type C). ESWs of Type C often alternate with Langmuir waves. © 2010 by the American Geophysical Union.

DOI： 10.1029/2010GL044529

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Language：English   Publishing type：Research paper (scientific journal)  

MAP-PACE (MAgnetic field and Plasma experiment-Plasma energy Angle and Composition Experiment) on SELENE (Kaguya) has completed its ∼1.5-year observation of low-energy charged particles around the Moon. MAP-PACE consists of 4 sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). ESA-S1 and S2 measured the distribution function of low-energy electrons in the energy range 6 eV-9 keV and 9 eV-16 keV, respectively. IMA and IEA measured the distribution function of low-energy ions in the energy ranges 7 eV/q-28 keV/q and 7 eV/q-29 keV/q. All the sensors performed quite well as expected from the laboratory experiment carried out before launch. Since each sensor has a hemispherical field of view, two electron sensors and two ion sensors installed on the spacecraft panels opposite each other could cover the full 3-dimensional phase space of low-energy electrons and ions. One of the ion sensors IMA is an energy mass spectrometer. IMA measured mass-specific ion energy spectra that have never before been obtained at a 100 km altitude polar orbit around the Moon. The newly observed data show characteristic ion populations around the Moon. Besides the solar wind, MAP-PACE-IMA found four clearly distinguishable ion populations on the dayside of the Moon: (1) Solar wind protons backscattered at the lunar surface, (2) Solar wind protons reflected by magnetic anomalies on the lunar surface, (3) Reflected/backscattered protons picked-up by the solar wind, and (4) Ions originating from the lunar surface/lunar exosphere. © 2010 Springer Science+Business Media B.V.

DOI： 10.1007/s11214-010-9647-x

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DOI： 10.1007/s11214-010-9655-x

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DOI： 10.1007/s11214-010-9652-0

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We study effect of the solar wind (SW) proton entry deep into the near-Moon wake that was recently discovered by the SELENE mission. Because previous lunar-wake models are based on electron dominance, no effect of SW proton entry has been taken into account. We show that the type-II entry of SW protons forms proton-governed region (PGR) to drastically change the electromagnetic environment of the lunar wake. Broadband electrostatic noise found in the PGR is manifestation of electron two-stream instability, which is attributed to the counter-streaming electrons attracted from the ambient SW to maintain the quasineutrality. Acceleration of the absorbed electrons up to ∼1 keV means a superabundance of positive charges of 10-510-7cm-3in the near-Moon wake, which should be immediately canceled out by the incoming high-speed electrons. This is a general phenomenon in the lunar wake, because PGR does not necessarily require peculiar SW conditions for its formation. Copyright 2010 by the American Geophysical Union.

DOI： 10.1029/2010GL043948

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Among electric field variations supposed to be associated with earthquakes, electric field variations coincident with the passage of seismic waves have been well documented and interpreted mostly in terms of the electrokinetic effect. Here we present two examples of electric field variations obtained in association with small artificial earthquakes caused by blasting and three examples for aftershocks of two large earthquakes of magnitude 6.9 and 7.2, respectively. The electric field turned out to be circularly polarized in some cases, whereas linearly polarized cases were also seen. Since it is unclear whether such a peculiar behavior is understood in terms of existing models, we propose another mechanism to explain circular polarization; here we call this mechanism as "seismic dynamo effect,'' which would be regarded as an extended model of the so-called induction effect. In our model we consider ions motion in pores filled with water in the ground, which is driven by ground motion in the Earth's magnetic field. With this model we show that circular polarization of electric field is realized in association with resonance between the frequency of ground velocity due to seismic wave and the cyclotron frequency of ions, such as HCO3- or Cl- contained in pores, for the Earth's magnetic field at the observation site. Ions with positive charge, such as Na+, also seem to be responsible for circular polarization of electric field with rotation direction opposite to that for ions with negative charge. We also show that in this model the magnitude of electric field can be estimated in terms of the number density of ions.

DOI： 10.1029/2008JB006117

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Language：English   Publishing type：Research paper (bulletin of university, research institution)  

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DOI： 10.1016/j.pepi.2008.09.014

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Publisher：Tokyo Geographical Society  

DOI： 10.5026/jgeography.114.2_Plate4a

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Language：Japanese   Publisher：The Volcanological Society of Japan  

DOI： 10.18940/vsj.2001.2.0_138

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

We have investigated the spatial variation of the stress field along the fault rupture zone of the 1999 Izmit earthquake (M(W) 7.4) using first-motion polarity data at seven distinct aftershock clusters. In our approach, the first-motion polarities of all the aftershocks in a cluster are simultaneously inverted to determine the stress tensor parameters and fault plane solutions of individual events, in accordance with the method of Horiuchi et al. (1995). Where post-seismic slip was significant (e.g., Sapanca, Sakarya-Akyazi, and Karadere segments), we obtained stress tensors with the fault parallel or fault normal maximum (sigma(1)) and minimum (sigma(3)) principal compressive stress axes, which may imply either low frictional coefficients or fault weakness. A stress tensor with similar features was derived from the Cinarcik cluster, where the aftershocks lie in a low-velocity zone beneath the geothermal area. The maximum principal stress axis tends to remain parallel to the trend of the pre-mainshock sigma(1) around the Yalova segment; this segment experienced little to no co-seismic displacements. The stress tensor around the Golcuk segment, where the largest surface displacement of 5.5 m was observed, was determined to be 20-25 degrees counterclockwise rotated, but the aftershock alignment remained fault parallel. We interpret these results in terms of the strong crust. On the other hand, both the aftershock alignment and the stress tensor were found to be rotated in the Izmit earthquake epicentral region despite the lower co-seismic displacements. We attribute this feature to the weak crust.

DOI： 10.5047/eps.2009.12.001

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

We have investigated the spatial variation of the stress field along the fault rupture zone of the 1999 Izmit earthquake (M(W) 7.4) using first-motion polarity data at seven distinct aftershock clusters. In our approach, the first-motion polarities of all the aftershocks in a cluster are simultaneously inverted to determine the stress tensor parameters and fault plane solutions of individual events, in accordance with the method of Horiuchi et al. (1995). Where post-seismic slip was significant (e.g., Sapanca, Sakarya-Akyazi, and Karadere segments), we obtained stress tensors with the fault parallel or fault normal maximum (sigma(1)) and minimum (sigma(3)) principal compressive stress axes, which may imply either low frictional coefficients or fault weakness. A stress tensor with similar features was derived from the Cinarcik cluster, where the aftershocks lie in a low-velocity zone beneath the geothermal area. The maximum principal stress axis tends to remain parallel to the trend of the pre-mainshock sigma(1) around the Yalova segment; this segment experienced little to no co-seismic displacements. The stress tensor around the Golcuk segment, where the largest surface displacement of 5.5 m was observed, was determined to be 20-25 degrees counterclockwise rotated, but the aftershock alignment remained fault parallel. We interpret these results in terms of the strong crust. On the other hand, both the aftershock alignment and the stress tensor were found to be rotated in the Izmit earthquake epicentral region despite the lower co-seismic displacements. We attribute this feature to the weak crust.

DOI： 10.5047/eps.2009.12.001

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

In contrast to many ground-based optical observations of the thin lunar alkali exosphere, in situ observations of the exospheric ions by satellite-borne plasma instruments have been quite rare. MAP-PACE-IMA onboard Japanese lunar orbiter SELENE (KAGUYA) succeeded in detecting Moon originating ions at 100 km altitude. Here we make the first report of the ion detection during intervals when the Moon was embedded in the Earth's magnetotail lobe. In the absence of plasma effects on the source process, ion species of H(+), He(++), He(+), C(+), O(+), Na(+), K(+) and Ar(+) are definitively identified. The ion fluxes were higher when the solar zenith angle was smaller, which is consistent with the idea that the solar photon driven processes dominates in supplying exospheric components. Citation: Tanaka, T., et al. (2009), First in situ observation of the Moon-originating ions in the Earth's Magnetosphere by MAP-PACE on SELENE (KAGUYA), Geophys. Res. Lett., 36, L22106, doi: 10.1029/2009GL040682.

DOI： 10.1029/2009GL040682

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

We study solar wind (SW) entry deep into the near-Moon wake using SELENE (KAGUYA) data. It has been known that SW protons flowing around the Moon access the central region of the distant lunar wake, while their intrusion deep into the near-Moon wake has never been expected. We show that SW protons sneak into the deepest lunar wake (anti-subsolar region at similar to 100 km altitude), and that the entry yields strong asymmetry of the near-Moon wake environment. Particle trajectory calculations demonstrate that these SW protons are once scattered at the lunar dayside surface, picked-up by the SW motional electric field, and finally sneak into the deepest wake. Our results mean that the SW protons scattered at the lunar dayside surface and coming into the night side region are crucial for plasma environment in the wake, suggesting absorption of ambient SW electrons into the wake to maintain quasi-neutrality. Citation: Nishino, M. N., et al. (2009), Solar-wind proton access deep into the near-Moon wake, Geophys. Res. Lett., 36, L16103, doi:10.1029/2009GL039444.

DOI： 10.1029/2009GL039444

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

We study solar wind (SW) intrusion into the near-Moon wake using SELENE (KAGUYA) data. It has been known that SW protons are gradually accelerated toward the wake center along magnetic field in the distant lunar wake, while SW intrusion into the near-Moon wake has never been measured. We show that the SW protons come into the lunar wake at similar to 100 km altitude in the direction perpendicular to the magnetic field, as they gain kinetic energy in one hemisphere while lose in the other hemisphere. Particle trajectory calculations and theoretical treatment demonstrate that proton Larmor motions and inward electric field around the wake boundary result in energy gain and loss of the SW protons. Our result shows emergence of proton particle dynamics around the near-Moon space, and suggests that the SW protons may relatively easily access the low-latitude and low-altitude region on the lunar night side. Citation: Nishino, M. N., et al. (2009), Pairwise energy gain-loss feature of solar wind protons in the near-Moon wake, Geophys. Res. Lett., 36, L12108, doi: 10.1029/2009GL039049.

DOI： 10.1029/2009GL039049

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

The Moon has no global intrinsic magnetic field and only has a very thin atmosphere. Ion measurements made from lunar orbit provide us with information regarding interactions between the solar wind and planetary surface, the surface composition through secondary ion mass spectrometry and the source and loss mechanisms of planetary tenuous atmosphere. An ion energy mass spectrometer MAP-PACE IMA onboard a lunar orbiter SELENE (KAGUYA) has detected low-energy ions at 100-km altitude. The MAP-PACE measurements have elucidated that the ions originate from the lunar surface and exosphere and that the ions are at least composed of He(+), C(+), O(+), Na(+) and K(+). Following the discovery of the lunar Na and K exospheres by the ground-based observation, MAP-PACE IMA have found the He, C and O exospheres around the Moon. Citation: Yokota, S., et al. (2009), First direct detection of ions originating from the Moon by MAP-PACE IMA onboard SELENE (KAGUYA), Geophys. Res. Lett., 36, L11201, doi:10.1029/2009GL038185.

DOI： 10.1029/2009GL038185

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Language：English   Publisher：ELSEVIER SCIENCE BV  

DOI： 10.1016/j.pepi.2008.09.014

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Language：English   Publisher：Springer Berlin  

The high-sensitivity fluxgate Lunar MAGnetometer (LMAG) is mounted on SELENE (KAGUYA) to investigate the near-surface electromagnetic environment and the evolution of the Moon through magnetic field observation. To avoid possible electromagnetic interferences, a triaxial fluxgate sensor (MGF-S) is installed at the far end of a 12-m-Iong mast. It is critical for the accurate observation to monitor MGF-S alignment in orbit, and thus we have calibrated the sensor alignment by measuring the known magnetic fields generated by the sensor alignment monitor coil (SAM-C) wound onto the mast canister. In-orbit calibration of the MGF-S alignment was performed twice each revolution during the initial check-out phase of the satellite. It is concluded that there is no systematic difference in the sensor alignment between the day-side and night-side. Applying a new technique based on the Davis-Smith method to the observed magnetic field data when KAGUYA was exposed to the solar wind, a zero offset of each axis was quickly and stably determined every month. As a result, LMAG has been calibrated with an accuracy that is sufficient for detection of the lunar magnetic anomaly at an altitude of 100 km and for high-resolution electron reflectometry. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS).

DOI： 10.1186/BF03352979

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

The high-sensitivity fluxgate Lunar MAGnetometer (LMAG) is mounted on SELENE (KAGUYA) to investigate the near-surface electromagnetic environment and the evolution of the Moon through magnetic field observation. To avoid possible electromagnetic interferences, a triaxial fluxgate sensor (MGF-S) is installed at the far end of a 12-m-long mast. It is critical for the accurate observation to monitor MGF-S alignment in orbit, and thus We have calibrated the sensor alignment by measuring the known magnetic fields generated by the sensor alignment monitor coil (SAM-C) Wound onto the mast canister. In-orbit calibration of the MGF-S alignment Was performed twice each revolution during the initial check-out phase of the satellite. It is concluded that there is no systematic difference in the sensor alignment between the day-side and night-side. Applying a new technique based oil the Davis-Smith method to the observed magnetic field data when KAGUYA was exposed to the solar wind, a zero offset of each axis was quickly and stably determined every month. As a result, LMAG has been calibrated with an accuracy that is sufficient for detection of the lunar magnetic anomaly at an altitude of 100 km and for high-resolution electron reflectometry.

DOI： 10.1186/BF03352979

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DOI： 10.1029/2008JB006117,

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DOI： 10.1029/2008JB006117,

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Language：English   Publisher：COPERNICUS PUBLICATIONS  

The Japanese Lunar Mission "Kaguya" carried out its first magnetic field and plasma measurements in the Earth's magnetotail on 22 December 2007. Fortuitously, three well-defined multiple onset substoms took place. Kaguya was located in the premidnight magnetotail at radial distances of 56RE and observed plasmoids and/or traveling compression regions (TCRs). Although the present study is based on limited data sets, important issues on multiple onset substorms can be examined. Each onset in a series of onsets releases a plasmoid, and magnetic reconnection likely proceeds to tail lobe field lines for each onset. Since the duration of each plasmoid is less than 5 min, these observations imply that magnetic reconnection for each onset can develop fully to the tail lobe field lines and be quenched within this timescale.

DOI： 10.5194/angeo-27-59-2009

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

Interaction between the solar wind and objects in the solar system varies largely according to the settings, such as the existence of a global intrinsic magnetic field and/or thick atmosphere. The Moon's case is characterized by the absence of both of them. Low energy ion measurements on the lunar orbit is realized more than 30 years after the Apollo period by low energy charged particle analyzers MAP-PACE on board SELENE(KAGUYA). MAP-PACE ion sensors have found that 0.1%similar to 1% of the solar wind protons are reflected back from the Moon instead of being absorbed by the lunar surface. Some of the reflected ions are accelerated above solar wind energy as they are picked-up by the solar wind convection electric field. The proton reflection that we have newly discovered around the Moon should be a universal process that characterizes the environment of an airless body. Citation: Saito, Y., et al. (2008), Solar wind proton reflection at the lunar surface: Low energy ion measurement by MAP-PACE onboard SELENE (KAGUYA), Geophys. Res. Lett., 35, L24205, doi:10.1029/2008GL036077.

DOI： 10.1029/2008GL036077

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Language：English   Publisher：ELSEVIER SCIENCE BV  

We have conducted a study of numerical dynamos in a rapidly rotating spherical shell with prescribed non-uniform heat-flux patterns at the outer boundary to examine effects of thermal structure at the core-mantle boundary (CMB) on the geodynamo, especially on the magnetic field strength. Large heat-flux heterogeneity with equatorial symmetry enhances strength of the dipolar magnetic field for a sufficiently small Ekman number, whereas that with equatorial anti-symmetry does not change the strength but affects the tilt of the magnetic dipole axis. The thermal wind induced by heat-flux heterogeneity at the CMB has strong influence on the flow structure and on the magnetic field intensity. These results suggest that thermally heterogeneous Structure of the lowermost mantle might give rise to an anomalously strong geomagnetic field such as that during the Cretaceous Normal Superchron. (c) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2008.06.017

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We have undertaken a numerical study of convection-driven MHD dynamos in a rapidly rotating spherical shell with the Ekman number, E, down to 2 × 1 0- 6 and the magnetic Prandtl number, Pm, down to 0.2. We focus on the characteristic scales of the flow and the magnetic field. Smaller-scale convection vortices responsible for generating the magnetic field appear at lower Ekman numbers, while the scale of the magnetic field shows less variation compared with the flow. As a result, scale separation between the flow and the magnetic field occurs as the Ekman number is decreased. Scale separation helps dynamos to maintain the magnetic field at P m &lt
1 through increase in the effective value of the magnetic Reynolds number. © 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.pepi.2008.03.005

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Language：English   Publisher：ELSEVIER SCIENCE BV  

We have undertaken a numerical study of convection-driven MHD dynamos in a rapidly rotating spherical shell with the Ekman number, E, down to 2 x 10(-6) and the magnetic Prandtl number, Pm, down to 0.2. We focus on the characteristic scales of the flow and the magnetic field. Smaller-scale convection vortices responsible for generating the magnetic field appear at lower Ekman numbers, while the scale of the magnetic field shows less variation compared with the flow. As a result, scale separation between the flow and the magnetic field occurs as the Ekman number is decreased. Scale separation helps dynamos to maintain the magnetic field at Pm < 1 through increase in the effective value of the magnetic Reynolds number. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.pepi.2008.03.005

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

Ground calibration experiments of the SELENE high sensitivity fluxgate Lunar Magnetometer (LMAG) have been performed in order to determine the alignment, sensitivity, and offset of the sensors (MGF-S). It is checked out that the sensors are orthogonal to each other within 0.4 degrees, and the linearity of the ambient magnetic field and the output from the sensors are confirmed. Also, the temperature dependences of the offset and sensitivity are examined but no clear signatures of temperature dependencies can be seen. SELENE has an in-flight calibration system in order to determine the direction of the magnetometer routinely. The magnetic fields generated by the sensor alignment monitor coil (SAM-C) system are used for the in-flight calibration. The magnetic field distributions generated by SAM-C are determined and the accuracy of determination of the magnetometer position and direction is estimated. Multiple measurements will allow us to determine the direction of MGF-S with about 0.1-degree accuracy. Appropriate corrections from the results of the ground and in-flight calibrations will allow us to recover the magnetic field near the moon with accuracy about 0.1 nT.

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

An objective scheme is presented for estimating the lunar crustal magnetic field from the LMAG (Lunar MAGnetometer) data of the SELENE ("KAGUYN') spacecraft. Our scheme improves the equivalent source method in three respects. The first improvement is that the source calculation is performed simultaneously with detrending. The second is that a great number of magnetic charges (magnetic monopoles) are used as the equivalent sources. The third is that the distribution of the magnetic charges is detremined by the damped least squares method, and the optimum smoothness is determined objectively by minimizing Akaike's Bayesian Information Criterion (ABIC). For testing the scheme, we apply it to the Lunar Prospector magnetometer data in the region centered at the Reiner Gamma magnetic anomaly. The magnetic field map at an altitude of 20 km is stably drawn from datasets for different altitudes (18 km and 34 km). The ABIC minimizing criterion successfully controls the smoothness due to the numerical damping and extracts as much information as possible from the given data. This scheme will help produce a coherent lunar magnetic anomaly map by integrating the observations from various altitudes of the SELENE and previous missions.

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Language：English   Publishing type：Book review, literature introduction, etc.   Publisher：SPRINGER  

Mariner 10 measurements proved the existence of a large-scale internal magnetic field on Mercury. The observed field amplitude, however, is too weak to be compatible with typical convective planetary dynamos. The Lorentz force based on an extrapolation of Mariner 10 data to the dynamo region is 10(-4) times smaller than the Coriolis force. This is at odds with the idea that planetary dynamos are thought to work in the so-called magne-tostrophic regime, where Coriolis force and Lorentz force should be of comparable magnitude. Recent convective dynamo simulations reviewed here seem to resolve this caveat. We show that the available convective power indeed suffices to drive a magnetostrophic dynamo even when the heat flow though Mercury's core-mantle boundary is subadiabatic, as suggested by thermal evolution models. Two possible causes are analyzed that could explain why the observations do not reflect a stronger internal field. First, toroidal magnetic fields can be strong but are confined to the conductive core, and second, the observations do not resolve potentially strong small-scale contributions. We review different dynamo simulations that promote either or both effects by (1) strongly driving convection, (2) assuming a particularly small inner core, or (3) assuming a very large inner core. These models still fall somewhat short of explaining the low amplitude of Mariner 10 observations, but the incorporation of an additional effect helps to reach this goal: The subadiabatic heat flow through Mercury's core-mantle boundary may cause the outer part of the core to be stably stratified, which would largely exclude convective motions in this region. The magnetic field, which is small scale, strong, and very time dependent in the lower convective part of the core, must diffuse through the stagnant layer. Here, the electromagnetic skin effect filters out the more rapidly varying high-order contributions and mainly leaves behind the weaker and slower varying dipole and quadrupole components (Christensen in Nature 444: 1056-1058, 2006). Messenger and BepiColombo data will allow us to discriminate between the various models in terms of the magnetic fields spatial structure, its degree of axisymmetry, and its secular variation.

DOI： 10.1007/s11214-007-9280-5

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Language：English   Publisher：BLACKWELL PUBLISHING  

Detailed source process of the 2000 November 15 Lake Van (eastern Turkey) earthquake (M-w = 5.6) was retrieved using the method of Kikuchi & Kanamori for source inversion of complex body waveforms. The event has been reported by USGS as a deep (67 km) subcrustal earthquake resulting from a rupture on a normal fault beneath the Bitlis Suture zone where continental collision is in action between the Arabian and Eurasian plates. However, our source model based on the analysis of complex body-waveforms suggests that the earthquake is a shallow crustal event comprised of two subevents on a predominantly reverse fault at depths of 12.5 and 15 km with a time interval of about 18 s. The seismic moment of the second subevent (M-o = 1.6 x 10(17) Nm; strike = 76 degrees, dip = 55 degrees and rake = 120 degrees) is larger than the first subevent (M-o = 1.0 x 10(17) Nm; strike = 87 degrees, dip = 62 degrees and rake = 110 degrees). Visual inspection of the strong ground motion records and waveform inversion analysis of the near-field records from four broad-band stations confirm the occurrence of the two subevents at shallow depths. We examined whether or not the observed complex waveforms of the main shock can be modelled in terms of a single point source embedded at an intermediate depth, and the result turned out to be further evidence for the multiple rupture. The plausibility of the focal depths and mechanisms of the two subevents was also examined by retrieving the source parameters of 14 aftershocks from near-field waveform data. Most of the aftershocks were relocated at depths around 15 km, which agrees with the shallow main shock. In addition, the analysis of near-field waveform data indicates subcrustal earthquake activity neither in the source region of the Lake Van earthquake nor in the Turkey-Iran border region. In the Turkey-Iran border region, an event on 1999 February 19 was reported to have taken place at a depth of 66 km (USGS) and 77 km (ISC), which also conflicts with our focal depth (18 km) determined through CMT inversion analysis of the broad-band records at the station GNI.

DOI： 10.1111/j.1365.246X.2007.03445.x

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

Magnetic polarity transitions in a Takahashi-Matsushima-Honkura dynamo model are analyzed. Distinctive differences in behavior of the axisymmetric poloidal magnetic field are found among a polarity reversal and excursions. including short polarity events. At the beginning of magnetic polarity transitions, the magnetic field with the reversed polarity is generated by anti-cyclonic convection columns deep within the outer core. In the case of excursion, it is soon advected by the radial flow toward a shallow interior of the core, and the transition can be detected at the core surface. However, the same process retrieves the original polarity from the deep interior, and the reversed field eventually vanishes. In the case of polarity reversal, on the other hand, the reversed polarity field is persistently generated deep within the core. It is then advected toward a shallow interior of the core, while the generation process of the reversed field occurs successively. The reversed polarity field near the core surface is collected by the downwelling flow associated with convection columns, as is the case for the original polarity field. The polarity reversal is completed by the advection process, the duration of which is consistent with the flow speed in the core.

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Language：English   Publisher：Springer Berlin  

Magnetic polarity transitions in a Takahashi-Matsushima-Honkura dynamo model are analyzed. Distinctive differences in behavior of the axisymmetric poloidal magnetic field are found among a polarity reversal and excursions, including short polarity events. At the beginning of magnetic polarity transitions, the magnetic field with the reversed polarity is generated by anti-cyclonic convection columns deep within the outer core. In the case of excursion, it is soon advected by the radial flow toward a shallow interior of the core, and the transition can be detected at the core surface. However, the same process retrieves the original polarity from the deep interior, and the reversed field eventually vanishes. In the case of polarity reversal, on the other hand, the reversed polarity field is persistently generated deep within the core. It is then advected toward a shallow interior of the core, while the generation process of the reversed field occurs successively. The reversed polarity field near the core surface is collected by the downwelling flow associated with convection columns, as is the case for the original polarity field. The polarity reversal is completed by the advection process, the duration of which is consistent with the flow speed in the core. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS).

DOI： 10.1186/BF03352729

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

CiNii Books

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The effects of small-scales (subgrid-scales) on large-scales (grid-scales) must be properly estimated in large eddy simulation. We reexamine a scale-similarity model based on spatial filter-width dependence in a small region of the Earth's core with periodic boundary conditions proposed by Matsushima (2004). The model, which has been found not to well reproduce spatial distribution of subgrid-scale fluxes, is then improved. Spatial distribution and amplitude of subgrid-scale fluxes estimated by the new model are compared with those obtained through direct numerical simulations (DNS) in confirmation of substantial improvement of the model. Numerical simulations at lower resolution with and without some subgrid-scale models are carried out to demonstrate that the present model is reliable and robust. Heat flux, kinetic energy and magnetic energy averaged over space and time are compared among results of DNS at high and low resolution and of numerical simulations with subgrid-scale models. The present model is found to perform better than the others examined in this study.

DOI： 10.1080/03091920600768397

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Language：English   Publisher：TAYLOR & FRANCIS LTD  

The effects of small-scales (subgrid-scales) on large-scales (grid-scales) must be properly estimated in large eddy simulation. We reexamine a scale-similarity model based on spatial filter-width dependence in a small region of the Earth's core with periodic boundary conditions proposed by Matsushima (2004). The model, which has been found not to well reproduce spatial distribution of subgrid-scale fluxes, is then improved. Spatial distribution and amplitude of subgrid-scale fluxes estimated by the new model are compared with those obtained through direct numerical simulations (DNS) in confirmation of substantial improvement of the model. Numerical simulations at lower resolution with and without some subgrid-scale models are carried out to demonstrate that the present model is reliable and robust. Heat flux, kinetic energy and magnetic energy averaged over space and time are compared among results of DNS at high and low resolution and of numerical simulations with subgrid-scale models. The present model is found to perform better than the others examined in this study.

DOI： 10.1080/03091920600768397

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Language：English  

Dynamo action possibly working in the fluid core of Mercury is examined using numerical models in a thin spherical shell. Dipolar (DP) dynamos are obtained in the regime of columnar flows outside the tangent cylinder (TC), whereas non-dipolar (NDP) dynamos dominated by the multipole components are found in the regime of flows both inside and outside the TC. It turns out that columnar-like convective motions not only outside but also inside the TC are responsible for the NDP dynamo. The electrically conducting inner core enhances the strength of large-scale magnetic field, but predominance of the NDP components remains due to the thin shell geometry. These results suggest that Mercury may have more complicated magnetic field than has been considered. Copyright 2006 by the American Geophysical Union.

DOI： 10.1029/2006GL025792

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

Dynamo action possibly working in the fluid core of Mercury is examined using numerical models in a thin spherical shell. Dipolar (DP) dynamos are obtained in the regime of columnar flows outside the tangent cylinder (TC), whereas non-dipolar (NDP) dynamos dominated by the multipole components are found in the regime of flows both inside and outside the TC. It turns out that columnar-like convective motions not only outside but also inside the TC are responsible for the NDP dynamo. The electrically conducting inner core enhances the strength of large-scale magnetic field, but predominance of the NDP components remains due to the thin shell geometry. These results suggest that Mercury may have more complicated magnetic field than has been considered.

DOI： 10.1029/2006GL025792

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

We show presence of a mini-magnetosphere above the Reiner Gamma magnetic anomaly (RGA) region in the solar wind, using Lunar Prospector magnetometer (MAG) measurement data. RGA is one of the strongest magnetic anomalies on the Moon. Two magnetic anomalies are found from six MAG datasets at 17 - 40 km altitudes in the lunar wake or the geomagnetic tail lobe and are well explained by a two-dipole model. When RGA was exposed to the solar wind plasma, two MAG datasets were obtained at 27 29 km altitudes. Although the magnetic anomalies survived against the plasma pressure, they were heavily distorted in comparison with the magnetic field of the two-dipole model. Flow directions and dynamic pressures of the solar wind plasma at those periods indicate that the distortions were caused by forming a mini-magnetosphere over the RGA region in the solar wind.

DOI： 10.1029/2005GL024097

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Language：English  

Turbulent motions in the Earth's core have a strong influence on diffusive processes for large-scale fields through the eddy diffusion. They are highly anisotropic due to the influence of the Earth's rotation and the magnetic field. It is therefore of significance to estimate the eddy diffusivity and to model subgrid-scale processes properly. A scale-similarity model, being one of the subgrid-scale models used in large-eddy simulation, is found to reproduce anisotropy. The purpose of this study is to reexamine scale similarity of magnetohydrodynamic (MHD) turbulence in the core and to demonstrate the validity of the subgrid-scale model developed by Matsushima [Matsushima, M., 2004. Scale similarity in the Earth's core. Earth Planets Space 56, 599-605.] in two ways. First, spatial correlation and root-mean-square amplitude of the turbulent flux estimated through use of the model are compared with those obtained through direct numerical simulations at the same time step. The model appears valid from the result. Second, numerical simulations of MHD turbulence including the model of subgrid scales are carried out, and their results of time evolution are compared with those obtained through numerical simulations excluding the model. The result also indicates the subgrid-scale model holds valid. © 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.pepi.2005.02.005

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Language：English   Publisher：ELSEVIER SCIENCE BV  

Turbulent motions in the Earth's core have a strong influence on diffusive processes for large-scale fields through the eddy diffusion. They are highly anisotropic due to the influence of the Earth's rotation and the magnetic field. It is therefore of significance to estimate the eddy diffusivity and to model subgrid-scale processes properly. A scale-similarity model, being one of the subgrid-scale models used in large-eddy simulation, is found to reproduce anisotropy. The purpose of this study is to reexamine scale similarity of magnetohydrodynamic (MHD) turbulence in the core and to demonstrate the validity of the subgrid-scale model developed by Matsushima [Matsushima, M., 2004. Scale similarity in the Earth's core. Earth Planets Space 56, 599-605.] in two ways. First, spatial correlation and root-mean-square amplitude of the turbulent flux estimated through use of the model are compared with those obtained through direct numerical simulations at the same time step. The model appears valid from the result. Second, numerical simulations of MHD turbulence including the model of subgrid scales are carried out, and their results of time evolution are compared with those obtained through numerical simulations excluding the model. The result also indicates the subgrid-scale model holds valid. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.pepi.2005.02.005

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Language：English  

High-resolution, low-viscosity geodynamo simulations have been carried out on the Earth Simulator, one of the fastest supercomputers, in a dynamic regime similar to that of Earth's core, that is, in a quasi-Taylor state. Our dynamo models exhibit features of the geodynamo not only in spatial and temporal characteristics but also in dynamics. Polarity reversals occurred when magnetic flux patches at high latitudes moved poleward and disappeared
patches with reversed field at low and mid-latitudes then moved poleward.

DOI： 10.1126/science.1111831

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Language：English   Publisher：AMER ASSOC ADVANCEMENT SCIENCE  

High-resolution, low-viscosity geodynamo simulations have been carried out on the Earth Simulator, one of the fastest supercomputers, in a dynamic regime similar to that of Earth's core, that is, in a quasi-Taylor state. Our dynamo models exhibit features of the geodynamo not only in spatial and temporal characteristics but also in dynamics. Polarity reversals occurred when magnetic flux patches at high latitudes moved poleward and disappeared; patches with reversed field at low and mid-latitudes then moved poleward.

DOI： 10.1126/science.1111831

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Language：English   Publisher：AMER INST PHYSICS  

We investigate convection-driven dynamos in a rotating spherical shell with the Rayleigh number Ra up to about 53 times the critical value Ra-C, emphasizing Rayleigh number dependence of the thermal convection and the magnetic field generated by dynamo action. The Rayleigh numbers used in calculations are chosen so as to be in a range which allows us to study the sequence of bifurcation. In the low-Ra-dynamo regime, the flow structure is characterized by columnar convection cells, which mainly generate the magnetic field that is predominantly dipolar. Force balance is essentially in a geostrophic state. Both the magnetic energy and the kinetic energy increase with increase in Ra. In the moderate-Ra-dynamo regime, convective motions appear inside the tangent cylinder (TC), where the azimuthal magnetic field is generated through the so-called omega effect. However, the magnetic energy shows saturation due to relatively inefficient magnetic field generation. In the high-Ra-dynamo regime, dominance of convection inside the TC is remarkable. The advection processes play important roles both in force balance and in magnetic field generation. The magnetic field is generated very inefficiently, leading to the reduced magnetic energy in spite of higher kinetic energy. These three dynamo regimes exhibit distinctive differences in the process of generating magnetic field and characteristic dissipation scales. (c) 2005 American Institute of Physics.

DOI： 10.1063/1.1972016

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Language：English   Publisher：ELSEVIER SCIENCE BV  

Wide-band (320-0.001 Hz) and long period (0.01-0.0001 Hz) magnetotelluric (MT) data were acquired along two profiles crossing the western part of the North Anatolian fault zone (NAFZ), Turkey, which consists of two main fault branches. The first profile (Izmit profile) crosses the epicentral area of the 17 August 1999 Izmit earthquake. In fact, the NIT measurements along this profile started just a few weeks before the occurrence of the mainshock and four instruments happened to be in operation in the vicinity of the fault when the earthquake took place. The second profile (Adapazari profile) is located about 30 km east of the first profile. Two-dimensional modeling shows the following results. First, the hypocenters of mainshock and aftershocks are located on the highly resistive side near the edge of a conductive zone. Second, the long-period NIT data show a low resistivity zone extending down to 50 km between the two fault branches. Such a deep conductive zone is interpreted as representing partial melting resulted from the past complex tectonics in this region, and it is related to the non-seismic after-slip in the layer below the seismogenic zone characterized by the highly resistive layer. (c) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/jpepi.2004.08.033

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Language：Japanese   Publisher：Tokyo Geographical Society  

The Earth possesses its intrinsic magnetic field generated by fluid motion in the electrically conducting core, known as the geodynamo. Provided the spatial distributions of the Earth's magnetic field and its temporal variations are known at the core surface, it seems to be possible to estimate the fluid motion there by solving an inverse problem. The magnetic diffusion term is much smaller than the advection term in the magnetic induction equation, and therefore it can be neglected for the time scale much shorter than the magnetic diffusion time. This is called as the frozen-flux approximation, and magnetic lines of force behave as if they are frozen-in fluid elements. Because of its fundamental non-uniqueness, additional constraints, for example, toroidal flow, steady flow, geostrophic flow, or a combination there of, are imposed in estimating the flow at the core surface. Estimated fluid motions have common features : large vortices at midlatitudes and westward flow near the equatorial region. It is now possible to carry out numerical simulations of three-dimensional magnetohydrodynamic dynamos in rotating spherical shells. Inversion methods are then tested for synthetic data of numerical models. It is concluded that some meaningful information on the core flow can be recovered, but that the resolution of the magnetic field at the core surface has a crucial effect on the flow structure. Hence, to construct a realistic geodynamo model, it is important to determine the magnetic field precisely with smaller length scales at the core surface. It is required to monitor the geomagnetic field by launching satellites for magnetic field measurements periodically, for example every five years.

DOI： 10.5026/jgeography.114.2_132

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Other Link： https://jlc.jst.go.jp/DN/JALC/00251527172?from=CiNii

DOI： 10.5026/jgeography.114.2_132

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

Turbulent motions in the core, being highly anisotropic because of the influence of the Earth's rotation and its magnetic field, cause the eddy diffusion of large-scale fields much more effectively than the molecular diffusion. Reliable estimates of the eddy diffusivities, or the subgrid-scale fluxes, are therefore of significance. In this paper, scale similarity of magnetohydrodynamic turbulence in a rapidly rotating system is investigated to model subgrid-scale processes, as used in large-eddy simulations. The turbulent flux has been computed by taking an ensemble average of results of direct numerical simulations, which are to be employed in this paper, over the computational box which represents a small region in the Earth's core. The anisotropy of turbulent flux computed after averaging over segments into which the box is divided remains unchanged even when the size of segments changes. Dependence of turbulent flux computed from fields to which a spatial filter is applied on its width indicates that subgrid-scale flux can be evaluated through extrapolation. This method will be useful for performing global geodynamo simulations taking into account subgrid-scale processes.

DOI： 10.5636/eps.56.6_599

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DOI： 10.5636/eps.56.6_599

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Language：English   Publisher：ELSEVIER SCIENCE BV  

Results of numerical simulations of three-dimensional, self-consistent, convection driven magnetohydrodynamic dynamos in a rotating spherical shell are reported. The electrical conductivity of the inner core and its solid body rotation relative to the reference frame are taken into account. Emphasis is put on the magnetic field generation inside the so-called tangent cylinder (TC) and its importance on the overall dynamo process. Convection inside the TC consisting of some upward and downward plumes takes place at a relatively high supercritical Rayleigh number. These plumes shear the toroidal field to the north-south direction, generating a new poloidal field, which does not grow significantly due to spatial and temporal variations in the plumes. The toroidal field often exhibits quadrupolar structure inside the TC and reverses its direction according to the direction of differential rotation. Such a quadrupolar symmetric toroidal field is destroyed by strong intermittent fluctuation in the meridional circulation inside the TC and the dipolar symmetric field recovers. It turns out, however, that the magnetic field generation inside the TC is at most several times weaker than the outside. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.pepi.2003.07.009

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

Magnetotelluric (MT) surveys were carried out along some profiles crossing the fault rupture zone associated with the Izmit earthquake which took place on 17 August 1999 in the western part of the North Anatolian Fault Zone (NAFZ). In this paper, we focus on the western part of the fault rupture zone where two different groups of seismicity followed the Izmit earthquake. One group was seen along a narrow belt and corresponds to aftershocks occurring along the fault rupture zone. The other was seen in a circular region and represents a swarm activity, presumably triggered by the occurrence of the Izmit earthquake. Two-dimensional inversion was performed for the MT data acquired along two profiles; one crosses the western end of the fault rupture zone and the other is located in the west of the swarm activity area. In the former case, aftershocks tended to occur in a resistive zone underlain by a moderately conductive zone, as was the case for the hypocenter area. In the latter case, the swarm activity tends to be confined in a conductive zone below a highly resistive zone. This activity is likely to be triggered through pore-pressure changes associated with the Izmit earthquake.

DOI： 10.5636/eps.55.7_437

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Language：English   Publisher：SEISMOLOGICAL SOC AMER  

In order to investigate the resistivity structure beneath the northern and the southern branches of the western part of the North Anatolian Fault Zone (NAFZ), which has been known as a seismic gap area, we started wideband magnetotelluric (NIT) measurements on 27 July 1999, alone, a north-south profile crossing the northern branch near Izmit and the southern branch near Iznik. When the Izmit (Kocaeli) earthquake (M-w 7.4) occurred on 17 August 1999, NIT fields were being measured at five sites, four of which happened to be located near the surface rupture zone associated with the mainshock, and large variations in the NIT fields were observed in association with seismic waves. We propose motional electromagnetic induction in the electrically conducting crust, which vibrates under the Earth's magnetic field, as one of the possible generation mechanisms for such NIT field variations. For more detailed studies of such a mechanism, we installed a wideband NIT instrument and a three-component seismometer at two of the four sites on 16 September 1999 and made simultaneous measurements during several aftershocks. The estimate of the order of the magnitude of motional electromagnetic response, derived from the NIT fields and ground motion observed during a large aftershock (M 4.5), supports our claim that the seismo-dynamo effect is a plausible mechanism for variations in the NIT fields associated with the seismic waves.

DOI： 10.1785/0120000807

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

Electric and magnetic data were obtained above the focal area in association with the 1999 Izmit, Turkey earthquake. The acquired data are extremely important for studies of electromagnetic phenomena associated with earthquakes, which have attracted much attention even without clear physical understanding of their characteristics. We have already reported that large electric and magnetic variations observed during the earthquake were simply due to seismic waves through the mechanism of seismic dynamo effect, because they appeared neither before nor simultaneously with the origin time of the earthquake but a few seconds later, with the arrival of seismic wave. In this letter we show the result of our further analyses. Our detailed examination of the electric and magnetic data disclosed small signals appearing less than one second before the large signals associated with the seismic waves. It is not yet solved whether this observational fact is simply one aspect of the seismic dynamo effect or requires a new mechanism.

DOI： 10.1186/BF03352449

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Language：English   Publisher：ELSEVIER SCIENCE BV  

We present the results of a benchmark study for a convection-driven magnetohydrodynamic dynamo problem in a rotating spherical shell. The solutions are stationary aside from azimuthal drift. One case of non-magnetic convection and two dynamos that differ in the assumptions concerning the inner core are studied. Six groups contributed numerical solutions which show good agreement. This provides an accurate reference standard with high confidence. (C) 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0031-9201(01)00275-8

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Language：English   Publisher：ELSEVIER SCIENCE BV  

Numerical simulations of three-dimensional self-consistent MHD dynamos in a rotating spherical shell are performed to examine the structures of the velocity and the magnetic fields and the mechanism of magnetic field generation. Emphasis is put on an important role of the boundary layer which arises for the no-slip boundary condition. The most important is precise computation in the boundary layers, in which the number of grid points must be large enough to ensure spatial resolution there. The result of computation shows that the dipole field is dominant and that the magnetic field is concentrated in the convection columns. Such results are the same as those derived from our previous study for the stress-free boundary condition. A marked difference is the strong toroidal magnetic field generated by strong shear flow inside the boundary layers. Also the effect of magnetic diffusion is strong and more significant than that of magnetic induction near the spherical surfaces. This suggests that the so-called frozen-flux hypothesis, which has usually been used to estimate core surface flows, does not necessarily hold for the cases in which significant boundary layers appear. (C) 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0031-9201(01)00283-7

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Language：English   Publisher：ELSEVIER SCIENCE BV  

Direct numerical simulations of local turbulence in the Earth's core are performed. A small region in the core is represented by a rectangular box with periodic boundary conditions. A uniform magnetic field is imposed, and small-scale motions are driven by a large-scale temperature gradient parallel to the gravitational field; the system is rapidly rotating. The turbulent heat flux is computed and represented by an anisotropic eddy diffusivity tensor. This tensor is compared with an alternative expression derived directly from the basic equations by applying a second moment closure model of the type used in turbulence theory. It is found that the two methods give consistent results, although some modifications are necessary to improve the agreement. It is believed that the results of the present study will be useful in quantifying the effects of anisotropic turbulence on global geodynamo models. (C) 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0031-9201(01)00282-5

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

Intraplate earthquake zones in the back arc of NE Japan were imaged by wide-band magnetotelluric (MT) soundings. The 90km long MT profile of 34 stations extends over the two topographic features, the Dewa Hills and the Ou Backbone Range, which were uplifted by thrust faults. MT data show two-dimensionality and strong TE/TM anisotropic responses at the periods around 100 s. After tensor decompositions with regional strike of N12 degreesE, two-dimensional inversion was carried out where static shift was also a model parameter. The final model is characterized by conductive blocks in the mid-crust to account for the anisotropic responses. Correlation of the conductors to the seismic scatterers and to the low velocity anomalies suggests that the conductors represent fluids. High seismicity clustering near the rims of conductors suggests that the intraplate seismicity results either from the migration of the fluids to less permeable crust or from local stress concentration near the structural boundary.

DOI： 10.1029/2001GL013269

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Direct numerical simulations of local turbulence in the Earth's core are performed. A small region in the core is represented by a rectangular box with periodic boundary conditions. A uniform magnetic field is imposed, and small-scale motions are driven by a large-scale temperature gradient parallel to the gravitational field
the system is rapidly rotating. The turbulent heat flux is computed and represented by an anisotropic eddy diffusivity tensor. This tensor is compared with an alternative expression derived directly from the basic equations by applying a second moment closure model of the type used in turbulence theory. It is found that the two methods give consistent results, although some modifications are necessary to improve the agreement. It is believed that the results of the present study will be useful in quantifying the effects of anisotropic turbulence on global geodynamo models. © 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0031-9201(01)00282-5

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Language：English   Publishing type：Rapid communication, short report, research note, etc. (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

On August 17, 1999, a destructive earthquake occurred in the western part of the North Anatolian Fault Zone, Turkey. The earthquake source region has been designated as a seismic gap and an M7-class earthquake has been supposed to occur someday in the future so as to fill this seismic gap. So far we have undertaken various kinds of observations in this area and we could obtain some valuable data before, during and after the mainshock. Here we report some of the preliminary results of our recent studies, which include field work started in late July this year and continued during and after the earthquake occurrence just in the earthquake source region and its vicinity, in addition to seismic observations carried out for several years before the mainshock. Much emphasis is put on magnetotelluric field data acquired during the mainshock; in fact, large variations caused by seismic waves were recorded. Such variations could be interpreted in terms of electromagnetic induction in the conducting crust caused by the velocity field interacting with the static magnetic field of the Earth. In particular, the first motion of seismic wave could be identified in the records and used for precise determination of the hypocenter of the mainshock.

DOI： 10.5636/eps.52.293

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Language：Japanese  

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A model of three-dimensional self-consistent MHD dynamo in a rotating spherical shell is investigated through numerical simulations, in which a fully spectral scheme is used to guarantee precise computation in space. Generation mechanisms of the magnetic field and the structures of the magnetic and the velocity fields are then examined with emphasis on a fundamental process in the Earth's core. It turns out that the magnetic field is confined in forward convective columns with respect to the drift direction of convection pattern. Detailed examination of generation mechanism of the magnetic field indicates that a strong toroidal magnetic field is generated by shear motion near the equatorial region close to the outer surface, whereas the poloidal magnetic field is rather maintained by fluid motion related to convective columns. Even when the magnetic and the velocity fields vary considerably, this relationship holds with some disruption during disordered states, and time variations of the magnetic field always show a time lag behind those of the velocity field. Although the so-called weak-field regime is investigated, fundamental phenomena shown in this paper should be of importance for understanding of the Earth and planetary dynamo processes.

DOI： 10.1016/S0031-9201(98)00152-6

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Language：English   Publisher：ELSEVIER SCIENCE BV  

A model of three-dimensional self-consistent MHD dynamo in a rotating spherical shell is investigated through numerical simulations, in which a fully spectral scheme is used to guarantee precise computation in space. Generation mechanisms of the magnetic field and the structures of the magnetic and the velocity fields art then examined with emphasis on a fundamental process in the Earth's core. It turns oat that the magnetic field is confined in forward convective columns with respect to the drift direction of convection pattern. Detailed examination of generation mechanism of the magnetic field indicates that a strong toroidal magnetic field is generated by shear motion near the equatorial region close to the outer surface, whereas the poloidal magnetic field is rather maintained by fluid motion related to convective columns. Even when the magnetic and the velocity fields vary considerably, this relationship holds with some disruption during disordered states, and time variations of the magnetic field always show a time lag behind those of the velocity field. Although the so-called weak-field regime is investigated, fundamental phenomena shown in this paper should be of importance for understanding of the Earth and planetary dynamo processes. (C) 1999 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0031-9201(98)00152-6

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

The anisotropy of local turbulence in the Earth's core is examined. It is recognized that small-scale motions in the core are strongly influenced by the Earth's rotation and its magnetic field. A small region of the core is simulated (the computational box), across which the prevailing large-scale (toroidal) magnetic field is supposed to be uniform and in which the temperature or compositional gradient providing the buoyancy that powers the turbulence is parallel to the (uniform) gravitational field. The simulations are used to estimate the turbulent fluxes of mean fields and their dependence on the latitude at which the computational box is situated. It is found that the effect of local turbulence on the diffusion of large-scale fields is significant, and that turbulent transport is anisotropic. It is believed that the results of the present study will prove useful in determining geophysically realistic diffusivities for use in future global geodynamo simulations.

DOI： 10.5636/eps.51.277

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DOI： 10.5636/eps.50.651

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

Long-period electromagnetic response must be estimated to derive information on the electrical conductivity distribution within the deep mantle of the Earth. For this, we estimated the electromagnetic response function for long-period geomagnetic variations, on the basis of the P-1 approximation. The data used for analyses are geomagnetic daily mean values for 10 years (1965-1974) from 59 stations distributed over the globe and for about 20 years (1960-1980) from 9 stations. It turned out that the P-1 approximation generally holds except for the periods corresponding to annual and semi-annual variations. We also found that accurate estimation is difficult for periods longer than a few years, probably because of contamination due to secular variations of core origin.

DOI： 10.5636/eps.50.651

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Language：Japanese   Publisher：海洋出版  

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Observations of the electric potential were made at three sites located near the Nojima fault on Awaji Island, aiming at examining whether any anomalous changes appear in association with aftershocks of the 1995 Hyogo-ken Nanbu earthquake of M 7.2. Unfortunately the observation area turned out to be highly contaminated by noise currents arising from the remote DC-operated railway systems. In order to avoid noise contamination, we attempted to apply the BAYTAP-G analysis method to the data obtained during the aftershock activity and found a gradual change which appeared several days before an aftershock of M 4.8. Time changes in the preferential direction of noise currents, are indicated due to the inhomogeneous resistivity structure in the vicinity of the fault, and it is suggested that they are likely to reflect changes in the electrical property of the fault.

DOI： 10.4294/jpe1952.44.397

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The velocity and the magnetic fields within the Earth's outer core are estimated from geomagnetic field data. This is performed by prescribing the radial dependence of the poloidal velocity field, into which the energy source for the geodynamo is implicitly incorporated, and by solving the Navier-Stokes and the magnetic induction equations. The conditions that time change of the velocity field is very slow and that the magnetic field and its time change must fit the observed geomagnetic field are imposed. The estimated velocity fields then have some common features with those estimated on the basis of the frozen-flux approximation. The toroidal magnetic field is stronger than the poloidal magnetic field, but both are of the same order of strength. Both generation balance in the induction equation and the strength of magnetic field in the core suggest that the geodynamo is likely to be of alpha(2) omega-type. Force balance in the core is estimated to be magnetostrophic.

DOI： 10.1016/0031-9201(95)03050-7

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Language：English   Publishing type：Rapid communication, short report, research note, etc. (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

We have attempted a blimp-borne magnetic survey over the Unzen Volcano which has been erupting since November, 1990. The first attempt, made in January, 1992, was unsuccessful, but the second one in March, 1992 was partly successful, yielding the distribution of the geomagnetic total intensity at an altitude of about 1,500 m. These total intensity data could be interpreted in terms of inhomogeneous magnetizations of rocks forming respective mountains of the Unzen Volcano. The east of Mt. Fugen, which is the site of current lava extrusion, shows a magnetization of 2 similar to 3 A/m. This result would be an important piece of information for a study of volcanomagnetic effect, in particular in constructing a thermal demagnetization model.

DOI： 10.5636/jgg.47.231

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The velocity and the magnetic fields within the Earth's outer core are estimated from geomagnetic field data. This is performed by prescribing the radial dependence of the poloidal velocity field, into which the energy source for the geodynamo is implicitly incorporated, and by solving the Navier-Stokes and the magnetic induction equations. The conditions that time change of the velocity field is very slow and that the magnetic field and its time change must fit the observed geomagnetic field are imposed. The estimated velocity fields then have some common features with those estimated on the basis of the frozen-flux approximation. The toroidal magnetic field is stronger than the poloidal magnetic field, but both are of the same order of strength. Both generation balance in the induction equation and the strength of magnetic field in the core suggest that the geodynamo is likely to be of α2ω-type. Force balance in the core is estimated to be magnetostrophic. © 1995.

DOI： 10.1016/0031-9201(95)03050-7

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An attempt is made to derive fluid motion in the Earth's outer core from geomagnetic field data. We implicitly incorporate the energy source for the geodynamo in the prescribed radial dependence of poloidal velocity field. Then the Navier-Stokes equation for the toroidal constituent and the induction equation for the toroidal and the poloidal magnetic fields are solved so as to fit the magnetic field at the core-mantle boundary estimated through downward continuation on the assumption that the mantle is an insulator. The basic standpoint is that the large-scale magnetic field is maintained by induction processes associated with large-scale fluid motion within the core. We also impose the condition that time variations of the velocity and the magnetic fields are very slow. The generation balance in the induction equation, for the derived velocity and magnetic fields, suggests that the geodynamo is likely to be of alpha(2) omega-type.

DOI： 10.5636/jgg.45.1481

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Language：English  

An attempt is made to derive fluid motion in the Earth's outer core from geomagnetic field data. We implicitly incorporate the energy source for the geodynamo in the prescribed radial dependence of poloidal velocity field. Then the Navier-Stokes equation for the toroidal constituent and the induction equation for the toroidal and the poloidal magnetic fields are solved so as to fit the magnetic field at the core-mantle boundary estimated through downward continuation on the assumption that the mantle is an insulator. The basic standpoint is that the large-scale magnetic field is maintained by induction processes associated with large-scale fluid motion within the core. We also impose the condition that time variations of the velocity and the magnetic fields are very slow. The generation balance in the induction equation, for the derived velocity and magnetic fields, suggests that the geodynamo is likely to be of α2ω-type. © 1993, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

DOI： 10.5636/jgg.45.1481

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

Time evolution of the magnetic field is examined numerically for some models of thermally driven MHD dynamo in a spherical shell. Special attention is paid to whether the polarity of the dipole magnetic field, in the models, reverses its sign during time evolution. Here we present the results for two typical models. In one model, the temperature is held constant at both boundaries of a spherical shell, whereas in the other, the distribution of temperature is represented by the degree 2 and order 2 constituent of spherical harmonics. The former shows an oscillatory variation and hence the polarity is reversed rather periodically The latter results in a rather stationary state and the magnetic field tends to stay at one polarity state. These numerical results imply that the thermal interaction between the core and the mantle can control the frequency of reversal.

DOI： 10.5636/jgg.45.1591

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

Fluid motion in the Earth's outer core has been derived from geomagnetic field data on the presumption that a strong omega-effect is operative in the core. In this case, it is important to properly estimate differential rotation which is expected to generate strong zonal toroidal magnetic fields. So far, we have relied on a physically simplified model in terms of virtual displacement of fluid particles. We here attempt to solve both the induction and the Navier-Stokes equations. The basic standpoint is that the non-axisymmetric poloidal magnetic field is maintained by the interaction between the strong zonal toroidal magnetic fields and a large-scale fluid motion within the outer core. The radial dependence of poloidal velocity field is unknown, but once a rather simple functional form is prescribed, the magnitude is determined so as to fit the magnetic field data. Then the toroidal velocity field can be derived by solving the Navier-Stokes equation. For simplicity, we assume that the steady state is realized at various epochs.
In order to check the validity of solutions, time-dependent behavior of the magnetic field is examined with the derived velocity field fixed. The time integration is performed using the fourth-order Runge-Kutta scheme. Unless the derived velocity field is appropriate, the magnetic field would vary rapidly contrary to the assumption of the steady state.
It turned out that the estimation of differential rotation based on virtual displacement of fluid particles is not valid. As long as a large-scale fluid motion and a large-scale magnetic field are considered, the omega-effect is not so effective as presumed and strong zonal toroidal magnetic fields are not generated. In order to derive fluid motion within the core, we should take all the interaction terms into consideration.

DOI： 10.5636/jgg.44.521

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

The magnetic field is one of the intrinsic properties of the Earth, originating in a magnetohydrodynamic (MHD) process in the liquid outer core. It also exhibits a variety of features of secular variation; most typically, reversals of the polarity of the axial dipole term and westward drifts of the non-zonal terms. Here we report on the results of our simulations of an MHD dynamo process in a rotating spherical shell and demonstrate that the above two typical features of the Earth's magnetic field can be understood quite naturally without any specific conditions. We also examine how the virtual geomagnetic pole (VGP) moves, in our MHD dynamo model, during transitional stages from one polarity to the other, and show that some features of the transitional behaviour derived from the paleomagnetic data can be seen in our dynamo model as well.

DOI： 10.5636/jgg.44.931

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

Fluid motion in the Earth's outer core has been derived from geomagnetic field data on the presumption that a strong omega-effect is operative in the core. In this case, it is important to properly estimate differential rotation which is expected to generate strong zonal toroidal magnetic fields. So far, we have relied on a physically simplified model in terms of virtual displacement of fluid particles. We here attempt to solve both the induction and the Navier-Stokes equations. The basic standpoint is that the non-axisymmetric poloidal magnetic field is maintained by the interaction between the strong zonal toroidal magnetic fields and a large-scale fluid motion within the outer core. The radial dependence of poloidal velocity field is unknown, but once a rather simple functional form is prescribed, the magnitude is determined so as to fit the magnetic field data. Then the toroidal velocity field can be derived by solving the Navier-Stokes equation. For simplicity, we assume that the steady state is realized at various epochs.
In order to check the validity of solutions, time-dependent behavior of the magnetic field is examined with the derived velocity field fixed. The time integration is performed using the fourth-order Runge-Kutta scheme. Unless the derived velocity field is appropriate, the magnetic field would vary rapidly contrary to the assumption of the steady state.
It turned out that the estimation of differential rotation based on virtual displacement of fluid particles is not valid. As long as a large-scale fluid motion and a large-scale magnetic field are considered, the omega-effect is not so effective as presumed and strong zonal toroidal magnetic fields are not generated. In order to derive fluid motion within the core, we should take all the interaction terms into consideration.

DOI： 10.5636/jgg.44.521

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DOI： 10.5636/jgg.41.963

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Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

DOI： 10.5636/jgg.41.963

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

DOI： 10.1029/JB093iB10p11631

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

DOI： 10.1029/GL015i007p00689

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Language：English   Publisher：BLACKWELL SCIENCE LTD  

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DOI： 10.1029/JB093iB10p11631

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DOI： 10.5636/jgg.40.1511

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DOI： 10.1029/GL015i007p00689

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Language：English   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

DOI： 10.5636/jgg.40.1511

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