齋藤 義文

©2016. American Geophysical Union. All Rights Reserved. We report evidence for reconnection between colliding reconnection jets in a compressed current sheet at the center of a magnetic flux rope at Earth's magnetopause. The reconnection involved nearly symmetric inflow boundary conditions with a strong guide field of two. The thin (2.5 ion-skin depth (di) width) current sheet (at ~12 di downstream of the X line) was well resolved by MMS, which revealed large asymmetries in plasma and field structures in the exhaust. Ion perpendicular heating, electron parallel heating, and density compression occurred on one side of the exhaust, while ion parallel heating and density depression were shifted to the other side. The normal electric field and double out-of-plane (bifurcated) currents spanned almost the entire exhaust. These observations are in good agreement with a kinetic simulation for similar boundary conditions, demonstrating in new detail that the structure of large guide field symmetric reconnection is distinctly different from antiparallel reconnection.

©2016. American Geophysical Union. All Rights Reserved. At 02:13 UT on 18 November 2015 when the geomagnetic dipole was tilted by −27°, the MMS spacecraft observed southward reconnection jets near the subsolar magnetopause under southward and dawnward interplanetary magnetic field conditions. Based on four-spacecraft estimations of the magnetic field direction near the separatrix and the motion and direction of the current sheet, the location of the reconnection line was estimated to be ~1.8 RE or further northward of MMS. The Geotail spacecraft at GSM Z~1.4 RE also observed southward reconnection jets at the dawnside magnetopause 30–40 min later. The estimated reconnection line location was northward of GSM Z~2 RE. This crossing occurred when MMS observed purely southward magnetic fields in the magnetosheath. The simultaneous observations are thus consistent with the hypothesis that the dayside magnetopause reconnection line shifts from the subsolar point toward the northern (winter) hemisphere due to the effect of geomagnetic dipole tilt.

Magnetic reconnection is a fundamental physical process in plasmas whereby stored magnetic energy is converted into heat and kinetic energy of charged particles. Reconnection occurs in many astrophysical plasma environments and in laboratory plasmas. Using measurements with very high time resolution, NASA's Magnetospheric Multiscale (MMS) mission has found direct evidence for electron demagnetization and acceleration at sites along the sunward boundary of Earth's magnetosphere where the interplanetary magnetic field reconnects with the terrestrial magnetic field. We have (i) observed the conversion of magnetic energy to particle energy; (ii) measured the electric field and current, which together cause the dissipation of magnetic energy; and (iii) identified the electron population that carries the current as a result of demagnetization and acceleration within the reconnection diffusion/dissipation region.

© 2016. American Geophysical Union. All Rights Reserved. Magnetic holes are ubiquitous in space plasmas, occurring in the solar wind, downstream of planetary bow shocks, and inside the magnetosphere. Recently, kinetic-scale magnetic holes have been observed near Earth's central plasma sheet. The Fast Plasma Investigation on NASA's Magnetospheric Multiscale (MMS) mission enables measurement of both ions and electrons with 2 orders of magnitude increased temporal resolution over previous magnetospheric instruments. Here we present data from MMS taken in Earth's nightside plasma sheet and use high-resolution particle and magnetometer data to characterize the structure of a subproton-scale magnetic hole. Electrons with gyroradii above the thermal gyroradius but below the current layer thickness carry a current sufficient to account for a ~10-20% depression in magnetic field magnitude. These observations suggest that the size and magnetic depth of kinetic-scale magnetic holes is strongly dependent on the background plasma conditions.

© 2016. American Geophysical Union. All Rights Reserved. Based on high-resolution measurements from NASA's Magnetospheric Multiscale mission, we present the dynamics of electrons associated with current systems observed near the diffusion region of magnetic reconnection at Earth's magnetopause. Using pitch angle distributions (PAD) and magnetic curvature analysis, we demonstrate the occurrence of electron scattering in the curved magnetic field of the diffusion region down to energies of 20 eV. We show that scattering occurs closer to the current sheet as the electron energy decreases. The scattering of inflowing electrons, associated with field-aligned electrostatic potentials and Hall currents, produces a new population of scattered electrons with broader PAD which bounce back and forth in the exhaust. Except at the center of the diffusion region the two populations are collocated and appear to behave adiabatically: the inflowing electron PAD focuses inward (toward lower magnetic field), while the bouncing population PAD gradually peaks at 90° away from the center (where it mirrors owing to higher magnetic field and probable field-aligned potentials).

© 2016, The Author(s). The Fast Plasma Investigation (FPI) was developed for flight on the Magnetospheric Multiscale (MMS) mission to measure the differential directional flux of magnetospheric electrons and ions with unprecedented time resolution to resolve kinetic-scale plasma dynamics. This increased resolution has been accomplished by placing four dual 180-degree top hat spectrometers for electrons and four dual 180-degree top hat spectrometers for ions around the periphery of each of four MMS spacecraft. Using electrostatic field-of-view deflection, the eight spectrometers for each species together provide 4pi-sr field-of-view with, at worst, 11.25-degree sample spacing. Energy/charge sampling is provided by swept electrostatic energy/charge selection over the range from 10 eV/q to 30000 eV/q. The eight dual spectrometers on each spacecraft are controlled and interrogated by a single block redundant Instrument Data Processing Unit, which in turn interfaces to the observatory’s Instrument Suite Central Instrument Data Processor. This paper describes the design of FPI, its ground and in-flight calibration, its operational concept, and its data products.

©2016. American Geophysical Union. All Rights Reserved. We present Magnetospheric Multiscale (MMS) mission measurements during a full magnetopause crossing associated with an enhanced southward ion flow. A quasi-steady magnetospheric whistler mode wave emission propagating toward the reconnection region with quasi-parallel and oblique wave angles is detected just before the opening of the magnetic field lines and the detection of escaping energetic electrons. Its source is likely the perpendicular temperature anisotropy of magnetospheric energetic electrons. In this region, perpendicular and parallel currents as well as the Hall electric field are calculated and found to be consistent with the decoupling of ions from the magnetic field and the crossing of a magnetospheric separatrix region. On the magnetosheath side, Hall electric fields are found smaller as the density is larger but still consistent with the decoupling of ions. Intense quasi-parallel whistler wave emissions are detected propagating both toward and away from the reconnection region in association with a perpendicular anisotropy of the high-energy part of the magnetosheath electron population and a strong perpendicular current, which suggests that in addition to the electron diffusion region, magnetosheath separatrices could be a source region for whistler waves.

©2016. American Geophysical Union. All Rights Reserved. Collisionless space plasma turbulence can generate reconnecting thin current sheets as suggested by recent results of numerical magnetohydrodynamic simulations. The Magnetospheric Multiscale (MMS) mission provides the first serious opportunity to verify whether small ion-electron-scale reconnection, generated by turbulence, resembles the reconnection events frequently observed in the magnetotail or at the magnetopause. Here we investigate field and particle observations obtained by the MMS fleet in the turbulent terrestrial magnetosheath behind quasi-parallel bow shock geometry. We observe multiple small-scale current sheets during the event and present a detailed look of one of the detected structures. The emergence of thin current sheets can lead to electron scale structures. Within these structures, we see signatures of ion demagnetization, electron jets, electron heating, and agyrotropy suggesting that MMS spacecraft observe reconnection at these scales.

©2016. American Geophysical Union. All Rights Reserved. We report Magnetospheric Multiscale observations of macroscopic and electron-scale current layers in asymmetric reconnection. By intercomparing plasma, magnetic, and electric field data at multiple crossings of a reconnecting magnetopause on 22 October 2015, when the average interspacecraft separation was ~10 km, we demonstrate that the ion and electron moments are sufficiently accurate to provide reliable current density measurements at 30 ms cadence. These measurements, which resolve current layers narrower than the interspacecraft separation, reveal electron-scale filamentary Hall currents and electron vorticity within the reconnection exhaust far downstream of the X line and even in the magnetosheath. Slightly downstream of the X line, intense (up to 3 μA/m2) electron currents, a super-Alfvénic outflowing electron jet, and nongyrotropic crescent shape electron distributions were observed deep inside the ion-scale magnetopause current sheet and embedded in the ion diffusion region. These characteristics are similar to those attributed to the electron dissipation/diffusion region around the X line.

©2016. American Geophysical Union. All Rights Reserved. We report ion velocity distribution functions (VDFs) observed by Magnetospheric Multiscale Mission (MMS) and present evidence for demagnetized ion Speiser motion during magnetopause reconnection. The demagnetization is observed in the vicinity of the X line, as well as near the current sheet midplane about tens of ion skin depths (di) away from the X line. Close to the X line before the outflow is built up, the VDFs are elongated, and the elongated part of VDFs rotates from the out-of-plane current direction toward the outflow directions downstream from the X line. Farther downstream, demagnetized ions exhibit a characteristic half-ring structure in the VDFs, as a result of the mixture of ions that have experienced different amounts of cyclotron turning around the magnetic field normal to the current sheet. Signatures of acceleration by electric fields are more pronounced in the VDFs near the X line than downstream.

©2016. The Authors. We report on field-aligned current observations by the four Magnetospheric Multiscale (MMS) spacecraft near the plasma sheet boundary layer (PSBL) during two major substorms on 23 June 2015. Small-scale field-aligned currents were found embedded in fluctuating PSBL flux tubes near the separatrix region. We resolve, for the first time, short-lived earthward (downward) intense field-aligned current sheets with thicknesses of a few tens of kilometers, which are well below the ion scale, on flux tubes moving equatorward/earthward during outward plasma sheet expansion. They coincide with upward field-aligned electron beams with energies of a few hundred eV. These electrons are most likely due to acceleration associated with a reconnection jet or high-energy ion beam-produced disturbances. The observations highlight coupling of multiscale processes in PSBL as a consequence of magnetotail reconnection.

©2016. American Geophysical Union. All Rights Reserved. In this letter the structure of the ion diffusion region of magnetic reconnection at Earth's magnetopause is investigated using the Magnetospheric Multiscale (MMS) spacecraft. The ion diffusion region is characterized by a strong DC electric field, approximately equal to the Hall electric field, intense currents, and electron heating parallel to the background magnetic field. Current structures well below ion spatial scales are resolved, and the electron motion associated with lower hybrid drift waves is shown to contribute significantly to the total current density. The electron heating is shown to be consistent with large-scale parallel electric fields trapping and accelerating electrons, rather than wave-particle interactions. These results show that sub-ion scale processes occur in the ion diffusion region and are important for understanding electron heating and acceleration.

©2015. American Geophysical Union. All Rights Reserved. We study hydrogen energetic neutral atom (ENA) emissions from the lunar surface, when the Moon is inside the terrestrial magnetosheath. The ENAs are generated by neutralization and backscattering of incident protons of solar wind origin. First, we model the effect of the increased ion temperature in the magnetosheath (>10 times larger than that in the undisturbed solar wind) on the ENA scattering characteristics. Then, we apply these models to ENA measurements by Chandrayaan-1 and simultaneous ion measurements by Kaguya at the Moon, in the magnetosheath. We produce maps of the ENA scattering fraction, covering a region at the lunar near-side that includes mare and highland surfaces and several lunar magnetic anomalies. We see clear signatures of plasma shielding by the magnetic anomalies. The maps are made at different lunar local times, and the results indicate an extended influence and altered morphology of the magnetic anomalies at shallower incidence angles of the magnetosheath protons. The scattering fraction from the unmagnetized regions remains consistent with that in the undisturbed solar wind (10%-20%). Moreover, the observed ENA energy spectra are well reproduced by our temperature-dependent model. We conclude that the ENA scattering process is unchanged in the magnetosheath. Similarly to the undisturbed solar wind case, it is only magnetic anomalies that provide contrast in the ENA maps, not any selenomorphological features such as mare and highland regions.

©2016. American Geophysical Union. All Rights Reserved. We present observations on 2 October 2015 when the Geotail spacecraft, near the Earth's equatorial plane, and the Magnetospheric Multiscale (MMS) spacecraft, at midsouthern latitudes, simultaneously encountered southward jets from dayside magnetopause reconnection under southward interplanetary magnetic field conditions. The observations show that the equatorial reconnection site under modest solar wind Alfvén Mach number conditions remained active almost continuously for hours and, at the same time, extended over a wide range of local times (≥4 h). The reconnection jets expanded toward the magnetosphere with distance from the reconnection site. Geotail, closer to the reconnection site, occasionally encountered large-amplitude mesoscale flux transfer events (FTEs) with durations about or less than 1 min. However, MMS subsequently detected no or only smaller-amplitude corresponding FTE signatures. It is suggested that during quasi-continuous spatially extended reconnection, mesoscale FTEs decay as the jet spatially evolves over distances between the two spacecraft of ≥350 ion inertial lengths.

©2016. American Geophysical Union. All Rights Reserved. The structure of asymmetric magnetopause reconnection is explored with multiple point and high-time-resolution ion velocity distribution observations from the Magnetospheric Multiscale mission. On 9 September 2015, reconnection took place at the magnetopause, which separated the magnetosheath and the magnetosphere with a density ratio of 25:2. The magnetic field intensity was rather constant, even higher in the asymptotic magnetosheath. The reconnected field line region had a width of approximately 540 km. In this region, streaming and gyrating ions are discriminated. The large extension of the reconnected field line region toward the magnetosheath can be identified where a thick layer of escaping magnetospheric ions was formed. The scale of the magnetosheath side of the reconnected field line region relative to the scale of its magnetospheric side was 4.5:1.

© 2015 Nakagawa et al.; licensee Springer. 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.

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.

© 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.

宇宙空間におけるプラズマの「その場」観測は,観測ロケットによる地球電離圏の観測に端を発し,人工衛星が地球近傍の周回軌道から,より遠い地球磁気圏の領域,そして他天体へとその探査領域を広げるに従ってその観測領域を広げてきた.太陽系プラズマの「その場」観測は,低周波から高周波の電場・磁場を含む「場」の観測と,eV程度の熱的・超熱的荷電粒子からMeVを超える高エネルギー荷電粒子を含む「粒子」の観測に大きく分けることができる.これらの「場」と「粒子」の「その場」観測を行う観測装置は,宇宙時代の幕開け以来,最新の技術を応用しながら探査する対象に適合する形で,高機能・高性能化,小型・軽量・省電力化が進められてきた.本節では,これらの「その場」観測装置の現状と,将来に向けての開発状況について述べる.

© 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.

The lunar exosphere is generated by several processes each of which generates neutral distributions with different spatial and temporal variability. Solar wind sputtering of the lunar surface is a major process for many regolith-derived species and typically generates neutral distributions with a cosine dependence on solar zenith angle. Complicating this picture are remanent crustal magnetic anomalies on the lunar surface, which decelerate and partially reflect the solar wind before it strikes the surface. We use Kaguya maps of solar wind reflection efficiencies, Lunar Prospector maps of crustal field strengths, and published neutral sputtering yields to calculate anisotropic solar wind sputtering maps. We feed these maps to a Monte Carlo neutral exospheric model to explore three-dimensional exospheric anisotropies and find that significant anisotropies should be present in the neutral exosphere depending on selenographic location and solar wind conditions. Better understanding of solar wind/crustal anomaly interactions could potentially improve our results. © 2014. American Geophysical Union. All Rights Reserved.

We present the observations of energetic neutral atoms (ENAs) produced at the lunar surface in the Earth's magnetotail. When the Moon was located in the terrestrial plasma sheet, Chandrayaan-1 Energetic Neutrals Analyzer (CENA) detected hydrogen ENAs from the Moon. Analysis of the data from CENA together with the Solar Wind Monitor (SWIM) onboard Chandrayaan-1 reveals the characteristic energy of the observed ENA energy spectrum (the e-folding energy of the distribution function) ∼100 eV and the ENA backscattering ratio (defined as the ratio of upward ENA flux to downward proton flux) <∼0.1. These characteristics are similar to those of the backscattered ENAs in the solar wind, suggesting that CENA detected plasma sheet particles backscattered as ENAs from the lunar surface. The observed ENA backscattering ratio in the plasma sheet exhibits no significant difference in the Southern Hemisphere, where a large and strong magnetized region exists, compared with that in the Northern Hemisphere. This is contrary to the CENA observations in the solar wind, when the backscattering ratio drops by ∼50% in the Southern Hemisphere. Our analysis and test particle simulations suggest that magnetic shielding of the lunar surface in the plasma sheet is less effective than in the solar wind due to the broad velocity distributions of the plasma sheet protons. Key Points We present ENA observations at the Moon in the Earth's plasma sheet Plasma sheet protons are backscattered as ENAs from the lunar surface Magnetic shielding in the plasma sheet is less effective than in the solar wind ©2014. American Geophysical Union. All Rights Reserved.

We present the observations of energetic neutral atoms (ENAs) produced at the lunar surface in the Earth's magnetotail. When the Moon was located in the terrestrial plasma sheet, Chandrayaan-1 Energetic Neutrals Analyzer (CENA) detected hydrogen ENAs from the Moon. Analysis of the data from CENA together with the Solar Wind Monitor (SWIM) onboard Chandrayaan-1 reveals the characteristic energy of the observed ENA energy spectrum (the e-folding energy of the distribution function) approximate to 100 eV and the ENA backscattering ratio (defined as the ratio of upward ENA flux to downward proton flux) <approximate to 0.1. These characteristics are similar to those of the backscattered ENAs in the solar wind, suggesting that CENA detected plasma sheet particles backscattered as ENAs from the lunar surface. The observed ENA backscattering ratio in the plasma sheet exhibits no significant difference in the Southern Hemisphere, where a large and strong magnetized region exists, compared with that in the Northern Hemisphere. This is contrary to the CENA observations in the solar wind, when the backscattering ratio drops by approximate to 50% in the Southern Hemisphere. Our analysis and test particle simulations suggest that magnetic shielding of the lunar surface in the plasma sheet is less effective than in the solar wind due to the broad velocity distributions of the plasma sheet protons.

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.

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.

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.

A sub-Alfve'nic jet in the tailward outflow region near the separatrix of the magnetic reconnection is observed by Geotail on 9 February 1995. Several dozens of electrostatic solitary waves/pulses (ESWs) are observed, respectively, on the current sheet-side and the lobe-side of the separatrix. The ESWs on the current sheet-side are of type-B with direction outward (toward to the tailward) while on the lobe-side they are of type-A directed to X-line. The amplitude of ESWs on the current sheet-side is about 6 times more than those on the lobe-side, suggesting that energies flowing outward from the reconnection X-line are much larger than those flowing inward. Moreover, observations show, that electron beams associated with ESWs, which are parallel to the ambient magnetic field, are much stronger on the current sheet-side than on the lobe-side of the separatrix. Furthermore, the direction of the electron beam on the lobe-side of the separatrix is mainly antiparallel to the ambient magnetic field and it is mainly parallel on the current sheet-side. Both are consistent with the propagation of ESWs which is in agreement with the generation mechanism of ESWs, which is suggested to be related to electron beams. These results are helpful for solving the issue of ESWs' generation mechanism associated with magnetic reconnection. It also provides an important clue for understanding the fast energy release during the magnetic reconnection process. Key Points Different types of ESWs are observed in both sides of the separatrix ESW in the CS-side is much larger than those in the lobe-side Electron beam are associated with ESWs ©2013. American Geophysical Union. All Rights Reserved.

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. By and/or B-z). 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 12 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 nightside 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 key) along the magnetic field associated with the 12 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 12 entry that we rename to T2MD). 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 (B-X-dominated) IMF condition. Our results indicate that, while the 12 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. (C) 2013 Elsevier Ltd. All rights reserved.

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.

Three-dimensional structure of magnetic reconnection in the near-Earth magnetotail is explored using Geotail observations from 1996 to 2012. Magnetic reconnection is identified as a simultaneous plasma flow and magnetic field reversal near the neutral sheet with intense out-of-plane electron currents. There are 30 events in the region of -32 &lt X &lt -18 RE (in the aberrated GSM coordinates) in the magnetotail. The dawn-dusk width (in the y direction) of the magnetic reconnection site is probably less than 8 R E with its center in the premidnight region. The magnetic field structure in the x-z plane does not change significantly in the full width of the magnetic reconnection site. The ion inflow-outflow structure shows a marked edge effect in the duskside. As inflow, ions move toward the equatorial plane with an additional dawnward motion. This dawnward motion is more evident in the duskside. As outflows, ions escape tailward and earthward with a duskward motion, as expected from the Speiser motion, except at the duskside edge. At the duskside edge, almost all ions move dawnward, and only part of high-energy ions can escape earthward and tailward. There is a possibility that the dawnward flow consists of adjacent plasma sheet plasmas transported by the pressure gradient force at the duskside edge. The present results would be the first step to construct a three-dimensional structure of magnetic reconnection in various cosmic plasmas. ©2013. American Geophysical Union. All Rights Reserved.

The ion-electron decoupling region where electron outflow speed differs from ion outflow speed is formed in the magnetic reconnection site. Ion and electron dynamics in the ion-electron decoupling region is derived with magnetic field and plasma observations by the spacecraft Geotail in near-Earth magnetotail magnetic reconnection. The ion-electron decoupling region has a spatial extent of approximately 11 λi (ion inertial length) along the GSM x direction, and the dawn-dusk current sheet with main current carriers of electrons exists over this region. An intense electron current layer with a spatial extent of 0.5-1 λi occupies in its center around the X line. High-speed electron outflow jets are formed just outside the central intense electron current layer. They are decelerated and become non-jet outflows with speed slightly higher than ion outflow speed. Electrons have flattop distribution functions indicating heating and acceleration in both the outflow jets and the non-jet outflows however, heating and acceleration are weak in the central intense current layer. Inflowing ions enter the central intense electron current layer, and these ions are accelerated up to 10 keV inside the electron outflow jet regions. Ion acceleration beyond 10 keV and thermalization operate mostly in the non-jet electron outflow regions. Electrons show thermal distributions without any heating/acceleration signatures immediately beyond the edge of the ion-electron decoupling region, while higher-energy ions pervade even beyond the edge and hot MHD plasma flows are produced. Key Points Ion and electron dynamics in magnetic reconnection is investigated Ion-electron decoupling region with the electron current layer is formed Electrons show flattop distributions in outflow jets and outflow non-jets ©2013. American Geophysical Union. All Rights Reserved.

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. (C) 2012 Elsevier Ltd. All rights reserved.

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.

We compare estimates for the ion fluxes of twelve expected constituents of the lunar exosphere with estimates for the ion fluxes ejected from the lunar surface by solar wind ions and electrons. Our estimates demonstrate that measurements of lunar ions will help constrain the abundances of many undetected species in the lunar exosphere, particularly Al and Si, because the expected ion flux levels from the exosphere exceed those from the surface. To correctly infer the relative abundances of exospheric ions and neutrals from Kaguya Ion Mass Analyzer (IMA) measurements, we must take into account the velocity distributions of local ions. The predicted spectrum underestimates the measured levels of O&lt sup&gt +&lt /sup&gt relative to other lunar ion species, a result that may suggest contributions by molecular ions to the measured O&lt sup&gt +&lt /sup&gt rates. © 2012 American Geophysical Union. All Rights Reserved.

We have analyzed nongyrotropic electron velocity distribution functions (VDFs) obtained near the lunar surface. Electron VDFs, measured at similar to 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.

The sounding rocket Investigation of Cusp Irregularities 2 (ICI-2) was launched into the cusp ionosphere over Svalbard to investigate the production of decameter scale irregularities in the electron plasma associated with HF radar backscatter. The main mission objective was to obtain high-resolution measurements of decameter scale electron plasma irregularities and to quantify the growth rate for the gradient drift instability (GDI). At the 5.7 kHz sampling rate of the absolute density measurements, ICI-2 has provided the first documentation in terms of absolute electron density measurements of how 10-m structures are located on km scale electron density gradients. ICI-2 traversed a cusp electron density structure created by ongoing soft precipitation. 10-m scale irregularities were generated at km scale density gradients. The estimated growth time for the GDI process was 10-50 seconds. Copyright 2012 by the American Geophysical Union.

Our previous study showed that the energy release associated with substorm expansion onsets is the most significant midway between the magnetic reconnection and initial dipolarization regions (-12 &gt X &gt -18 R E in the premidnight sector) in the magnetotail. In the present paper, we have statistically studied the substorm-associated energy balance and transport in the magnetotail, focusing on the midway region as well as the near-Earth initial dipolarization region (X &gt ∼-12 RE). We find that a large amount of energy is released in the midway region, associated with onsets, but only a part of this energy is transported to the near-Earth initial dipolarization region mainly in the form of the thermal flux and the wave Poynting flux. It is possible that the energy carried by fast earthward flows and waves from the reconnection region is not sufficient for the thermal energy increase and the outward transported energy in the initial dipolarization region, although the magnetic flux may be sufficiently carried. A considerably large amount of the magnetic energy comes from the lobes in the form of the Poynting flux also in the initial dipolarization region. © 2012. American Geophysical Union. All Rights Reserved.

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.

At similar to 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 Delta E/q (Delta E: 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.

A rich set of new measurements has greatly expanded our understanding of the Moonplasma interaction over the last sixteen years, and helped demonstrate the fundamentally kinetic nature of many aspects thereof. Photon and charged particle impacts act to charge the lunar surface, forming thin Debye-scale plasma sheaths above both sunlit and shadowed hemispheres. These impacts also produce photoelectrons and secondary electrons from the surface, as well as ions from the surface and exosphere, all of which in turn feed back into the plasma environment. The solar wind interacts with sub-ion-inertial-scale crustal magnetic fields to form what may be the smallest magnetospheres in the solar system. Proton gyro-motion, solar wind pickup of protons scattered from the dayside surface, and plasma expansion into vacuum each affect the dynamics and structure of different portions of the lunar plasma wake. The Moon provides us with a basic plasma physics laboratory for the study of fundamental processes, some of which we cannot easily observe elsewhere. At the same time, the Moon provides us with a test bed for the study of processes that also operate at many other solar system bodies. We have learned much about the Moonplasma interaction, with implications for other space and planetary environments. However, many fundamental problems remain unsolved, including the details of the coupling between various parts of the plasma environment, as well as between plasma and the surface, neutral exosphere, and dust. In this paper, we describe our current understanding of the lunar plasma environment, including illustrative new results from Lunar Prospector and Kaguya, and outstanding unsolved problems. © 2010 Elsevier Ltd.

Kelvin-Helmholtz instability (KHI) is a fundamental fluid dynamical process that develops in a velocity shear layer. It is excited on the tail-flanks of the Earth's magnetosphere where the flowing magnetosheath plasma and the stagnant magnetospheric plasma sit adjacent to each other. This instability is thought to induce vortical structures and play an important role in plasma transport there. While KHI vortices have been detected, the earlier observations were performed only on one flank at a time and questions related to dawn-dusk asymmetry were not addressed. Here, we report a case where KHI vortices grow more or less simultaneously and symmetrically on both flanks, despite all the factors that may have broken the symmetry. Yet, energy distributions of ions in and around the vortices show a remarkable dawn-dusk asymmetry. Our results thus suggest that although the initiation and development of the KHI depend primarily on the macroscopic properties of the flow, the observed enhancement of ion energy transport around the dawn side vortices may be linked to microphysical processes including wave-particle interactions. Possible coupling between macro- and micro-scales, if it is at work, suggests a role for KHI not only within the Earth's magnetosphere (e.g., magnetopause and geomagnetic tail) but also in other regions where shear flows of magnetized plasma play important roles. (C) 2010 Elsevier Ltd. All rights reserved.

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&#039;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.

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-Alfvenic (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. (C) 2011 Elsevier Ltd. All rights reserved.

We have studied successive substorm expansions that occurred in spite of a period of prolonged northward interplanetary magnetic field (IMF). After the northward IMF persisted for almost ∼19 h, a series of 11 very weak to moderate substorm expansions occurred during the following 6-h interval of northward IMF on 19 January 1998, separated from each other by a few tens of minutes. Most of these substorm expansions did not accompany significant changes of the solar wind and the IMF. As for typical substorms that occur under southward IMF conditions, spacecraft and ground-based instruments observed auroral breakups and enhancements of the westward auroral electrojet and large-scale convection in the ionosphere, together with plasmoids and dipolarizations in the magnetotail, in association with the substorm expansion onsets. These signatures suggest that the mechanism of substorm expansion onsets is the same as for typical substorms, even under prolonged northward IMF conditions. We suggest that the large IMF By affects large-scale convection and energy accumulation in the magnetotail and that the enhanced convection due to the large IMF By effect and substorm expansions can be an important factor for the promotion of succeeding substorm expansion onsets. Copyright 2011 by the American Geophysical Union.

The Geotail spacecraft made in situ observations of magnetic reconnection on 15 May 2003 in the near-Earth magnetotail at a radial distance of 28 R E when a moderate substorm started on the ground. For this event, the intense cross-tail electron current layer was detected in association with the simultaneous plasma flow and magnetic field reversals, and the scale length along the GSM x axis was obtained. This observation enables us to deduce scales for the basic structure of magnetic reconnection in the near-Earth magnetotail. In the center of the electron current layer (a possible X line), the speed of the dawnward electron flow carrying cross-tail current exceeds the maximum of the electron outflow speed (earthward/tailward), and it is highly super Alfvénic. The full extent of this central intense cross-tail electron current layer is approximately 1 λi (ion inertial length) in the x direction, which corresponds to 0.2 RE in the magnetotail. Electron outflow speed reaches its maximum, which is also super Alfvénic, at distances of less than 1 λi from the X line, and ion outflow speed reaches its maximum farther away from the center. Electron outflow speed decreases in the downstream region, and it becomes the same as the ion speed at distances of 4 λi. The full extent of the ion-electron decoupling region is 8 λi in the x direction, which corresponds to 1.5 RE in the magnetotail, and the outer region belongs to the MHD regime. Inside the ion-electron decoupling region, ions are accelerated up to 10 keV during inflow processes and further accelerated beyond 40 keV toward the duskward direction near the center and along the x axis slightly away from the center. These observations of the ion and electron dynamics in the close vicinity of the X line are fairly consistent with results from the two-dimensional particle-in-cell simulation described here and others. The present Geotail results provide the observational basis for the structure of magnetic reconnection in the near-Earth magnetotail. Copyright 2011 by the American Geophysical Union.