齋藤 義文

Bepi Colombo is a joint mission between ESA and JAXA that is scheduled for launch in 2014 and arrival at Mercury in 2020. A comprehensive set of particle sensors will be flown onboard the two probes that form Bepi Colombo. These sensors will allow a detailed investigation of the structure and dynamics of the charged particle environment at Mercury. Onboard the Mercury Magnetospheric Orbiter (MMO) the Mercury Electron Analyzers (MEA) sensors constitute the experiment dedicated to fast electron measurements between 3 and 25,500 eV. They consist of two top-hat electrostatic analyzers for angle-energy analysis followed by microchannel plate multipliers and collecting anodes. A notable and new feature of MEA is that the transmission factor of each analyzer can be varied in-flight electronically by a factor reaching up to 100, thus allowing to largely increasing the dynamical range of the experiment. This capability is of importance at Mercury where large changes of electron fluxes are expected from the solar wind to the various regions of the Mercury magnetosphere. While the first models are being delivered to JAXA, an engineering model has been tested and proven to fulfill the expectations about geometrical factor reduction and energy-angular transmission characteristics. Taking advantage of the spacecraft rotation with a 4 s period, MEA will provide fast three-dimensional distribution functions of magnetospheric electrons, from energies of the solar wind and exospheric populations (a few eVs) up to the plasma sheet energy range (some tens of keV). The use of two sensors viewing perpendicular planes allows reaching a spin period time resolution, i.e., 1 s, to obtain a full 3D distribution. © 2010 Elsevier Ltd. All rights reserved.

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 (similar to 10 mV/m) around the Moon when it is in the plasma sheet. Citation: Harada, Y., et al. (2010), Interaction between terrestrial plasma sheet electrons and the lunar surface: SELENE (Kaguya) observations, Geophys. Res. Lett., 37, L19202, doi:10.1029/2010GL044574.

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.

MAP-PACE (MAgnetic field and Plasma experiment-Plasma energy Angle and Composition Experiment) on SELENE (Kaguya) has completed its similar to 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 day-side 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.

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 10-510-7 cm -3 in 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.

Using Geotail data, we have studied the response of the plasma sheet in the magnetotail to sudden enhancements of the solar wind pressure. We have selected three events in which there were no indications of substorm expansions around the corresponding sudden impulses (SIs) on the ground. It is shown that the ion number density and temperature, as well as the ion pressure, increase significantly due to the plasma sheet compression. The specific entropy does not significantly change, suggesting that the plasma sheet compression is nearly adiabatic. This plasma behavior is in contrast to the nonadiabatic substorm-associated processes. Hence examining the correlation between the number density and the temperature, and especially the specific entropy is critical for distinguishing between substorm-associated and SI-associated changes of the plasma sheet. The northward magnetic field in the plasma sheet also increases, or at least does not decrease, associated with the compression, implying that the lateral magnetotail compression suppresses the triggering of magnetic reconnection and therefore substorm expansion onsets.

The Mercury Ion Analyzer (MIA) is one of the plasma instruments on board the Mercury Magnetopsheric Orbiter (MMO) and measures 3-dimensional distribution function of ions around Mercury with energy range of 5 eV up to 30 keV. MIA is designed to make observations both in the magnetosphere and in the solar wind. We simulate here the telemetry output of MIA based on the design of the Engineering Model (EM), from which we derive the original plasma parameters, such as bulk velocity, density, and temperature of protons, expected around Mercury. We conclude that under the current design of the sensor and observation modes MIA works well with sufficient accuracy to understand the various plasma environments around Mercury, ranging from the hot and cold plasma sheet in the magnetosphere to the solar wind between 0.3 and 0.47 from the sun.

We have studied plasma (ion) pressure changes that occurred in association with the dipolarization in the near-Earth plasma sheet around substorm onsets. Using Geotail data, we have performed a superposed epoch analysis in addition to detailed examinations of two individual cases with special emphasis on the contribution of high-energy particles to the plasma pressure. It is found that, unlike previously reported results, the plasma pressure does increase in association with the initial dipolarization at X &gt ∼-12 RE and -2 &lt Y &lt 6 RE, with the increase largely due to high-energy particles. Outside the initial dipolarization region, particularly tailward and duskward of this region, the plasma pressure begins to decrease owing to the magnetic reconnection before onset or before the dipolarization region reaches there. At later times, the plasma pressure tends to increase there, related to the expanding dipolarization region, but the contribution of high-energy particles is not very large. These observations suggest the following. The rarefaction wave scenario proposed in the current disruption model is questionable. The radial and azimuthal pressure gradients may strengthen between the initial dipolarization and outside regions, possibly resulting in stronger braking of fast earthward flows and changes in field-aligned currents. The characteristics of the dipolarization may differ between the initial dipolarization and tailward regions, which would be possibly reflected in the auroral features. Furthermore, we have examined the specific entropy and the ion β. The specific entropy increases in the plasma sheet in the dipolarization region as well as in the midtail region in conjunction with substorm onsets, suggesting from the ideal MHD point of view that the substorm processes are nonadiabatic. The ion β is found to peak at the magnetic equator in the initial dipolarization region around dipolarization onsets. Copyright 2010 by the American Geophysical Union.

Mercury is one of the least explored planets in our solar system. Until the recent flyby of Mercury by MESSENGER, no spacecraft had visited Mercury since Mariner 10 made three flybys: two in 1974 and one in 1975. In order to elucidate the detailed plasma structure and dynamics around Mercury, an orbiter BepiColombo MMO (Mercury Magnetospheric Orbiter) is planned to be launched in 2013 as a joint mission between ESA and ISAS/JAXA. Mercury Plasma Particle Experiment (MPPE) was proposed in order to investigate the plasma/particle environment around Mercury. MPPE is a comprehensive instrument package for plasma, high-energy particle and energetic neutral atom measurements. It consists of seven sensors: two Mercury electron analyzers (MEA1 and MEA2). Mercury ion analyzer (MIA), Mercury mass spectrum analyzer (MSA),. high-energy particle instrument for electron (HEP-ele), high-energy particle instrument for ion (HEP-ion), and energetic neutrals analyzer (ENA). Since comprehensive full three-dimensional simultaneous measurements of low to high-energy ions and electrons around Mercury as well as measurements of energetic neutral atoms will not be realized before BepiColombo/MMO's arrival at Mercury, it is expected that many unresolved problems concerning the Mercury magnetosphere will be elucidated by the MPPE observation. (C) 2008 Elsevier Ltd. All rights reserved.

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.

We study solar wind (SW) entry deep into the nearMoon 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 ∼ 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. Copyright 2009 by the American Geophysical Union.

火星の大気と表層環境の劇的な変遷に深く関与した可能性が高いと考えられる「宇宙空間への大気散逸」.そのプロセスの全貌に迫り,環境変遷に果たしてきた役割の理解を飛躍的に向上させる,MELOS複合探査による火星大気散逸観測,および,大気進化研究の科学要求を概観する.

The Mercury Magnetopsheric Orbiter (MMO) is one of the spacecraft of the BepiColombo mission the mission is scheduled for launch in 2014 and plans to revisit Mercury with modern instrumentation. MMO is to elucidate the detailed plasma structure and dynamics around Mercury, one of the least-explored planets in our solar system. The Mercury Plasma Particle Experiment (MPPE) on board MMO is a comprehensive instrument package for plasma, high-energy particle, and energetic neutral particle atom measurements. The Mercury Ion Analyzer (MIA) is one of the plasma instruments of MPPE, and measures the three dimensional velocity distribution of low-energy ions (from 5 eV to 30 keV) by using a top-hat electrostatic analyzer for half a spin period (2 s). By combining both the mechanical and electrical sensitivity controls, MIA has a wide dynamic range of count rates for the proton flux expected around Mercury, which ranges from 106 to 1012 cm-2 s-1 str-1 keV-1, in the solar wind between 0.3 and 0.47 AU from the sun, and in both the hot and cold plasma sheet of Mercury's magnetosphere. The geometrical factor of MIA is variable, ranging from 1.0 × 10-7 cm2 str keV/keV for large fluxes of solar wind ions to 4.7 × 10-4 cm2 str keV/keV for small fluxes of magnetospheric ions. The entrance grid used for the mechanical sensitivity control of incident ions also work to significantly reduce the contamination of solar UV radiation, whose intensity is about 10 times larger than that around Earth's orbit. © 2009 COSPAR.

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.

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 ∼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. Copyright 2009 by the American Geophysical Union.

We investigated the temporal and spatial development of the near-Earth magnetotail during substorms based on multi-dimensional superposed-epoch analysis of Geotail data. The start time of the auroral break-up (t=0) of each substorm was determined from auroral data obtained by the Polar and IMAGE spacecraft. The key parameters derived from the plasma, magnetic-field, and electric-field data from Geotail were sorted by their meridional X(GSM)-Z(proxy) coordinates. The results show that the Poynting flux toward the plasmasheet center starts at least 10 min before the substorm onset, and is further enhanced at X̃-12RE (Earth radii) around 4 min before the onset. Simultaneously, large-amplitude fluctuations occurred, and earthward flows in the central plasma sheet between X̃-11RE and X̃-19R E and a duskward flow around X=-10RE were enhanced. The total pressure starts to decrease around X=-16RE about 4 min before the onset of the substorm. After the substorm onset, a notable dipolarization is observed and tailward lows commence, characterised by southward magnetic fields in the form of a plasmoid. We confirm various observable-parameter variations based on or predicted by the relevant substorm models however, none of these can explain our results perfectly. Therefore, we propose a catapult (slingshot) current-sheet relaxation model, in which an earthward convective flow produced by catapult current-sheet relaxation and a converted duskward flow near the Earth are enhanced through flow braking around 4 min before the substorm onset. These flows induce a ballooning instability or other instabilities, causing the observed current disruption. The formation of the magnetic neutral line is a natural consequence of the present model, because the relaxation of a highly stretched catapult current-sheet produces a very thin current at its tailward edge being surrounded by intense earthward and tailward magnetic fields which were formerly the off-equatorial lobe magnetic fields. This location is the boundary between a highly stressed catapult current sheet and a Harris-type current sheet characterized by little stress. In addition, the flows induced around the boundary toward the current-sheet center may enhance the formation of the magnetic neutral line and the efficiency of magnetic reconnection. After magnetic reconnection is induced, it plays a significant role in driving the substorm. © Author(s) 2009.

From the viewpoint of plasma particle measurements in the radiation belt, background noise is a serious problem. High-energy particles penetrating the sensor shielding generate spurious signals, and their count rate often can be comparable to the true signals. In order to attenuate such background noise during medium-energy (5-83 keV) electron measurements, we propose the double energy analyses (DEA) method. DEA is conducted by a combination of an electrostatic analyser (ESA) and avalanche photo-diodes (APDs) ESA and APD independently determine the energy of each incoming particle. By using the DEA method, therefore, the penetrating particles can be rejected when the two energy determinations are inconsistent spurious noise are caused only when the deposited energy at an APD is by chance consistent with the measured energy by ESA. We formulate the noise count rate and show the advantage of DEA method quantitatively. © 2008 COSPAR.

BEPI COLOMBO is a joint mission between ESA and JAXA that is scheduled for launch in 2014 and arrival at Mercury in 2020. A comprehensive set of ion sensors will be flown onboard the two probes that form BEPI COLOMBO. These ion sensors combined with electron analyzers will allow a detailed investigation of the structure and dynamics of the charged particle environment at Mercury. Among the ion sensors, the Mass Spectrum Analyzer (MSA) is the experiment dedicated to composition analysis onboard the Mercury Magnetospheric Orbiter (MMO). It consists of a top-hat for energy analysis followed by a Time-Of-Flight (TOF) section to derive the ion mass. A notable feature of MSA is that the TOF section is polarized with a linear electric field that provides an enhanced mass resolution, a capability that is of importance at Mercury since a variety of species originating from the planet surface and exosphere is expected. MSA exhibits two detection planes: (i) one with moderate mass resolution but a high count rate making MSA appropriate for plasma analysis, (ii) another with a high (above 40) mass resolution though a low count rate making it appropriate for planetology science. Taking advantage of the spacecraft rotation, MSA will provide three-dimensional distribution functions of magnetospheric ions, from energies characteristic of exospheric populations (a few eVs or a few tens of eVs) up to the plasma sheet energy range (up to ∼40 keV/q) in one spin (4 s). © 2008 COSPAR.

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 56REand 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.

MAP-PACE (MAgnetic field and Plasma experiment - Plasma energy Angle and Composition Experiment) is one of the scientific instruments onboard the KAGUYA (SELENE) satellite. 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 measure the distribution function of low energy electrons below 15keV, while IMA and IEA measure the distribution function of mass identified low energy ions below 28keV/q. Since KAGUYA is a three-axis stabilized spacecraft, a pair of electron sensors (ESA-S1 and S2) and a pair of ion sensors (IMA and IEA) are necessary for obtaining three-dimensional distribution function of electrons and ions. Low energy ion measurements on the lunar orbit have been realized more than 30 years after the Apollo period. In addition, nobody has ever measured the mass identified three-dimensional distribution function of low energy ions at 100km altitude. PACE discovered surprisingly active low energy ion environment around the Moon. Instead of being absorbed by the lunar surface, quite a large amount of solar wind ions are reflected back from the Moon. The reflected solar wind ions are accelerated above solar wind energy picked up by the electric field in the solar wind.

Energetic ions are at times observed in the upstream region of the Earth's bow shock, and their origin is considered to be in the interaction with the shock front. While the energy of the solar wind ions is a few keV at most, the energy of the back streaming ions ranges from ̃5 keV to several MeV In the present study we investigate back streaming energetic ions in the upstream of the Earth's bow shock observed by Geotail during two coronal mass ejection events. The observed local magnetic field rotated significantly during the events. Using the bow shock model and the observed magnetic field data, we found that the energetic ions appeared only when the upstream magnetic field was connected to the bow shock. The energetic ions showed two distinct distribution function characteristics, namely, the field-aligned beam (FAB) and the loss cone-shaped distribution. While the former is occasionally detected, the latter having higher energies (30 keV to several hundred keV, compared to &lt 18 keV for FAB) has not been reported before. Using a bow shock model, we can also estimate the shock angle at the point on the shock surface that the upstream field line is connected to and find that the distribution function shape transits from the FAB to the loss cone-shaped distribution as the shock angle becomes larger (transition at ΘBn., = 70°- 80°). We discuss the possible mechanisms responsible for the production of the newly found member of the energetic upstream ion family. Copyright 2009 by the American Geophysical Union.

We examine traversais on 20 November 2001 of the equatorial magnetopause boundary layer simultaneously at ∼ 1500 magnetic local time (MLT) by the Geotail spacecraft and at ∼1900 MLT by the Cluster spacecraft, which detected rolled-up MHDscale vortices generated by the Kelvin-Helmholtz instability (KHI) under prolonged northward interplanetary magnetic field conditions. Our purpose is to address the excitation process of the KHI, MHD-scale and ion-scale structures of the vortices, and the formation mechanism of the low-latitude boundary layer (LLBL). The observed KH wavelength (&gt 4 × 104 km) is considerably longer man predicted by the linear theory from the thickness (∼1000 km) of the dayside velocity shear layer. Our analyses suggest that the KHI excitation is facilitated by combined effects of the formation of the LLBL presumably through high-latitude magnetopause reconnection and compressional magnetosheath fluctuations on the dayside, and that breakup and/or coalescence of the vortices are beginning around 1900 MLT. Current layers of thickness a few times ion inertia length ∼100 km and of magnetic shear ∼60° existed at the trailing edges of the vortices. Identified in one such current sheet were signatures of local reconnection: Alfvénic outflow jet within a bifurcated current sheet, nonzero magnetic field component normal to the sheet, and field-aligned beam of accelerated electrons. Because of its incipient nature, however, this reconnection process is unlikely to lead to the observed dusk-flank LLBL. It is thus inferred that the flank LLBL resulted from other mechanisms, namely, diffusion and/or remote reconnection unidentified by Cluster. Copyright 2009 by the American Geophysical Union.

In order to reach the new horizon of the space physics research, the Plasma Universe, via in-situ measurements in the Earth's magnetosphere, SCOPE will perform formation flying observations combined with high-time resolution electron measurements. The simultaneous multi-scale observations by SCOPE of various plasma dynamical phenomena will enable data-based study of the key space plasma processes from the cross-scale coupling point of view. Key physical processes to be studied are magnetic reconnection under various boundary conditions, shocks in space plasma, collisionless plasma mixing at the boundaries, and physics of current sheets embedded in complex magnetic geometries. The SCOPE formation is made up of 5 spacecraft and is put into the equatorial orbit with the apogee at 30Re (Re: earth radius). One of the spacecraft is a large mother ship which is equipped with a full suite of particle detectors including ultra-high time resolution electron detector. Among other 4 small spacecraft, one remains near (̃10km) the mother ship and the spacecraft-pair will focus on the electron-scale physics. Others at the distance of 100̃3000km (electron ̃ ion spatial scales) from the mother ship will monitor plasma dynamics surrounding the mother-daughter pair. There is lively on-going discussion on Japan-Europe international collaboration (ESA's Cross-Scale), which would certainly make better the coverage over the scales of interest and thus make the success of the mission, i.e., clarifying the multi-scale nature of the Plasma Universe, to be attained at an even higher level. © 2009 American Institute of Physics.

The Bepi Colombo mission to Mercury is a joint mission between ESA and JAXA that is scheduled for launch in 2013 and arrival at Mercury in 2019. A comprehensive set of ion sensors will be flown onboard the two Bepi Colombo probes which, in addition to electron sensors, will allow in-depth analysis of the charged particle environment of Mercury. We review some features of ion transport at Mercury, from their solar wind or exospheric source until injection into the inner magnetotail. We also briefly describe the various Bepi Colombo sensors dedicated to their analysis. © 2009 American Institute of Physics.

The future magnetospheric exploration missions (ex. SCOPE: cross Scale COupling in the Plasma universE) aim to obtain electron 3D distribution function with very fast time resolution below 10 ms to investigate the electron dynamics that is regarded as pivotal in understanding the space plasma phenomena such as magnetic reconnection. This can be achieved by developing a new plasma detector system which is fast in signal processing with small size, light weight and low power consumption. The new detector system consists of stacked micro channel plates and a position sensitive multi-anode detector with on-anode analogue ASIC (Application Specific Integrated Circuits). Multi-anode system usually suffers from false signals caused by mainly two effects. One is the effect of the electrostatic crosstalk between the discrete anodes since our new detector consists of many adjacent anodes with small gaps to increase the detection areas. Our experimental results show that there exists electrostatic crosstalk effect of approximately 10% from the adjacent anodes. The effect of 10% electrostatic crosstalk can be effectively avoided by a suitable discrimination level of the signal processing circuit. Non negligible charge cloud size on the anode also causes false counts. Optimized ASIC for in-situ plasma measurement in the Earth's magnetosphere is under development. The initial electron cloud at the MCP output has angular divergence. Furthermore, space charge effects may broaden the size of the charge cloud. We have obtained the charge cloud size both experimentally and theoretically. Our test model detector shows expected performance that is explained by our studies above. © 2009 American Institute of Physics.

The Earth's bow shock is known to produce non-thermal electrons which are generally observed as a 'spike' in their flux profile. Here, in this paper, we present an analysis of electron and whistler wave properties for a quasi-perpendicular shock crossing that is supercritical, but subcritical to the so-called whistler critical Mach number, Mwcrit, above which whistler waves cannot propagate upstream. We have found that the amplitudes of whistler waves increased exponentially as a function of time prior to the shock encounter, while the suprathermal (&gt 2 keV) electron flux similarly increased with time, although with differing e-folding time scales. Comparison of the electron energy spectrum measured within the ramp with predictions from diffusive shock acceleration theory was poor, but the variation of pitch angle distribution showed scattering of non-thermal electrons in the upstream region. While not finding a specific mechanism to account for the electron diffusion, we suggest that the whistlers seen probably account for the differences observed between this 'gradual' event and the 'spike' events seen at shocks with no upstream whistlers. Copyright © The Society of Geomagnetism and Earth.

We have obtained a state-of-the-art picture of substorm-associated evolution of the near-Earth magnetotail and the inner magnetosphere for understanding the substorm triggering mechanism. We performed superposed epoch analysis of Geotail, Polar, and GOES data with 2-min resolution, utilizing a total of 3787 substorms for each of which auroral breakup was determined from Polar UVI or IMAGE FUV auroral imager data. The decrease of the north-south magnetic field associated with plasmoids and the initial total pressure decrease suggest that the magnetic reconnection first occurs in the premidnight tail, on average, at X ∼ -16 to -20 RE at least 2 min before auroral onset. The magnetic reconnection site is located near the tailward edge of a region of considerably taillike magnetic field lines and intense cross-tail current, which extends from X ∼ -5 to -20 RE in the premidnight sector. Then the plasmoid substantially evolves tailward of X ∼ -20 R E immediately after onset. Almost simultaneously with the magnetic reconnection, the dipolarization begins first at X ∼ -7 to -10 RE 2 min before onset. The dipolarization region then expands tailward as well as in the dawn-dusk directions and earthward. We find that the total pressure generally enhances in association with the dipolarization, with the contribution of high-energy particles. Also, energy release is more significant between the regions of the magnetic reconnection and the initial dipolarization. The present results will be helpful as a reference guide to developing the overall picture of magnetotail evolution and studying the causal relationship between the magnetic reconnection and the dipolarization as well as detailed mechanisms of each of the two processes on the basis of multispacecraft observations. Copyright 2009 by the American Geophysical Union.

The information on energy spectra of 1-100 keV electrons is expected to provide an important clue to understand heating and acceleration mechanisms of magnetospheric plasmas. However, electrons of several keV to several tens of keV are not properly verified by observations owing to the problems in the measurement techniques. This study aims to bridge this gap by applying Avalanche Photodiodes (APDs) to the detection of electrons. The internal gain of APDs enables high-resolution detection of low-energy electrons down to several keV. We have tested an APD: Type spl 3989, Hamamatsu Photonics Co. Ltd. The APD responded to 2-40 keV electrons with the fine peaks of the out put pulse height distributions. Although the experiment is limited to 40 keV, electrons up to about 60 keV are predicted to be detectable with this APD from the simulation. We also have successfully made a verification test by the sounding rocket of ISAS/JAXA targeting medium energy electrons.

In regions such as the reconnection sites and the ring current, plasmas are highly accelerated and their energies sometimes exceed the uppermost energy level of low-energy plasma sensors (typically similar to 40 keV). In order to study acceleration mechanisms in such key regions, in-situ observations with continuous energy coverage from low (similar to eV) to medium (similar to 10- similar to 200 keV) or to even higher energies are necessary. In fact, ERG and Cross scale missions are planned to explore the above regions with plasma instrument packages that require covering the majority of the energy range. We, therefore, develop a medium energy ion mass spectrometer, which consists of an Electrostatic Analyser (ESA), a Time-of-Flight mass spectrometer (ToF), and solid-state detectors (SSDs). It can simultaneously and independently measure energy-per-charge (E/q), velocity (v), and energy (E) of incoming ions, thus deducing E, m, and q. In addition, the coincidence method via the combination of ToF start-stop signals and SSD signals is useful to reject background noise that is caused by radiation belt electrons and/or solar energetic protons. In order to enable electrostatic analyses with a practical sensor size, we have developed a novel "cusp type" electrostatic analyser. This design provides us a relatively small instrument that has an energy range up to similar to 200 keV/q with a full solid angle coverage (using the spacecraft spin motion). This kind of electrostatic analyser may also be used for electron measurements.

SCOPE is the magnetospheric explorer mission using a distributed sensing system to investigate space-time correlation of particle behaviors in the magnetotail. The mission needs an autonomous inter-satellite link system enabling commanding, data transfer, ranging, and timing control support for data sampling in unison in a distributed system. TDM/TDMA system has been proposed so as to fit to SCOPE specification. We have developed a simulator to validate our design for SCOPE and obtained successful results. As an instrument to work within a limited resources of small sized satellites like SCOPE, data link, ranging performance, timing accuracy satisfy requirements of SCOPE mission. We describe our inter-satellite link system and achievements obtained through the simulator experiments.

We have newly developed an electron energy analyzer FESA (Fast Electron energy Spectrum Analyzer) for a future magnetospheric satellite mission SCOPE. The SCOPE mission is designed in order that observational studies from the cross-scale coupling viewpoint are enabled. One of the key observations necessary for the SCOPE mission is high-time resolution electron measurement. Eight FESAs on a spinning spacecraft are capable of measuring three dimensional electron distribution function with time resolution of 8 msec. FESA consists of two electrostatic analyzers that are composed of three nested hemispherical deflectors. Single FESA functions as four top-hat type electrostatic analyzers that can measure electrons with four different energies simultaneously. By measuring the characteristics of the test model FESA, we proved the validity of the design concept of FESA. Based on the measured characteristics, we designed FESA optimized for the SCOPE mission. This optimized analyzer has good enough performance to measure three dimensional electron distribution functions around the magnetic reconnection region in the Earth's magnetotail.

We have studied the response of large-scale ionospheric convection to substorm expansion onsets on the basis of two weak substorms of 1 May 2001, during which a large part of the dawn cell of the two-cell ionospheric convection pattern was monitored by the Super DARN radars. Ionospheric convection began to enhance first in a localized region of the equatorward part of the dawn cell ∼2 minutes before the expansion onsets of both substorms and then enhanced in the entire dawn cell successively. The enhanced convection persisted throughout their expansion phase, possibly even near the footprint of a plasma sheet region without fast flows observed by Geotail. These observations suggest that ionospheric convection begins to enhance just before substorm expansion onset and then enhances in the entire cell, possibly regardless of the presence of fast earthward flows in the corresponding plasma sheet region of the magnetotail. The global enhancement of ionospheric convection is consistent with that of magnetotail convection, which also begins just before onset. © 2008 by the American 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.

The plasma number density in the near-Earth plasma sheet depends on the solar wind number density and the north-south component of interplanetary magnetic field (IMF Bz) with time lag and duration of several hours. We examined the three-dimensional structure of such dependences by fitting observations of plasma sheet and solar wind to an empirical model equation. Analyses were conducted separately for northward and southward IMF conditions. Effects of solar wind speed and IMF orientation were also examined by further subdivision of the dataset. Based on obtained results, we discuss (i) the relative contribution of the ionosphere and solar wind to plasma sheet mass supply, (ii) the entry mechanisms for magnetosheath particles, and (iii) the plasma transport in the plasma sheet. We found that solar wind number density dependence is weaker and IMF Bz dependence is stronger for faster solar wind with southward IMF, which suggests the contribution of ionospheric particles. Further from the Earth, different interplanetary conditions result in different structures of solar wind dependence, which indicate different solar wind entry mechanisms: (1) southward IMF results in a strong dependence on solar wind number density in the flank high-latitude region, (2) slow solar wind with northward IMF leads to lower-latitude peaks of solar wind number density dependence in the flank region, (3) fast solar wind with northward IMF results in a strong dependence on solar wind number density at the down-tail dusk flank equator, and (4) solar wind number density dependence is stronger in the downstream of quasi-parallel bow shock. These features are attributable to (1) low-latitude dayside reconnection entry, (2) high-latitude dayside reconnection entry, (3) entry due to decay of Kelvin-Helmholtz vortices, and (4) diffusive entry mediated by kinetic Alfven waves, respectively. Effect of IMF B z and its time lags show plasma sheet reconfiguration associated with enhanced convective transport under southward IMF. Duration of IMF B z effect under northward IMF is interpreted in terms of turbulent diffusive transport.

During a storm recovery phase on 15 May 2005, the Geotail spacecraft repeatedly observed high-energy (&gt 180 keV) oxygen ions in the dayside magnetosheath near the equatorial plane. We focused on the time period from 11:20 UT to 13:00 UT, when Geotail observed the oxygen ions and the interplanetary magnetic field (IMF) was constantly northward. The magnetic reconnection occurrence northward and duskward of Geotail is indicated by the Walén analysis and convective flows in the magnetopause boundary layer. Anisotropic pitch angle distributions of ions suggest that high-energy oxygen ions escaped from the northward of Geotail along the reconnected magnetic field lines. From the low-energy particle precipitation in the polar cap observed by DMSP, which is consistent with magnetic reconnection occurring between the magnetosheath field lines and the magnetospheric closed field lines, we conclude that these oxygen ions are of ring current origin. Our results thus suggest a new escape route of oxygen ions during northward IMF. In the present event, this escape mechanism is more dominant than the leakage via the finite Larmor radius effect across the dayside equatorial magnetopause. © 2008 European Geosciences Union.

月は8割以上の時間を太陽風中で過ごすが,その際に夜側にウェイクと呼ばれる真空に近い領域が形成される.太陽風の電子は一部が夜側の月面に到達できるが,イオンは熱速度が太陽風速度と比較して圧倒的に遅いためウェイク側の月面には到達できないと考えられている.ところが今回,月周回衛星「かぐや」に搭載されたプラズマ・磁場観測装置(MAP)は,月の真夜中側100km高度のほぼ中央部分(太陽直下点の反対側付近の低緯度領域)で太陽風起源とみられるイオンを観測した.これらのイオンは一部が夜側の月面から飛来しており,夜側へと加速された太陽風が月面に衝突して反射したものを観測したと考えられる.また,ウェイク境界で太陽風イオンが夜側へ向かって加速される現場も観測された.これらの現象は何らかの理由でウェイク境界付近の電場が強まった結果として起きたものと考えられる.

With two electron beam sources, we have tested two new Hamamatsu [Hamamatsu Photonics K.K., Shizuoka, Japan 〈http://www.hamamatsu.com/〉] avalanche photodiodes (APDs) of spl 3988 and spl 6098 to detect electron beams up to 100 keV. Though our previous results showed the effectiveness and the advantage of an APD to measure 2-40 keV electrons, its upper limit was not high enough to detect so-called medium-energy electrons. In addition to the limitation of its detectable range, the response at different energies was also not linear. These newly developed APDs, which have thicker depletion-layers, provide full coverage of this missing range along with a good linearity. The depletion-layer thickness was increased to 140 μ m for both APDs, the dead-layer of spl 3988 became 10 times thicker than that of spl 6098. The thin-surface dead-layer and thick depletion-layer of spl 6098 allows the detection of electrons from 3 keV up to 100 keV with a good linearity and with an excellent energy resolution of 4 keV at 100-keV electrons. The wide dynamic range from 3 keV to 100 keV of those APDs will increase their appeal in detecting electrons for space plasma research. © 2008 Elsevier B.V. All rights reserved.

The dawn-dusk locations of reconnection in the near-earth magnetotail at the time of isolated auroral breakup are studied to clarify whether breakup is always accompanied by reconnection. The near-earth reconnection is identified by tailward plasma flows faster than 200 km/s with southward magnetic field. We first identified 66 breakups in the Polar ultraviolet imager observations of the nightside polar ionosphere. We then studied tailward flows during breakups using Geotail in situ observations of the plasma sheet between 25 and 31 R E down the tail. It was found that the dawn-dusk (Y) locations of relatively fast (≥400 km/s) tailward flows were associated with breakup magnetic local time (MLT) by a regression line of YAGSM = -5.7 × (MLT + 0.6) RE with a correlation coefficient of 0.8. Most tailward flows were observed within 5 RE of the modeled Y locations, where tailward flows occurred in 88% of the 26 cases of breakups between 22 and 0 MLT. It is thus inferred that in most cases, breakup is accompanied by tailward flow near the breakup MLT with its dawn-dusk dimension ∼10 R E. There were only two events without tailward flows in the region where flows have been expected. These two events were an earthward flow event and a traveling compression region event, which are not inconsistent with the initiation of the near-earth reconnection. Auroral breakup is thus likely to always be accompanied by near-earth reconnection near breakup MLT. It is also inferred that reconnection and breakup occur simultaneously within a few minutes, assuming a time delay between reconnection onset and the arrival of tailward flows at satellite locations. Copyright 2008 by the American Geophysical Union.

Spacecraft observations indicate that low-frequency (0.006-0.025 Hz) fluctuations of the magnetic field appear a few min prior to a substorm-associated dipolarization. These fluctuations are examined on the basis of the linear magnetohydrodynamic (MHD) fluid theory. We propose a "low-frequency fitting method" for identifying the characteristics of the MHD waves, such as the mode, the wavevector, and the frequency. Using the magnetic field and the ion velocity data, the frequency in the plasma rest frame is obtained by removing the Doppler shift. The fitting method takes the inhomogeneity of the ambient magnetic field into account, so that the parameter which is equivalent to the sum of the field line curvature and the gradient scale length of the ambient magnetic field is obtained as an output. We applied the method to a selected dipolarization event in which Geotail remained in the vicinity of the magnetic equator and also in the same magnetic local time as the onset region of an auroral breakup. The low-frequency fluctuations were detected from ∼4 min before the local dipolarization onset. What has been found are as follows: (1) the parallel magnetic field fluctuations had a strong correlation with the perpendicular ion velocity fluctuations, which indicates that the observed waves were explained as magnetosonic modes. The slow magnetosonic wave was identified ∼3 min before the dipolarization onset, whereas the fast magnetosonic wave was identified ∼1.5 min before the dipolarization onset. This fast wave was estimated to be propagating tailward with a phase velocity of 400 km/s and a period of 70 s in the plasma frame. (2) The perpendicular fluctuations of the magnetic field and the ion velocity were only weakly correlated, which indicates the presence of a mode that cannot be captured by the present method. Using the variance analysis, we show that this mode is likely to be a drift mode, which had almost zero frequency in the plasma frame and was propagating duskward together with the plasma bulk flow. A possible interpretation of the observed waves is briefly discussed with a relevance to previously proposed substorm initiation models. Particularly, the drift wave can be interpreted as a linear stage of the ballooning instability. Copyright 2008 by the American Geophysical Union.

We have developed a new energy/mass spectrometer for medium energy range (similar to 10-200 keV/q) ion measurements in the Earth's magnetosphere and interplanetary space. The wide field-of-view (similar to 360 degrees fan) enables acquisition of 3-D distribution functions for all the major ions, by utilizing spacecraft spin motions. The g-factor is much larger than the previous ion mass spectrometers in the medium energy range. The mass analysis unit that measures ion time-of-flights is well designed to realize a lightweight and simple signal processing. Laboratory experiments with a test model show that the performance of mass spectroscopy agrees with numerical simulations. Medium energy ion mass spectrometer with this new design will surely be useful for upcoming space missions in the inner magnetosphere, reconnection regions, and other energetic plasma structures/phenomena in space.

We present in situ observations consistent with the ballooning mode in the vicinity of the magnetic equator at XGSM = -10 to -13 RE prior to substorm-associated dipolarization onsets. The ballooning instability is expected to have a wavevector along the Y direction and to give variation to the curvature of the ambient magnetic field lines. The magnetic field fluctuations appearing in the Bx component are transported by the ambient plasma drift in the Y direction. A discrete frequency band would be identified in time series data if the mode has a discrete wavelength. The ballooning mode of this property was identified at the magnetic equator a few min before dipolarization onsets only when the plasma β was large (20 to 70). Using low-energy ion velocity data, we show that the mode has almost zero frequency in the plasma rest frame so that ωsc ∼ ky · vy, where ωsc is the frequency in the spacecraft frame, and ky and vy are the wavenumber and the ambient plasma flow in the Y direction, respectively. This enables us to estimate the wavelengths of the ballooning mode, which were found to be of the order of the ion Larmor radius. Copyright 2008 by the American Geophysical Union.

We examine photoelectron distributions detected by the low-energy-particle (LEP) instrument onboard the GEOTAIL spacecraft by means of both data analysis and numerical simulations. Statistical data analysis shows asymmetries in the photoelectron distributions. For photoelectrons incident normal to the spacecraft spin axis, a higher flux is observed in the dawnward than in the duskward sector of the LEP. The distribution significantly depends on the ratio of the photoelectron energy to the spacecraft potential. Our numerical simulations reveal that the asymmetry is caused by the electrostatic potential around the thin antenna located at +18° anticlockwise (viewed from the top) relative to the LEP. Photoelectrons in the dawnward sector are preferentially carried from the sunlit surface by this potential. For upward/downward incident photoelectrons, a higher flux of upward photoelectrons is observed in the antisunward than in the sunward sector, whereas downward photoelectrons show a weak asymmetry. Our numerical simulations demonstrate that the greater flux of upward photoelectrons is caused by the electrons emitted from the sunlit surface they are attracted to the antisunward sector. Based on these results, the asymmetries in the photoelectron distribution measured around GEOTAIL are found to be caused by the asymmetric positioning of the thin antennas relative to the LEP. © 2008 IEEE.