浅村 和史

The software-type wave–particle interaction analyzer (S-WPIA) is an instrument package onboard the Arase satellite, which studies the magnetosphere. The S-WPIA represents a new method for directly observing wave–particle interactions onboard a spacecraft in a space plasma environment. The main objective of the S-WPIA is to quantitatively detect wave–particle interactions associated with whistler-mode chorus emissions and electrons over a wide energy range (from several keV to several MeV). The quantity of energy exchanges between waves and particles can be represented as the inner product of the wave electric-field vector and the particle velocity vector. The S-WPIA requires accurate measurement of the phase difference between wave and particle gyration. The leading edge of the S-WPIA system allows us to collect comprehensive information, including the detection time, energy, and incoming direction of individual particles and instantaneous-wave electric and magnetic fields, at a high sampling rate. All the collected particle and waveform data are stored in the onboard large-volume data storage. The S-WPIA executes calculations asynchronously using the collected electric and magnetic wave data, data acquired from multiple particle instruments, and ambient magnetic-field data. The S-WPIA has the role of handling large amounts of raw data that are dedicated to calculations of the S-WPIA. Then, the results are transferred to the ground station. This paper describes the design of the S-WPIA and its calculations in detail, as implemented onboard Arase.[Figure not available: see fulltext.].

Arase is a small scientific satellite program conducted by the Institute of Space and Astronautical Science/Japan Aerospace Exploration Agency, which is dedicated to the detailed study of the radiation belts around Earth through in situ observations. In particular, the goal is to directly observe the interaction between plasma waves and particles, which cause the generation of high-energy electrons. To observe the waves and particles in detail, we must record large volumes of burst data with high transmission rates through onboard mission network systems. For this purpose, we developed a high-speed and highly reliable mission network based on SpaceWire, as well as a new and large memory data recorder equipped with a data search function based on observation time (the time index, TI, is the satellite time starting from when the spacecraft is powered on.) with respect to the orbital data generated in large quantities. By adopting a new transaction concept of a ring topology network with SpaceWire, we could secure a redundant mission network system without using large routers and having to suppress the increase in cable weight. We confirmed that their orbit performs as designed.[Figure not available: see fulltext.].

The exploration of energization and radiation in geospace (ERG) satellite, nicknamed "Arase," is the second satellite in a series of small scientific satellites created by the Institute of Space and Astronautical Science of the Japan Aerospace Exploration Agency. It was launched on December 20, 2016, by the Epsilon launch vehicle. The purpose of the ERG project is to investigate how high-energy (over MeV) electrons in the radiation belts surrounding Earth are generated and lost by monitoring the interactions between plasma waves and electrically charged particles. To measure these physical processes in situ, the ERG satellite traverses the heart of the radiation belts. The orbit of the ERG is highly elliptical and varies due to the perturbation force: the apogee altitude is approximately 32,200-32,300 km, and the perigee altitude is 340-440 km. In this study, we introduce the scientific background for this project and four major challenges that need to be addressed to effectively carry out this scientific mission with a small satellite: (1) dealing with harsh environmental conditions in orbit and electromagnetic compatibility issues, (2) spin attitude stabilization and avoiding excitation of the libration by flexible structures, (3) attaining an appropriate balance between the mission requirements and the limited resources of the small satellite, and (4) the adaptation and use of a flexible standardized bus. In this context, we describe the development process and the flight operations for the satellite, which is currently working as designed and obtaining excellent data in its mission.

The Exploration of energization and Radiation in Geospace (ERG) Science Center serves as a hub of the ERG project, providing data files in a common format and developing the space physics environment data analysis software and plug-ins for data analysis. The Science Center also develops observation plans for the ERG (Arase) satellite according to the science strategy of the project. Conjugate observations with other satellites and ground-based observations are also planned. These tasks contribute to the ERG project by achieving quick analysis and well-organized conjugate ERG satellite and ground-based observations.[Figure not available: see fulltext.]

In the inner magnetosphere, the Arase spacecraft has observed electromagnetic ion cyclotron (EMIC) emissions with both rising and falling frequencies. The instantaneous frequency analyses on the electromagnetic fields of the EMIC rising tone emission have been performed by the Hilbert-Huang transform. The time variation of the instantaneous frequency shows a good agreement with the nonlinear theory for the frequency evolutions. Rapid instantaneous frequency modulation is also found during the rising tone emission. We estimate the peak-to-peak time of the fluctuation in the frequency and find that the fluctuation is caused around a half of the particle trapping time. From the motion of the phase-bunched particle around the resonant velocity, it is expected that the nonlinear resonant current, which induces the falling frequency is formed in half the trapping time.Plain Language Summary The Arase spacecraft observed nonlinear EMIC rising and falling tone emissions in the inner magnetosphere. Instantaneous frequency of nonlinear EMIC rising tone emission is analyzed by Hilbert-Huang Transformation. Fast frequency modulation of the rising tone emission is found in the instantaneous frequency, which can be caused by the phase-bunched particles in the phase space.

©2018. American Geophysical Union. All Rights Reserved. Super Dual Auroral Radar Network (SuperDARN) observations show that ionospheric flow fluctuations of millihertz or lower-frequency range with horizontal velocities of a few hundred meters per second appeared in the subauroral to midlatitude region during a magnetic storm on 27 March 2017. A set of the radars have provided the first ever observations that the fluctuations propagate azimuthally both westward and eastward simultaneously, showing bifurcated phase propagation associated with substorm expansion. Concurrent observations near the conjugate site in the inner magnetosphere made by the Arase satellite provide evidence that multiple drifting clouds of electrons in the near-Earth equatorial plane were associated with the electric field fluctuations propagating eastward in the ionosphere. We interpret this event in terms of mesoscale pressure gradients carried by drifting ring current electrons that distort field lines one after another as they drift through the inner magnetosphere, causing eastward propagating ionospheric electric field fluctuations.

Differential energy flux of electrons precipitating into the high-latitude ionosphere can be estimated from incoherent scatter radar observations of the ionospheric electron density profile. We present a method called ELSPEC for electron spectrum estimation from incoherent scatter radar measurements, which is based on integration of the electron continuity equation and spectrum model selection by means of the Akaike information criterion. This approach allows us to use data with almost arbitrary time resolutions, enables spectrum estimation with dense energy grids, avoids noise amplifications in numerical derivatives, and yields statistical error estimates for all the output parameters, including the number and energy fluxes and upward field-aligned currents carried by the precipitating electrons. The technique is targeted for auroral energies, 1-100 keV, which ionize the atmosphere mainly between 80 and 150 km altitudes. We validate the technique by means of a simulation study, which shows that Maxwellian, kappa, and mono-energetic spectra, as well as combinations of those, can be reproduced. Comparison study for two conjugate satellite measurements to the EISCAT UHF radar are shown, for Reimei and Swarm, showing an agreement with the results. Finally, an example of a 2-hr measurement by the EISCAT radar is shown, during which we observe a variety of precipitation characteristics, from soft background precipitation to mono-energetic spectra with peak energies up to 60 keV. The upward field-aligned current varies from 0 to 10 mu Am-2 and the total energy flux from 0 to 250 mWm(-2).

© 2017, The Author(s). The medium-energy particle experiments–ion mass analyzer (MEP-i) was developed for the exploration of energization and radiation in geospace (ERG) mission (Arase), in order to measure the three-dimensional distribution functions of the inner-magnetospheric ions in the medium energy range between 10 and 180 keV/q. The energy, mass, and charge state of each ion are determined by a combination of an electrostatic energy/charge analyzer, a time-of-flight mass/charge analyzer, and energy-sensitive solid-state detectors. This paper describes the instrumentation of the MEP-i, data products, and observation results during a magnetic storm.

In this report, we describe the low-energy electron instrument LEPe (low-energy particle experiments-electron analyzer) onboard the Arase (ERG) spacecraft. The instrument measures a three-dimensional distribution function of electrons with energies of similar to 19 eV-19 keV. Electrons in this energy range dominate in the inner magnetosphere, and measurement of such electrons is important in terms of understanding the magnetospheric dynamics and waveparticle interaction. The instrument employs a toroidal tophat electrostatic energy analyzer with a passive 6-mm aluminum shield. To minimize background radiation effects, the analyzer has a background channel, which monitors counts produced by background radiation. Background counts are then subtracted from measured counts. Electronic components are radiation tolerant, and 5-mm-thick shielding of the electronics housing ensures that the total dose is less than 100 kRad for the one-year nominal mission lifetime. The first in-space measurement test was done on February 12, 2017, showing that the instrument functions well. On February 27, the first all-instrument run test was done, and the LEPe instrument measured an energy dispersion event probably related to a substorm injection occurring immediately before the instrument turn-on. These initial results indicate that the instrument works fine in space, and the measurement performance is good for science purposes.

We discuss the surface-scattering of solar wind protons at Mercury based on observed scattering characteristics from lunar regolith. The properties of the impinging plasma are expected to be different between different regions on Mercury, and between Mercury and the Moon. Here, we review the expected Hermean plasma conditions and lunar empirical scattering models. We present observed and modeled energy spectra for scattered protons and hydrogen energetic neutral atoms (ENAs) for three cases of very different plasma conditions at the Moon. Then, we simulate scattering from the Hermean surface by applying the empirical models to four different scenarios of plasma precipitation on Mercury. The results suggest that surface-scattering is a strong source of ENAs at Mercury (up to similar to 10(8) cm(-2) s(-1)), which can be very useful for remote-sensing of the plasma conditions at the surface. Protons scattered from the surface back into space are also expected with high fluxes up to similar to 10(7) cm(-2) s(-1), and may be important for wave generation and the filling in of the loss cone of mirroring and quasi-trapped populations. Scattered protons at the cusp region (of similar to 10(6) cm(-2) s(-1)) can potentially be detected by orbiters as outflowing protons within the loss cone. (C) 2017 Elsevier Inc. All rights reserved.

Electromagnetic plasma waves are thought to be responsible for energy exchange between charged particles in space plasmas. Such an energy exchange process is evidenced by phase space holes identified in the ion distribution function and measurements of the dot product of the plasma wave electric field and the ion velocity. We develop a method to identify ion hole formation, taking into consideration the phase differences between the gyromotion of ions and the electromagnetic ion cyclotron (EMIC) waves. Using this method, we identify ion holes in the distribution function and the resulting nonlinear EMIC wave evolution from Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations. These ion holes are key to wave growth and frequency drift by the ion currents through nonlinear wave-particle interactions, which are identified by a computer simulation in this study.

The lithium-ion secondary batteries have been widely used for the space programs, today. Among them, REIMEI was one of the first satellites using lithium-ion secondary battery. In 2005, the satellite was launched, and injected into the low earth polar orbit. Eleven years has passed since the launch and over 60,000 cycles of charge and discharge was experienced in space. The lithium-ion secondary cell of the REIMEI battery was designed using spinel manganese oxide type material for the positive electrode, and the graphitized type carbon for the negative electrode. The cell case was made of aluminium laminated film and the structure was reinforced by the epoxy resin and aluminium housing. After the operation of eleven years, the cells still maintain the appropriate uniform balance and operative. In order to identify the internal condition of the battery/cell, we calculated the ac impedance by the pulse duration to the on-board battery.

For five days of each lunar orbit, the Moon is shielded from solar wind bombardment by the Earths magnetosphere, which is filled with terrestrial ions. Although the possibility of the presence of terrestrial nitrogen and noble gases in lunar soil has been discussed based on their isotopic composition, complicated oxygen isotope fractionation in lunar metal(2,3) (particularly the provenance of a O-16-poor component) remains an enigma(4,5) . Here, we report observations from the Japanese spacecraft Kaguya of significant numbers of 110 keV O+ ions, seen only when the Moon was in the Earths plasma sheet. Considering the penetration depth into metal of O+ ions with such energy, and the O-16-poor mass-independent fractionation of the Earths upper atmosphere 6 , we conclude that biogenic terrestrial oxygen has been transported to the Moon by the Earth wind (at least 2.6 x104 ions cm(-2) s(-1)) and implanted into the surface of the lunar regolith, at around tens of nanometres in depth (3,4) . We suggest the possibility that the Earths atmosphere of billions of years ago may be preserved on the present-day lunar surface.

The time resolution of low-energy charged particle measurements is becoming higher and higher. In order to realize high time resolution measurements, a 1-D circular delay line anode has been developed as a high-speed microchannel plate (MCP) anode. The maximum count rate of the 1-D circular delay line anode is around 1x10(7)/s/360 degrees, which is much higher than the widely used resistive anode, whose maximum count rate is around 1 x 10(6)/s/360 degrees. In order to achieve much higher speeds, an MCP anode with application-specific integrated circuit (ASIC) has been developed. We have decided to adopt an anode configuration in which a discrete anode is formed on a ceramic substrate, and a bare ASIC chip is installed on the back of the ceramic. It has been found that the anode can detect at a high count rate of 2 x 10(8)/s/360 degrees. Developments in both delay line and discrete anodes, as well as readout electronics, will be reviewed.

When quaternions are used for representing the attitude of a spinning spacecraft in an attitude estimation filter, several problems appear due to their rapid variations. These problems include numerical integration errors and violation of the linear approximations for the filter. In this study, we propose representing the attitude of a spinning spacecraft using a set of spin parameters. These parameters consist of the spin-axis orientation unit vector in the inertial frame and the spin phase angle. This representation is advantageous as the spin axis direction components in the inertial frame do not change rapidly and the phase angle changes with a constant rate in the absence of a torque. The attitude matrix and the kinematics equations are derived in terms of spin parameters. As the equations are highly nonlinear an Unscented Kalman Filter (UKF) is implemented to estimate the spacecraft's attitude in spin parameters. The estimation results are compared with those of a quaternion based UKF in different scenarios using the simulated data for JAXA's ERG spacecraft.

The ERG (Exploration of energization and Radiation in Geospace) is Japanese geospace exploration project. The project focuses on relativistic electron acceleration mechanism of the outer belt and dynamics of space storms in the context of the cross-energy coupling via wave-particle interactions. The project consists of the satellite observation team, the ground-based network observation team, and integrated-data analysis/simulation team. The satellite was launched on December 20 2016 and has been nicknamed, "Arase". This paper describes overview of the project and future plan for observations.

We present first measurements of energetic neutral atoms that originate from solar wind plasma having interacted with the lunar nightside surface. We observe two distinct energetic neutral atom (ENA) distributions parallel to the terminator, the spectral shape, and the intensity of both of which indicate that the particles originate from the bulk solar wind flow. The first distribution modifies the dayside ENA flux to reach approximate to 6 degrees into the nightside and is well explained by the kinetic temperature of the solar wind protons. The second distribution, which was not predicted, reaches from the terminator to up to 30 degrees beyond the terminator, with a maximum at approximate to 102 degrees in solar zenith angle. As most likely wake transport processes for this second distribution we identify acceleration by the ambipolar electric field and by the negatively charged lunar nightside surface. In addition, our data provide the first observation indicative of a global solar zenith angle dependence of positive dayside surface potentials.

Lithium-ion secondary cells are widely used for the space applications, today. Among these applications, REIMEI, which was launched in 2005, was one of the first satellites using lithium-ion battery. The off-the-shelf type cells designed using spinel manganese oxide for the positive and the graphitized carbon for the negative electrode were used. The cell case was made of aluminum laminated film and the structure was reinforced by the aluminum case filled with epoxy resin. Today, ten years has passed, and the battery experienced 55,000 cycles for charge and discharge. The current distribution between two batteries almost coincided together even after the long term operation, which revealed the stable performance of the lithium-ion secondary cells under the microgravity in space.

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.

Characterised by a surface bound exosphere and localised crustal magnetic fields, the Moon was considered as a passive object when solar wind interacts with it. However, the neutral particle and plasma measurements around the Moon by recent dedicated lunar missions, such as Chandrayaan-1, Kaguya, Chang’E-1, LRO, and ARTEMIS, as well as IBEX have revealed a variety of phenomena around the Moon which results from the interaction with solar wind, such as backscattering of solar wind protons as energetic neutral atoms (ENA) from lunar surface, sputtering of atoms from the lunar surface, formation of a “mini-magnetosphere” around lunar magnetic anomaly regions, as well as several plasma populations around the Moon, including solar wind protons scattered from the lunar surface, from the magnetic anomalies, pick-up ions, protons in lunar wake and more. This paper provides a review of these recent findings and presents the interaction of solar wind with the Moon in a new perspective.

The lunar surface is very efficient in reflecting impinging solar wind ions as energetic neutral atoms (ENAs). A global analysis of lunar hydrogen ENAs showed that on average 16% of the solar wind protons are reflected, and that the reflected fraction can range from less than 8% to more than 24%, depending on location. It is established that magnetic anomalies reduce the flux of backscattered hydrogen ENAs by screening-off a fraction of the impinging solar wind. The effects of the surface properties, such as porosity, roughness, chemical composition, and extent of weathering, were not known. In this paper, we conduct an in-depth analysis of ENA observations of the South Pole-Aitken basin to determine which of the surface properties might be responsible for the observed variation in the integral ENA flux. The South Pole-Aitken basin with its highly variable surface properties is an ideal object for such studies. It is very deep, possesses strikingly elevated concentrations in iron and thorium, has a low albedo and coincides with a cluster of strong magnetic anomalies located on the northern rim of the basin. Our analysis shows that whereas, as expected, the magnetic anomalies can account well for the observed ENA depletion at the South Pole-Aitken basin, none of the other surface properties seem to influence the ENA reflection efficiency. Therefore, the integral flux of backscattered hydrogen ENAs is mainly determined by the impinging plasma flux and ENA imaging of backscattered hydrogen captures the electrodynamics of the plasma at the surface. We cannot exclude minor effects by surface features. (C) 2015 Elsevier Ltd. All rights reserved.

We investigate the origin of the fine structure of the energy spectrum of precipitating electrons for the pulsating aurora (PsA) observed by the low-altitude Reimei satellite. The Reimei satellite achieved simultaneous observations of the optical images and precipitating electrons of the PsA from satellite altitude (similar to 620km) with resolution of 40ms. The main modulation of precipitation, with a few seconds, and the internal modulations, with a few hertz, that are embedded inside the main modulations are identified above similar to 3keV. Moreover, stable precipitations at similar to 1keV are found for the PsA. A precipitation gap is discovered between two energy bands. We identify the origin of the fine structure of the energy spectrum for the precipitating electrons using the computer simulation on the wave-particle interaction between electrons and chorus waves. The lower band chorus (LBC) bursts cause the main modulation of energetic electrons, and the generation and collapse of the LBC bursts determines on-off switching of the PsA. A train of rising tone elements embedded in the LBC bursts drives the internal modulations. A close set of upper band chorus (UBC) waves causes the stable precipitations at similar to 1keV. We show that a wave power gap around the half gyrofrequency at the equatorial plane in the magnetosphere between LBC and UBC reduces the loss rate of electrons at the intermediate energy range, forming a gap of precipitating electrons in the ionosphere.

Pulsating auroras often appear in forms of geo-stable or slowly convecting patches. These patches can maintain their rough shape and size over many sequences of luminosity pulsations, yet they slowly drift with ionospheric ExB convection. Because of these characteristics, there has long been a speculation that the pulsating auroral patch (PAP) is connected to flux tubes filled with enhanced cold plasma. In this study, we perform a survey on pulsating auroral events when the footprints of low-Earth-orbit satellites traversed the PAPs, with a focus on the low-energy particle signatures associated with the PAPs. As a result, we identified, in a majority (similar to 2/3) of events, the existence of a low-energy ion precipitation structure that is collocated with the PAP, with core energies ranging from several tens of eV up to a few hundred eV. This result supports the hypothesis that a PAP connects to flux tubes filled with enhanced cold plasma. We further propose that the plasma outflows from the ionosphere are the origin of such cold plasma flux tubes. We suggest that the PAP is formed by a combination of high-energy electrons of a magnetospheric origin, the low-energy plasma structure of an ionospheric origin, and certain ELF/VLF waves that are intensified and modulated in interactions with both the hot and cold plasma populations.

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.

This paper reports two event studies on characteristic pitch angle variations of inverted-V electrons observed by the Reimei satellite. At edges of the inverted-V structures, the electron pitch angles are collimated between 0 degrees and 20 degrees while the electric field of the potential distribution is commonly depicted perpendicular to the local magnetic field. As Reimei moved toward the center of the inverted-V regions, the electron pitch angles broadened up to approximate to 120 degrees, and their energies rapidly increased and continuously changed to those of the energetic inverted-V component. At the higher latitudes of the inverted-V structure, diffuse electrons with the isotropic distribution were also observed. Estimations of the electron density and temperature indicate that the source region of the beam electrons is the topside ionosphere by comparison with those of the diffuse electrons and the energetic inverted-V electrons. For the auroral emissions, in the first event, some horizontal auroral motions were observed which may be accompanied by the horizontal drift motion of the whole structure of the parallel potential drop. This motion could supply electrons in the topside ionosphere into the potential structure, and then the beam electrons are continuously formed at the low altitudes. In the second event, on the other hand, at both edges of the auroral band, the auroral emissions did not expand while the beam electrons were observed. One of the probable reasons to produce the beam electrons is the inertial Alfven waves although they are inconsistent with previous studies because the velocity dispersions were not observed.

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 key 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. (C) 2014 Elsevier Ltd. All rights reserved.

We present the first direct measurement of neutral oxygen in the lunar exosphere, detected by the Chandrayaan-1 Energetic Neutral Analyzer (CENA). With the lunar surface consisting of about 60% of oxygen in number, the neutral oxygen detected in CENA's energy range (11eV-3.3keV) is attributed to have originated from the lunar surface, where it was released through solar wind ion sputtering. Fitting of CENA's mass spectra with calibration spectra from ground and in-flight data resulted in the detection of a robust oxygen signal, with a flux of 0.2 to 0.4 times the flux of backscattered hydrogen, depending on the solar wind helium content and particle velocity. For the two solar wind types observed, we derive subsolar surface oxygen atom densities of N-0= (1.1 0.3) 10(7)m(-3) and (1.4 0.4) 10(7)m(-3), respectively, which agree well with earlier model predictions and measured upper limits. From these surface densities, we derive column densities of N-C= (1.5 0.5) 10(13)m(-2)and (1.6 0.5) 10(13)m(-2). In addition, we identified for the first time a helium component. This helium is attributed to backscattering of solar wind helium (alpha particles) from the lunar surface as neutral energetic helium atoms, which has also been observed for the first time. This identification is supported by the characteristic energy of the measured helium atoms, which is roughly 4 times the energy of reflected solar wind hydrogen, and the correlation with solar wind helium content.

The lithium-ion secondary cells/batteries are commonly used for the spacecraft, today. 'REIMEI' satellite is also an example using lithium-ion secondary battery. It used 3 Ah-class off-the-shelf lithium-ion secondary cells, which used the spinel LiMnO4 for the positive and graphite for the negative electrode. Seven cells were connected in series, and two series strings were connected in parallel. The satellite was launched in August, 2005, and injected into the low earth polar orbit. Initially, the battery performance was simulated based on the dependency of the cell performance on temperature. Considering the impedance and discharge performance depending on temperature, the end-of-discharge-voltage during the operation had been precisely controlled. Seven years operation of the lithium-ion secondary battery under the micro-gravity conditions have been demonstrated through the REIMEI operation in space.

This paper reports two event studies on characteristic pitch angle variations of inverted-V electrons observed by the Reimei satellite. At edges of the inverted-V structures, the electron pitch angles are collimated between 0° and 20° while the electric field of the potential distribution is commonly depicted perpendicular to the local magnetic field. As Reimei moved toward the center of the inverted-V regions, the electron pitch angles broadened up to ∼120°, and their energies rapidly increased and continuously changed to those of the energetic inverted-V component. At the higher latitudes of the inverted-V structure, diffuse electrons with the isotropic distribution were also observed. Estimations of the electron density and temperature indicate that the source region of the beam electrons is the topside ionosphere by comparison with those of the diffuse electrons and the energetic inverted-V electrons. For the auroral emissions, in the first event, some horizontal auroral motions were observed which may be accompanied by the horizontal drift motion of the whole structure of the parallel potential drop. This motion could supply electrons in the topside ionosphere into the potential structure, and then the beam electrons are continuously formed at the low altitudes. In the second event, on the other hand, at both edges of the auroral band, the auroral emissions did not expand while the beam electrons were observed. One of the probable reasons to produce the beam electrons is the inertial Alfvén waves although they are inconsistent with previous studies because the velocity dispersions were not observed. Key Points Beam electrons were observed at edges of inverted-V structures Beam electrons were observed whether the auroral motion was observed or not Estimations on the origin of beam electrons show it is the topside ionosphere ©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) &lt ∼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 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.

The auroral emission of the first negative system of N-2(+) at 427.8 nm is analyzed using simultaneous measurements from the ground with ALIS (Auroral Large Imaging System) and from space with optical (MAC) and particle (ESA) instruments of the Reimei satellite. The study has two main objectives. The first is validation of the absolute calibration of the ALIS and the Reimei MAC cameras. The other task is to evaluate different cross sections of the electron excitation of N-2(+) that are used for the modeling of the auroral 1N system emissions. The simultaneous measurements of the 427.8 nm emission by ALIS and Reimei imagers show excellent agreement, indicating that the calibration of the two instruments is correct. Comparison of the 427.8 nm emission intensity calculated using the incident electron flux measured by the Reimei particle instruments with intensities measured by the optical imagers show that the best match is reached with the cross section from Shemansky and Liu (2005).

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.

Since the Moon is not shielded by a global magnetic field or by an atmosphere, solar wind plasma impinges onto the lunar surface almost unhindered. Until recently, it was assumed that almost all of the impinging solar wind ions are absorbed by the surface. However, recent Interstellar Boundary Explorer, Chandrayaan-1, and Kaguya observations showed that the interaction process between the solar wind ions and the lunar surface is more complex than previously assumed. In contrast to previous assumptions, a large fraction of the impinging solar wind ions is backscattered as energetic neutral atoms. Using the complete Chandrayaan-1 Energetic Neutral Analyzer data set, we compute a global solar wind reflection ratio of 0.160.05 from the lunar surface. Since these backscattered neutral particles are not affected by any electric or magnetic fields, each particle's point of origin on the lunar surface can be determined in a straight-forward manner allowing us to create energetic neutral atom maps of the lunar surface. The energetic neutral atom measurements recorded by the Chandrayaan-1 Energetic Neutral Analyzer cover approximate to 89% of the lunar surface, whereby the lunar farside is almost completely covered. We analyzed all available energetic neutral atom measurements recorded by the Chandrayaan-1 Energetic Neutral Analyzer to create the first global energetic neutral hydrogen maps of the lunar surface.

In the upcoming JAXA/ERG satellite mission, Wave Particle Interaction Analyzer (WPIA) will be installed as an onboard software function. We study the statistical significance of the WPIA for measurement of the energy transfer process between energetic electrons and whistler-mode chorus emissions in the Earth's inner magnetosphere. The WPIA measures a relative phase angle between the wave vector E and velocity vector v of each electron and computes their inner product W, where W is the time variation of the kinetic energy of energetic electrons interacting with plasma waves. We evaluate the feasibility by applying the WPIA analysis to the simulation results of whistler-mode chorus generation. We compute W using both a wave electric field vector observed at a fixed point in the simulation system and a velocity vector of each energetic electron passing through this point. By summing up W-i of an individual particle i to give W-int, we obtain significant values of W-int as expected from the evolution of chorus emissions in the simulation result. We can discuss the efficiency of the energy exchange through wave-particle interactions by selecting the range of the kinetic energy and pitch angle of the electrons used in the computation of W-int. The statistical significance of the obtained W-int is evaluated by calculating the standard deviation sigma(W) of W-int. In the results of the analysis, positive or negative W-int is obtained at the different regions of velocity phase space, while at the specific regions the obtained W-int values are significantly greater than sigma(W), indicating efficient wave-particle interactions. The present study demonstrates the feasibility of using the WPIA, which will be on board the upcoming ERG satellite, for direct measurement of wave-particle interactions.

The formation of electric potential over lunar magnetized regions is essential for understanding fundamental lunar science, for understanding the lunar environment, and for planning human exploration on the Moon. A large positive electric potential was predicted and detected from single point measurements. Here, we demonstrate a remote imaging technique of electric potential mapping at the lunar surface, making use of a new concept involving hydrogen neutral atoms derived from solar wind. We apply the technique to a lunar magnetized region using an existing dataset of the neutral atom energy spectrometer SARA/CENA on Chandrayaan-1. Electrostatic potential larger than +135 V inside the Gerasimovic anomaly is confirmed. This structure is found spreading all over the magnetized region. The widely spread electric potential can influence the local plasma and dust environment near the magnetic anomaly. Citation: Futaana, Y., S. Barabash, M. Wieser, C. Lue, P. Wurz, A. Vorburger, A. Bhardwaj, and K. Asamura (2013), Remote energetic neutral atom imaging of electric potential over a lunar magnetic anomaly, Geophys. Res. Lett., 40, 262-266, doi:10.1002/grl.50135.

This paper describes a technique for intercalibrating particle and optical measurements from the Reimei microsatellite using an ionospheric model. Reimei has three auroral cameras ("MAC"), together with electron and ion energy spectrum analysers ("ESA/ISA"). The maximum electron energy measured is 12 keV, which means that during high-energy events, the particle data are often missing an important part of the energy flux. Although the total electron energy flux can be estimated from the optical measurements, the MAC data must be accurately calibrated, which is complicated by an unknown and variable background from sources such as the moon and snow reflection. Using unsaturated ESA measurements of the complete electron spectrum as input for an ionospheric model, the coincident camera observations can be calibrated, allowing estimates to be made of the total electron energy flux at other times during the same event, when the maximum energy is well above that measured by ESA. (C) 2012 Elsevier Ltd. All rights reserved.

SARA, the Sub-KeV Atom Analyzer, on board Chandrayaan-1 recorded the first image of a minimagnetosphere above a lunar magnetic anomaly using energetic neutral atoms (ENAs). It was shown that this magnetosphere, which is located near the Gerasimovich crater, is able to reduce the solar wind ion flux impinging onto the lunar surface by more than 50%. Following this first observation, we investigated all magnetic anomalies that are in the SARA data set. We searched for a possible correlation between the solar wind plasma parameters (dynamic pressure, magnetic field), the local magnetic field, and the reduction in the reflected hydrogen ENA flux (henceforth called shielding efficiency). Having analyzed all observations by SARA, we discovered that the Gerasimovich magnetic anomaly is topologically a very simple, large-scale magnetic structure, which is favorable for this kind of investigation. Most other magnetic anomalies on the lunar surface have more small-scale features in their magnetic field structure, which complicates the interpretation of the observed data. We find a clear correlation between the plasma parameters and the shielding efficiency for the Gerasimovich case. For the other observed anomalies only about half of the cases showed such a correlation. We therefore conclude that the solar wind ions-magnetic anomaly interaction is in general more complex than in the Gerasimovich case.

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.

We present an empirical model of the energy spectra for hydrogen energetic neutral atoms (ENA) backscattered from the lunar surface based on Chandrayaan-1 Energetic Neutral Atom (CENA) observations. The observed energy spectra of the backscattered ENAs are well reproduced by Maxwell-Boltzmann distribution functions. The backscatter fraction is constant and independent of any solar wind parameters and the impinging solar wind angle. The calculated backscatter fraction is 0.19, and the 25% and 75% percentiles are 0.16 and 0.21. The empirical parameters of the Maxwell-Boltzman distribution derived from the CENA imager have no correlations with the upstream solar wind parameters, except for a good correlation between the solar wind velocity and the temperature of the backscattered ENAs. These results suggest that the reflected ENAs have experienced several collisions during the interaction with the loose lunar grains, and are then released into space. The mathematical model of the energy spectra of the backscattered ENAs is expressed by a function of the solar wind flux and velocity, which can be used for future investigations of regolith-solar wind interaction.

We are in progress to develop a system for automatic operation of a satellite in order to reduce human load at satellite steady operation phase. The ground station for small satellite REIMEI (INDEX : INnovative-technology Demonstration EXperiment) is used as a test bench for verification of the proposed method. In our new automatic operation system, a scheduler software as a substitutive operator manages all the operations through a unified procedure, including sending command, receiving telemetry, and driving antenna in accordance with an operation time line which is prepared before the operation pass. The scheduler also performs diagnostics of satellite anomaly based upon the received telemetry data and status of the ground station. In case that some anomaly of the satellite is detected, the scheduler initiates an emergency schedule that was prepared depending on the emergency level. The automatic operation system is nearly completed for downlink operations of the data recorder that account for 75% of REIMEI steady operation. This approach is very effective to reduce psychological and physical load of operators.

The results from the Sub-keV Atom Reflecting Analyzer (SARA) experiment onboard Chandrayaan-1 have revealed several hitherto unknown and interesting aspects about the interaction of solar wind with the Moon. The SARA experiment had two sensors - CENA and SWIM. The Chandrayaan-1 energetic neutrals analyzer (CENA), detected energetic neutral atoms (ENAs), and the Solar Wind Monitor (SWIM) measured ions of solar wind origin. In this review, we summarize the observations made by the SARA experiment, which are: (1) substantial (similar to 20%) and sustained backscattering of solar wind protons from lunar surface as energetic neutral hydrogen,(1) (2) mini-magnetosphere around magnetic anomalies on Moon using the backscattered ENAs,(2) (3) reflection of solar wind protons from the Moon surface,(3) (4) huge (similar to 50%) deflection of solar wind protons over strong magnetic anomalies,(4) and (5) presence of protons in the near-lunar plasma wake.(5) These results have implications on the lunar plasma environment, implantation of solar wind hydrogen on lunar surface, and behavior of small scale magnetic anomalies on planetary bodies. The SARA observations suggest that similar processes may happen on other airless bodies covered with regolith in the solar system as well as in extra-solar system. This paper presents a review of the results obtained from the SARA observation.

Black aurora is a small-scale (typically a few to 10 km) black structure seen in diffuse aurora, and its generation process has been studied with immense interest. We report the precise characteristics of black aurora based on simultaneous image and particle measurement data and possible generation process. Thirteen black auroral events are identified from the Reimei satellite data, and the relationship between particle and auroral images around the satellite's magnetic footprints is investigated in detail. We found that a number of small-scale deficiencies were embedded in precipitating electrons from the central plasma sheet with energies greater than 2-7 keV and that each deficiency corresponded exactly to black arcs and black patches at the magnetic footprint. Therefore, black arcs and black patches are not associated with a field-aligned potential (such as a divergent potential structure) but probably originate from the suppression of pitch angle scattering. In the black auroral region, low-energy (2-5 keV) inverted-V-type downward electrons (spanning channels that are several tens of kilometers wide) often appear to overlap with high-energy (several keV) plasma sheet electrons.

We present the results of a statistical study using optical images from ALIS (Auroral Large Imaging System) to investigate the spatial and temporal variations of structures in diffuse aurora. Analysis of conjugate Reimei data shows that such fine structures are a result of modulation of high-energy precipitating electrons. Pitch angle diffusion into the loss cone due to interaction of whistler mode waves with plasma sheet electrons is the most feasible mechanism leading to high-energy electron precipitation. This suggests that the fine structure is an indication of modulations of the efficiency of the wave-particle interaction. The scale sizes and variations of these structures, mapped to the magnetosphere, can give us information about the characteristics of the modulating wave activity. We found the scale size of the auroral stripes and the spacing between them to be on average 1314 km, which corresponds to 3-4 ion gyro radii for protons with an energy of 7 keV. The structures move southward with a speed close to zero in the plasma convection frame.