浅村 和史

The Sub-keV Atom Reflecting Analyzer instrument on board the lunar orbiter Chandrayaan-1 provided a large number of measurements of lunar energetic neutral atoms (ENAs). These ENAs were formerly solar wind ions, which were neutralized and backscattered from the lunar surface. The angles under which the ENAs are scattered strongly depend on the solar wind ions' incidence angle, which corresponds to the solar zenith angle (SZA). Our large dataset provides us with a complete coverage of the SZA and almost complete coverage of the scattering angles. When combining all available measurements, four distinct features are discernible with SZA increase: amplitude decrease, less azimuthal uniformity, bigger ratio of sunward versus anti-sunward flux and shallower scattering. We analyzed more than 290'000 measurements and derived a mathematical description of the features and their dependencies on the SZA. Citation: Schaufelberger, A., P. Wurz, S. Barabash, M. Wieser, Y. Futaana, M. Holmstrom, A. Bhardwaj, M. B. Dhanya, R. Sridharan, and K. Asamura (2011), Scattering function for energetic neutral hydrogen atoms off the lunar surface, Geophys. Res. Lett., 38, L22202, doi:10.1029/2011GL049362.

High resolution imaging within regions of auroral luminosity reveal complex, highly structured dynamic and often vortical forms which evolve on time scales of the order of several seconds and less. These features are inherently multi-scale in nature with different sizes moving and evolving at different rates. Recent analyses have shown how the scale dependency of these motions can provide new insights into the nature of energy transport across scales occurring in current sheets through the auroral acceleration region. However the processes driving this transport and thus facilitating particle acceleration and the formation of bright and dynamic aurora remain unknown. This is a basic issue not only for advancing understanding of auroral arc formation but moreover for understanding dissipation and particle acceleration in current sheets generally. In this Frontier article we show how dedicated space-borne auroral imagery combined with magnetically conjugate field and particle measurements can be used to advance understanding of this universal physical process. By coupling these measurements with numerical simulations we show how flow shear, magnetic reconnection and tearing may launch a cascade toward smaller scales and conspire to form, shape and structure auroral forms. The simulations show that these processes evolve toward a robust scaling of structured magnetic fields (B(x)) with wavenumber (k(y)) perpendicular to the geomagnetic field where B(x)(2)(k(y))/Delta k(y) similar to k(y)(-7/3) as observed. Citation: Chaston, C. C., K. Seki, T. Sakanoi, K. Asamura, M. Hirahara, and C. W. Carlson (2011), Cross-scale coupling in the auroral acceleration region, Geophys. Res. Lett., 38, L20101, doi:10.1029/2011GL049185.

This paper describes the outline and the five years' on-orbit results of the small scientific satellite REIMEI for aurora observations and demonstrations of advanced small satellite technologies. REIMEI is a small satellite with 72 kg mass, and is provided with three-axis attitude control capabilities for aurora observations. REIMEI was launched into a nearly sun synchronous polar orbit on Aug. 23rd, 2005, from Baikonur, Kazakhstan, by Dnepr rocket. REIMEI satellite has been satisfactorily working on the orbit for five years at present as of January, 2011. Three-axis control is achieved with accuracy of 0.1 degrees (3 sigma). Multi-spectrum images of aurora are taken with 8 Hz rate and 2 km spatial resolution to investigate the aurora physics. REIMEI is performing the simultaneous observation of aurora images and particle measurements. REIMEI indicates that even a small satellite launched as a piggy-back can successfully perform unique scientific mission purposes. (C) 2011 Elsevier Ltd. All rights reserved.

To clarify the characteristics and generation process of black auroras, we investigated 13 black auroral events using simultaneous imaging and particle data from the Reimei satellite obtained between November 2005 and October 2006. The formation and motion of black auroras were determined from successive monochromatic auroral images around the satellite's magnetic footprints, while the auroral intensities at the footprints were compared with precipitating electrons. 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 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. Drifting black patches were also observed. We estimated the speed and direction of the drift by minimum mean squared error analysis.

We conducted high-speed imaging observations of flickering aurora at 100 Hz sampling rate using electron multiplying charge-coupled device in Alaska during 2009-2010 winter season. We detected various types of flickering aurora, including drifting and rotating features at a frequency below 15 Hz. We identified, for the first time, flickering stripes and some other unusual flickering events at frequency of higher than 20 Hz on the imaging observations. A dispersion relation derived from a statistical analysis of observed images is compared with the theoretical dispersion curve of O(+) electromagnetic ion cyclotron (EMIC) waves. The frequencies and spatial scales calculated from a coherence/phase analysis based on an interference theory are consistent with the wave dispersion relation derived from the statistical analysis, suggesting that the obtained results are essentially consistent with the scenario that the interference of EMIC waves produces the observed dispersion relation of flickering aurora. Furthermore, flickering frequencies higher than 20 Hz are confirmed from our observations, which are higher than expected frequency of O(+) EMIC waves at altitudes of several thousand kilometers. We therefore suggest that high-frequency waves such as He(+) and H(+) EMIC waves may also contribute to produce a significant fraction of flickering aurora.

Using image and particle data sets obtained from observations by the Reimei satellite, we carried out time-of-flight (TOF) analysis for 29 pulsating aurora events to understand the precise properties of pulsating auroras and the possible generation process. While the sources identified using a standard TOF model were distributed almost continuously from magnetic latitudes 50 degrees to -20 degrees, the sources identified using a different TOF model that takes into account whistler mode wave propagation were confined to the equatorial region up to about 15 degrees. The latter source distribution agrees with previous statistical studies of whistler mode chorus waves. In addition, the cold plasma density of the source region and the wave frequency can be estimated from the latter TOF analysis. The estimated cold plasma densities and wave frequencies normalized by the equatorial cyclotron frequency ranged in 0.20-21.7 cm(-3) and 0.22-0.65, respectively. The estimated wave frequency showed clear dependence on the invariant latitudes of the pulsating aurora source region and increased up to the frequency range of the upper band chorus as the distance from the Earth decreased (up to about 5-6 R-E). These results suggest that both lower and upper band chorus wave contribute to the electron scattering of pulsating auroras, which depends on the radial distance of the source region.

ジオスペースERG探査計画の紹介

We present state-of-the-art multiple instrument observations of an isolated substorm on October 12, 2007. The auroral breakup was observed simultaneously by Reimei, THEMIS ASI, and PFISR. The footprint of Geotail was also near the breakup. These observations allow for detailed study of the breakup location in terms of large-and small-scale auroral morphology, particle precipitation, and ionospheric convection, which has not previously been achieved. It also allows for detailed identification of the sequence leading to the breakup. We report the first spaceborne high spatial and temporal resolution images of part of a breakup arc and a wave-like auroral enhancement captured by Reimei. Observations suggest a sudden plasma sheet thinning initiated similar to 10 min before the onset. Wave-like auroral enhancements were observed twice at the most equatorward arc similar to 3 min and similar to 1 min before the breakup. These enhancements are likely due to some near-Earth instability, such as ballooning instability. Unlike the usual substorm sequence, this most equatorward arc did not develop into the breakup arc but remained almost stable until being engulfed by the auroral equatorward expansion from higher latitude after onset. The wave-like auroral enhancement was associated with three fine inverted V arcs and embedded within energetic ion precipitation. Following this enhancement, an arc, likely a poleward boundary intensification, formed at higher latitude just adjacent to the plasma sheet boundary layer (PSBL). This arc then extended southwestward and led to the breakup arc, which was located poleward of the wavy structures. Assuming longitudinal homogeneity of ion precipitation over 1, this breakup arc was located in a region without ion precipitation just poleward of the energetic ion precipitation. These observations suggest the possible existence of a low-entropy flow channel associated with the arc adjacent to the PSBL, which might be associated with instability in the near-Earth plasma sheet responsible for the auroral breakup.

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 propose a model for the energy dispersion of electron precipitation associated with pulsating auroras, considering the wave-particle interactions with propagating whistler mode waves from the equator. Since the resonant energy depends on the magnetic latitude, the pitch angle scattering of different energy electrons can occur continuously along the field line. Considering the energy-dependent path length and the precipitation start time of the precipitating electrons, the transit time of whistler mode waves, and the frequency drift, we calculated the precipitation of electrons observed at the topside ionosphere. Note that higher energy electrons precipitate into the ionosphere of the opposite hemisphere earlier than lower energy electrons. As a result, an energy dispersion of precipitating electrons is observed at the topside ionosphere, even though the modulation of low energy electrons occurs prior to that of high energy electrons. Using the model, we conducted a time-of-flight (TOF) analysis of precipitating electrons observed by the REIMEI satellite, assuming an interaction with the whistler mode chorus rising tone. Our TOF analysis suggests that the modulation region of the pitch angle scattering is near the magnetic equator, whereas previous models expected that the modulation region is far from the magnetic equator. The estimated parameters, such as wave-frequency and latitudinal distribution of the modulation region, are consistent with previous statistical studies of whistler waves at the magnetosphere.

We present small and meso-scale properties of a substorm onset arc observed simultaneously by the Reimei and THEMIS satellites together with ground-based observations by the THEMIS GBO system. The optical observations revealed the slow equatorward motion of the growth-phase arc and the development of a much brighter onset arc poleward of it. Both arcs showed the typical particle signature of electrostatic acceleration in an inverted-V structure together with a strong Alfven wave acceleration signature at the poleward edge of the onset arc. Two THEMIS spacecraft encountered earthward flow bursts around the times the expanding optical aurora reached their magnetic footprints in the ionosphere. The particle and field measurements allowed for the reconstruction of the field-aligned current system and the determination of plasma properties in the auroral source region. Auroral arc properties were extracted from the optical and particle measurements and were used to compare measured values to theoretical predictions of the electrodynamic model for the generation of auroral arcs. Good agreement could be reached for the meso-scale arc properties. A qualitative analysis of the internal structuring of the bright onset arc suggests the operation of the tearing instability which provides a 'rope-like' appearance due to advection of the current in the sheared flow across the arc. We also note that for the observed parameters ionospheric conductivity gradients due to electron precipitation will be unstable to the feedback instability in the ionospheric Alfven resonator that can drive structuring in luminosity over the range of scales observed.

Significant proton fluxes were detected in the near-wake region of the Moon by an ion mass spectrometer on board Chandrayaan-1. The energy of these nightside protons is slightly higher than the energy of the solar wind protons. The protons are detected close to the lunar equatorial plane at a 140 degrees solar zenith angle, that is, similar to 50 degrees behind the terminator at a height of 100 km. The protons come from just above the local horizon and move along the magnetic field in the solar wind reference frame. We compare the observed proton flux with the predictions from analytical models of an electrostatic plasma expansion into a vacuum. The observed velocity is higher by a factor of 2 to 3 than the velocity predicted by analytical models. The simple analytical models cannot explain the observed ion dynamics along the magnetic field in the vicinity of the Moon.

The Analyzer of Space Plasmas and Energetic Atoms (ASPERA-4) instrument on Venus Express is used to determine bow shock position at Venus using ion data alone, using data recorded during a solar minimum from the Ion Mass Analyzer (IMA) which is part of the ASPERA-4 package. Previous models constructed from solar minimum data using Venus Express, Pioneer Venus Orbiter (PVO) and Venera 9 and 10 are also compared to the current fit. An important feature of this new fit is a statistical accuracy introduced in the form of a probability weighting function for the data points, based on the time spent in particular locations. The bow shock curve is then compared to two-dimensional ion maps. These verify the accuracy of this and previous solar minimum fit curves based on PVO and Venus Express magnetic data. Comparing all bow shock models to the 2D ion maps shows that a combination of models produces the best fit. Since all the fitted curves show differences in position they are investigated relative to the solar conditions pertaining at the times when the individual data sets were measured. The sub solar point and terminator distance were thus found to vary linearly with sunspot number and hence with solar activity. This relationship, which was already known to exist between solar maximum and solar minimum, is now shown to exist between different solar minima and even within the same minimum. This indicates a need for the mechanisms for bow shock maintenance and variance to be more closely modeled.

[1] We present observations of highly structured, thin auroras cased by precipitation of nonaccelerated electrons on the basis of optical and particle measurements performed by the Reimei satellite near the equatorward edge of the main auroral oval. The aurora has the following characteristics: (1) A full width at half maximum (FWHM) value is as low as only similar to 1.8 km from optical measurements, and similar to 0.6 km from particle measurements at the ionospheric altitude, which is much smaller than previously determined. (2) The FWHM value of 0.6 km corresponds to 9 km in the equatorial plane, which is similar to 10 times smaller than the gyroradius of typical protons trapped in the near-Earth plasma sheet. (3) At high energies greater than similar to 1 keV, the velocity distribution function of precipitating electrons is comparable to that of the trapped ones and does not demonstrate any plateau or positive gradient in the distribution. (4) The aurora was observed in geomagnetically quiet condition. (5) A geosynchronous satellite observed a significant increase in the plasma pressure of hot electrons in comparison with that of hot ions. The structuring of the aurora may be attributed to scattering processes of hot electrons. If this were the case, hot electrons would be scattered by whistler mode chorus, or electrostatic electron cyclotron harmonic waves, and the structured aurora would be a visual manifestation of the highly structured, cold plasma that determines the growth of the waves scattering the hot electrons.

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.

Using high resolution auroral imagery and particle measurements from the Reimei spacecraft we distinguish 'Alfvenic' and 'quasi-static' auroral forms and implement a new multi-scale wavelet analysis technique to examine how the vorticity in these forms varies as a function of scale transverse to the geomagnetic field. We find from this analysis that the complex motions of aurorae can be described by power-laws relating spatial scale to optical vorticity. We demonstrate how these relationships naturally arise from the 'conductance' of geomagnetic field-lines and the physics of Alfven waves. Citation: Chaston, C. C., K. Seki, T. Sakanoi, K. Asamura, and M. Hirahara (2010), Motion of aurorae, Geophys. Res. Lett., 37, L08104, doi: 10.1029/2009GL042117.

The Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian Chandrayaan-1 spacecraft has produced for the first time an image of a lunar magnetic anomaly in backscattered hydrogen atoms. The image shows that a partial void of the solar wind, a mini-magnetosphere, is formed above the strong magnetic anomaly near the Crisium antipode. The mini-magnetosphere is 360 km across at the surface and is surrounded by a 300-km-thick region of enhanced plasma flux that results from the solar wind flowing around the mini-magnetosphere. The mini-magnetosphere is visible only in hydrogen atoms with energy exceeding 150 eV. Fluxes with energies below 100 eV do not show corresponding spatial variations. While the high-energy atoms result from the backscattering process, the origin of the low-energy component is puzzling. These observations reveal a new class of objects, mini-magnetospheres, and demonstrate a new observational technique to study airless bodies, imaging in backscattered neutral atoms. Citation: Wieser, M., S. Barabash, Y. Futaana, M. Holmstrom, A. Bhardwaj, R. Sridharan, M. B. Dhanya, A. Schaufelberger, P. Wurz, and K. Asamura (2010), First observation of a mini-magnetosphere above a lunar magnetic anomaly using energetic neutral atoms, Geophys. Res. Lett., 37, L05103, doi:10.1029/2009GL041721.

© 2008 Elsevier Ltd. All rights reserved. ‘Search for Exospheric Refilling and Emitted Natural Abundances’ (SERENA) is an instrument package that will fly on board the BepiColombo/Mercury Planetary Orbiter (MPO). It will investigate Mercury’s complex particle environment that is composed of thermal and directional neutral atoms (exosphere) caused by surface release and charge-exchange processes, and of ionized particles caused by photo-ionization of neutrals as well by charge exchange and surface release processes. In order to investigate the structure and dynamics of the environment, an in-situ analysis of the key neutral and charged components is necessary, and for this purpose the SERENA instrument shall include four units: two neutral particle analyzers (Emitted Low Energy Neutral Atoms (ELENA) sensor and Start from a Rotating FIeld mass spectrometer (STROFIO)) and two ion spectrometers (Miniature Ion Precipitation Analyzer (MIPA) and Planetary Ion Camera (PICAM)). The scientific merits of SERENA are presented, and the basic characteristics of the four units are described, with a focus on novel technological aspects.

SARA experiment aboard the first Indian lunar mission Chandrayaan-1 had the objective to explore the solar wind-lunar interaction using energetic neutral atoms (ENA) from the lunar surface as diagnostic tool. SARA consisted of an ENA imaging mass analyzer CENA (Chandrayaan-1 Energetic Neutral Analyzer) and an ion mass analyser SWIM (Solar Wind Monitor), along with a digital processing unit (DPU) which commands and controls the sensors and provides the interface to the spacecraft. Both sensors have provided excellent observational data. CENA has observed ENAs from the lunar surface and found that ~20% of the incident solar wind ions get backscattered as ENAs from the lunar surface. This is contrary to the previous assumptions of almost complete absorption of solar wind by the lunar surface. The observation is relevant for other airless bodies in the solar system.

We present state-of-the-art multiple instrument observations of an isolated substorm on October 12, 2007. The auroral breakup was observed simultaneously by Reimei, THEMIS ASI, and PFISR. The footprint of Geotail was also near the breakup. These observations allow for detailed study of the breakup location in terms of large- and small-scale auroral morphology, particle precipitation, and ionospheric convection, which has not previously been achieved. It also allows for detailed identification of the sequence leading to the breakup. We report the first spaceborne high spatial and temporal resolution images of part of a breakup arc and a wave-like auroral enhancement captured by Reimei. Observations suggest a sudden plasma sheet thinning initiated ∼10 min before the onset. Wave-like auroral enhancements were observed twice at the most equatorward arc ∼3 min and ∼1 min before the breakup. These enhancements are likely due to some near-Earth instability, such as ballooning instability. Unlike the usual substorm sequence, this most equatorward arc did not develop into the breakup arc but remained almost stable until being engulfed by the auroral equatorward expansion from higher latitude after onset. The wave-like auroral enhancement was associated with three fine inverted V arcs and embedded within energetic ion precipitation. Following this enhancement, an arc, likely a poleward boundary intensification, formed at higher latitude just adjacent to the plasma sheet boundary layer (PSBL). This arc then extended southwestward and led to the breakup arc, which was located poleward of the wavy structures. Assuming longitudinal homogeneity of ion precipitation over 1, this breakup arc was located in a region without ion precipitation just poleward of the energetic ion precipitation. These observations suggest the possible existence of a low-entropy flow channel associated with the arc adjacent to the PSBL, which might be associated with instability in the near-Earth plasma sheet responsible for the auroral breakup. Copyright 2010 by the American Geophysical Union.

Significant proton fluxes were detected in the near-wake region of the Moon by an ion mass spectrometer on board Chandrayaan-1. The energy of these nightside protons is slightly higher than the energy of the solar wind protons. The protons are detected close to the lunar equatorial plane at a 140° solar zenith angle, that is, ∼50° behind the terminator at a height of 100 km. The protons come from just above the local horizon and move along the magnetic field in the solar wind reference frame. We compare the observed proton flux with the predictions from analytical models of an electrostatic plasma expansion into a vacuum. The observed velocity is higher by a factor of 2 to 3 than the velocity predicted by analytical models. The simple analytical models cannot explain the observed ion dynamics along the magnetic field in the vicinity of the Moon. Copyright 2010 by the American Geophysical Union.

We propose a model for the energy dispersion of electron precipitation associated with pulsating auroras, considering the wave-particle interactions with propagating whistler mode waves from the equator. Since the resonant energy depends on the magnetic latitude, the pitch angle scattering of different energy electrons can occur continuously along the field line. Considering the energy-dependent path length and the precipitation start time of the precipitating electrons, the transit time of whistler mode waves, and the frequency drift, we calculated the precipitation of electrons observed at the topside ionosphere. Note that higher energy electrons precipitate into the ionosphere of the opposite hemisphere earlier than lower energy electrons. As a result, an energy dispersion of precipitating electrons is observed at the topside ionosphere, even though the modulation of low energy electrons occurs prior to that of high energy electrons. Using the model, we conducted a time-of-flight (TOF) analysis of precipitating electrons observed by the REIMEI satellite, assuming an interaction with the whistler mode chorus rising tone. Our TOF analysis suggests that the modulation region of the pitch angle scattering is near the magnetic equator, whereas previous models expected that the modulation region is far from the magnetic equator. The estimated parameters, such as wave-frequency and latitudinal distribution of the modulation region, are consistent with previous statistical studies of whistler waves at the magnetosphere. © 2010 by the American Geophysical Union.

We present small and meso-scale properties of a substorm onset arc observed simultaneously by the Reimei and THEMIS satellites together with ground-based observations by the THEMIS GBO system. The optical observations revealed the slow equatorward motion of the growth-phase arc and the development of a much brighter onset arc poleward of it. Both arcs showed the typical particle signature of electrostatic acceleration in an inverted-V structure together with a strong Alfvn wave acceleration signature at the poleward edge of the onset arc. Two THEMIS spacecraft encountered earthward flow bursts around the times the expanding optical aurora reached their magnetic footprints in the ionosphere. The particle and field measurements allowed for the reconstruction of the field-aligned current system and the determination of plasma properties in the auroral source region. Auroral arc properties were extracted from the optical and particle measurements and were used to compare measured values to theoretical predictions of the electrodynamic model for the generation of auroral arcs. Good agreement could be reached for the meso-scale arc properties. A qualitative analysis of the internal structuring of the bright onset arc suggests the operation of the tearing instability which provides a 'rope-like' appearance due to advection of the current in the sheared flow across the arc. We also note that for the observed parameters ionospheric conductivity gradients due to electron precipitation will be unstable to the feedback instability in the ionospheric Alfvn resonator that can drive structuring in luminosity over the range of scales observed. Copyright 2010 by the American Geophysical Union.

The Analyzer of Space Plasmas and Energetic Atoms (ASPERA-4) instrument on Venus Express is used to determine bow shock position at Venus using ion data alone, using data recorded during a solar minimum from the Ion Mass Analyzer (IMA) which is part of the ASPERA-4 package. Previous models constructed from solar minimum data using Venus Express, Pioneer Venus Orbiter (PVO) and Venera 9 and 10 are also compared to the current fit. An important feature of this new fit is a statistical accuracy introduced in the form of a probability weighting function for the data points, based on the time spent in particular locations. The bow shock curve is then compared to two-dimensional ion maps. These verify the accuracy of this and previous solar minimum fit curves based on PVO and Venus Express magnetic data. Comparing all bow shock models to the 2D ion maps shows that a combination of models produces the best fit. Since all the fitted curves show differences in position they are investigated relative to the solar conditions pertaining at the times when the individual data sets were measured. The sub solar point and terminator distance were thus found to vary linearly with sunspot number and hence with solar activity. This relationship, which was already known to exist between solar maximum and solar minimum, is now shown to exist between different solar minima and even within the same minimum. This indicates a need for the mechanisms for bow shock maintenance and variance to be more closely modeled. Copyright 2010 by the American Geophysical Union.

We present observations of highly structured, thin auroras cased by precipitation of nonaccelerated electrons on the basis of optical and particle measurements performed by the Reimei satellite near the equatorward edge of the main auroral oval. The aurora has the following characteristics: (1) A full width at half maximum (FWHM) value is as low as only ∼1.8 km from optical measurements, and ∼0.6 km from particle measurements at the ionospheric altitude, which is much smaller than previously determined. (2) The FWHM value of 0.6 km corresponds to 9 km in the equatorial plane, which is ∼10 times smaller than the gyroradius of typical protons trapped in the near-Earth plasma sheet. (3) At high energies greater than ∼1 keV, the velocity distribution function of precipitating electrons is comparable to that of the trapped ones and does not demonstrate any plateau or positive gradient in the distribution. (4) The aurora was observed in geomagnetically quiet condition. (5) A geosynchronous satellite observed a significant increase in the plasma pressure of hot electrons in comparison with that of hot ions. The structuring of the aurora may be attributed to scattering processes of hot electrons. If this were the case, hot electrons would be scattered by whistler mode chorus, or electrostatic electron cyclotron harmonic waves, and the structured aurora would be a visual manifestation of the highly structured, cold plasma that determines the growth of the waves scattering the hot electrons. Copyright 2010 by the American Geophysical Union.

The first Indian lunar mission Chandrayaan-1 was launched on 22 October 2008. The Sub-keV Atom Reflecting Analyzer (SARA) instrument onboard Chandrayaan-1 consists of an energetic neutral atom (ENA) imaging mass analyzer called CENA (Chandrayaan-1 Energetic Neutrals Analyzer), and an ion-mass analyzer called SWIM (Solar wind Monitor). CENA performed the first ever experiment to study the solar wind-planetary surface interaction via detection of sputtered neutral atoms and neutralized backscattered solar wind protons in the energy range similar to 0.01-3.0 keV. SWIM measures solar wind ions, magnetosheath and magnetotail ions, as well as ions scattered from lunar surface in the similar to 0.01-15 keV energy range. The neutral atom sensor uses conversion of the incoming neutrals to positive ions, which are then analyzed via surface interaction technique. The ion mass analyzer is based on similar principle. This paper presents the SARA instrument and the first results obtained by the SWIM and CENA sensors. SARA observations suggest that about 20% of the incident solar wind protons are backscattered as neutral hydrogen and similar to 1% as protons from the lunar surface. These findings have important implications for other airless bodies in the solar system.

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.

We present results from ground-based auroral observations coordinated with the Japanese satellite, Reimei, that took place during the winters of 2006, 2007 and 2008 at Poker Flat, Alaska. Comparable temporal and spatial resolution for the optical and in situ particle data, allowed for investigation of small scale and/or rapidly time-varying auroral structures. Four satellite passes through diffuse auroral structures were identified. The structures within the aurora, whether stationary or time-varying (pulsating aurora), were most closely correlated with the highest energy precipitating electrons measured by these detectors (8 to 12 keV). This relation is found to be consistent across all four examples, revealing that the electron precipitation responsible for these diffuse auroral structures is primarily that of the >= 8 keV electrons.

We report on measurements of extremely high reflection rates of solar wind particles from regolith-covered lunar surfaces. Measurements by the Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian Chandrayaan-1 spacecraft in orbit around the Moon show that up to 20% of the impinging solar wind protons are reflected from the lunar surface back to space as neutral hydrogen atoms. This finding, generally applicable to regolith-covered atmosphereless bodies, invalidates the widely accepted assumption that regolith almost completely absorbs the impinging solar wind. (C) 2009 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.

Aurora occasionally exhibits short-lived appearance of a bright red border at the bottom of an auroral arc, band, or curtain. This is called a type B red aurora. Based on simultaneous measurements of auroral emissions from N(2) 1PG ("red" aurora) and O ((1)S) ("green" aurora) as well as incident electrons by the Reimei satellite, we show the following two individual observations. Energy flux and average energy of incident electrons (1) were not always higher in the red-dominated aurora than in the green-dominated aurora, and (2) were not correlated with the intensity of "green" auroras, but with the intensity of "red" auroras. These observational facts suggest that for the reddening of auroras, intense electron precipitation is unnecessary. A rapid movement, or appearance of electron precipitation is sufficient for the reddening because of the difference in lifetimes of N(2) 1PG and O ((1)S). Citation: Ebihara, Y., T. Sakanoi, K. Asamura, M. Hirahara, and A. Ieda (2009), Optical and particle observations of type B red aurora, Geophys. Res. Lett., 36, L20105, doi: 10.1029/2009GL041037.

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

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 similar to 100 km altitude in the direction perpendicular to the magnetic field, as they gain kinetic energy in one hemisphere while lose in the other hemisphere. Particle trajectory calculations and theoretical treatment demonstrate that proton Larmor motions and inward electric field around the wake boundary result in energy gain and loss of the SW protons. Our result shows emergence of proton particle dynamics around the near-Moon space, and suggests that the SW protons may relatively easily access the low-latitude and low-altitude region on the lunar night side. Citation: Nishino, M. N., et al. (2009), Pairwise energy gain-loss feature of solar wind protons in the near-Moon wake, Geophys. Res. Lett., 36, L12108, doi: 10.1029/2009GL039049.

Previous observations have shown that pulsating aurora sometimes occurs with patches of a vertical extent that is thinner than would be expected for aurora caused by collisional processes, implying that local ionospheric processes are important in causing the narrow luminosity enhancement. However, Poker Flat Incoherent Scatter Radar (PFISR) data from four pulsating aurora events, during the Rocket Observations of Pulsating Aurora (ROPA) mission in January and February 2007, show that the electron density profile associated with the pulsating patches had a thickness of similar to 15-25 km in all four cases and that, therefore, these are not examples of such thin enhancements. A numerical model of the associated volume emission rates for the night of the ROPA launch supports this conclusion. In the process of modeling the volume emission rates, the PFISR data are inverted to calculate the associated electron energy distribution for comparison with in situ electron measurements from ROPA and the REIMEI satellite. The modeled distribution shows a diffuse plasma sheet population which gradually decreases in energy over the course of the event, resulting in similar to 6-8 keV precipitation by the end of the PFISR data interval, in agreement with the ROPA/REIMEI measurements. (C) 2008 Elsevier Ltd. All rights reserved.

The combination of an electrostatic analyzer and a silicon strip solid detector is proposed for measuring the energy and charge state of medium energy (10-200 keV/q) ions in space. Based on laboratory experiments, it is shown that a single-sided silicon strip detector (SSSD) has low-noise levels that provide sufficient energy resolution for charge state measurements of medium energy ions. It is also demonstrated that energy loss at a dead-layer is a critical factor for the energy resolution and measurement energy threshold; the dead-layer is measured to be similar to 370 mm. A single-sided silicon strip detector with a thin dead-layer is important for the development of medium energy ion measurements into next generation satellite-borne missions. (C) 2009 Elsevier B.V. All rights reserved.

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. (C) 2008 COSPAR. Published by Elsevier Ltd. All rights reserved.

Coincident particle and optical measurements from the Reimei spacecraft suggest that sheared flows through an inverted-V arc may be unstable to the emission of Alfven waves. The particle measurements reveal time dispersed field-aligned electron bursts typical of low energy electrons accelerated by inertial Alfven waves (IAWs). These Alfven wave accelerated electrons are embedded within multi-keV inverted-V electron structures. The optical measurements at the footprint of the inverted-V structures reveal counter propagating and folded/vortical discrete auroral forms moving with speeds of similar to 14-18 km/s. We show that the flow shear inferred from this motion exceeds that required for instability to the emission of Alfven waves on scales of the order of an electron inertial length. The emission of these waves provides a likely means for driving the low energy dispersed electron bursts we observe. Citation: Asamura, K., et al. (2009), Sheared flows and small-scale Alfven wave generation in the auroral acceleration region, Geophys. Res. Lett., 36, L05105, doi:10.1029/2008GL036803.

The SARA instrument (Sub-keV Atom Reflecting Analyser) comprises a low energy neutral atom (LENA) sensor for the energy range 10 eV-3.3 keV and an ion mass spectrometer (10 eV-15 keV). It is the first ever experiment to study the solar wind-planetary surface interaction via measurements of the sputtered atoms and neutralized back-scattered solar wind hydrogen. The neutral atom sensor uses conversion of the incoming neutrals to positive ions, which are then analysed via surface interaction technique. The ion mass spectrometer is based on the same principle. SARA performs LENA imaging of the Moon's elemental surface composition including that of permanently shadowed areas, and imaging of the lunar surface magnetic anomalies. It will also investigate processes of space weathering and sputtered sources of the exospheric gases.

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.

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.

Low-energy neutral atom (LENA) observations bring us important information on particle environments around planetary objects such as Mercury and the Moon. In this paper, we report on a new development of a LENA instrument of light weight (similar to 2 kg) for planetary explorations. The instrument is capable of energy and mass discrimination with a large sensitivity by utilizing surface ionization followed by an electrostatic analyzer and a time-of-flight velocity spectrometer. The performance of the instrument is investigated by numerical simulations. This enables us to obtain detailed performance characterization of LENA measurements by the instrument. We also made trajectory tracing of photons entering the instrument to examine photon rejection capability. The simulations show that the energy range is from similar to 10 eV to >3.3 keV and the angular resolutions are 10 deg x 25 deg for 25-eV LENAs, which are sufficient for planetary LENA observations. Laboratory tests of a prototype model of the instruments developed with this study are now ongoing. According to the initial tests, the measurement principle of the instrument has been verified. This LENA instrument has been selected for both the Indian Moon exploration mission Chandrayaan-1 and the European-Japanese Mercury exploration mission BepiColombo.

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.

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.

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

Simultaneous observation with the Sondrestrom incoherent-scatter (IS) radar and the Reimei satellite was conducted on 3 October 2007. The objective was to measure horizontal patterns of the ionospheric structure in the vicinity of an auroral arc. The IS radar was scanned azimuthally with a fixed elevation angle, and the satellite narrow-view camera (557.7 nm) was directed downward taking pictures every 0.12 s. A stationary auroral arc was captured at 05:17 UT (02:54 MLT) with both instruments simultaneously. Gross ionospheric features measured with the IS radar around the arc were in good agreement with the expected magnetosphere-ionosphere current system. Of particular interest was the horizontal pattern in the ion speed and temperature in the F region. The ion speed within the arc was close to zero; by contrast the larger ion speed (350-400 m s-1) on the poleward side was parallel to the arc and almost no horizontal shear within about 20 km width perpendicular to the arc. This area was separated into two parts by the ion temperature: one was associated with clear enhancements in excess of 1200 K, and another was with more moderate enhancements (less than ∼1000 K). The widths of the two areas were approximately 10 km each. The horizontal shear seen in the ion temperature suggested the presence of a narrow thermospheric wind shear of about 10 km width. This paper suggests that the possible cause for the thermospheric wind shear was ion drag associated with localized soft particle precipitation or F-region ionization. Copyright 2009 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.