浅村 和史

This paper describes the initial on-orbit results of the small scientific satellite "INDEX" (REIMEI) for aurora observation and demonstration of advanced satellite technologies. REIMEI is a small satellite with 72kg mass, and is provided with three-axis attitude controlled capabilities for aurora observation. REIMEI was launched into a nearly sun synchronous polar orbit on Aug. 23rd, 2005 (UT) from Baikonur, Kazakhstan by Dnepr rocket. After REIMEI was separated from the Dnepr rocket, REIMEI successfully performed a sun-pointing acquisition with spinning motion. A week later REIMEI switched into three-axis attitude control mode. REIMEI satellite functions works satisfactorily in the orbit. The first imaging observations of aurora were successfully performed above the southern polar region in Sep. 16th, 2005. Multi-spectrum images of aurora were taken with 8Hz rate and 2 km spatial resolution.

We have developed an ion energy mass spectrometer for use aboard three-axis stabilized spacecraft. This spectrometer measures the three-dimensional distribution function of mass-discriminated ions with a high sampling rate using electrostatic energy analysis and time-of-flight mass analysis. Three-axis stabilized spacecraft make it difficult to obtain complete coverage of all possible plasma arrival directions. We have added angular scanning deflectors to a cylindrically symmetric analyzer to provide a hemispherical (2pi str) field of view. Ion analyzers need suitable sensitivity with respect to surrounding space plasma fluxes, whose intensities vary greatly depending especially on plasma regions such as the solar wind and the planetary magnetospheres. To obtain a wide range of sensitivity, we equipped the analyzer with sensitivity control electrodes. Ions originating from planetary atmosphere and surface include various ion species such as Na+, Mg+, Al+, and Fe+. The time-of-flight device of our spectrometer applies a peculiar electric field, called a linear electric field, which increases linearly with the penetration length of incident ions to enable mass resolution higher than that of conventional time-of-flight techniques. In this electric field, ions bounce in simple harmonic motion, where the energy and flight path no longer affect the flight time and thus the mass resolution. We have designed and fabricated the ion energy mass spectrometer, and have evaluated its performance through laboratory experiments. (C) 2005 American Institute of Physics.

The Analyzer of Space Plasma and Energetic Atoms (ASPERA) on board the Mars Express spacecraft found that solar wind plasma and accelerated ionospheric ions may be observed all the way down to the Mars Express pericenter of 270 kilometers above the dayside planetary surface. This is very deep in the ionosphere, implying direct exposure of the martian topside atmosphere to solar wind plasma forcing. The low-altitude penetration of solar wind plasma and the energization of ionospheric plasma may be due to solar wind irregularities or perturbations, to magnetic anomalies at Mars, or both.

SS-520-2 sounding rocket flew through the dayside high-latitude auroral region in the vicinity of the cusp. One of the science instruments LEP-ESA/ISA simultaneously observed electron acceleration and ion deceleration when inverted-V type electron precipitations were observed. The correspondence between the electron acceleration energy and the ion deceleration energy clearly indicated the existence of the quasi-static field-aligned potential drop above the sounding rocket. Moreover, ion deceleration started 9.5 seconds later than electron acceleration. By considering a simple temporal model that field-aligned potential drop gradually grew in 10 seconds at 4200 km altitude region, we have succeeded in explaining the observed characteristics of the electron acceleration and ion deceleration.

The INDEX satellite is a microsatellite which will be inserted into a low-altitude (680 km) polar orbit. A low-energy plasma particle instrument, which consists of two sensor heads (electron/ions sensors; ESA/ISA) and a multi-spectral auroral camera (MAC) will be installed in the INDEX - satellite in order to investigate formation mechanisms of fine-scale structures of optical auroral arc emissions. One of advantages of the INDEX mission is that the satellite is capable to observe fine-scale auroral arc emissions and corresponding auroral particles simultaneously by controlling an attitude of the satellite. Since the satellite velocity is relatively fast, a high time-resolution is necessary for the plasma measurement. The time resolution of the plasma instruments onboard the INDEX satellite is 20 ms, which corresponds to a spatial scale of similar to150m. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

To investigate the fine-scale auroral structures, high time and spatial resolution imaging observations of optical auroras will be made by a multi-spectral auroral camera (MAC) onboard the INDEX satellite which will be launched by an H2A rocket as a piggyback satellite into a polar orbit at an altitude of similar to700 km. Monochromatic auroral image data at emissions of N-2(+) first negative band (427.8 nm), OI (557.7 nm), and N-2 first positive band (670 nm) are obtained by MAC with the field-of-view (FOV) of 7.6degrees using three independent CCD cameras in combination with interference filters. MAC will operate in the nightside auroral region by two operation modes in the following. (1) Simultaneous measurement with particle sensors (ESA/ISA). In this mode, MAC observes an imaging area of similar to80x80 kin (at a 100 km altitude) around a magnetic footprint with spatial and time resolutions of similar to1.2 km and 120 msec, respectively. (2) Auroral height distribution measurement. The attitude of INDEX satellite is changed to direct the FOV of MAC on the limb of the Earth. In this mode, MAC observes an imaging area of similar to270x270 km (at a 2000 km distance from the satellite) with spatial and time resolutions of similar to4 km and 1 sec, respectively. In this paper, the science mission, the instrumentation, and observation modes concerning on MAC will be presented. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

Altitude profiles of energetic neutral atom (ENA) fluxes precipitating into the upper atmosphere are simulated numerically by using a Monte Carlo method with source magnetospheric protons distributed between L shells 4 and 7, taking account of collisional interactions with atmospheric particles. Numerical results are in reasonable agreement with the profile of similar to 10 keV neutral atom fluxes recently measured under a geomagnetically quiet condition. Through collisional interactions, ENAs can be converted to ions and then the ionized ENAs can be reneutralized and repeat this cycle. Therefore the precipitating ENAs lose their original directionality because of gyromotions of the ionized ENAs. However, the source information is still seen in ENA data measured at low altitudes where collisions are significant, because their pitch angles are almost conserved with the forward scattering approximation for each collision in the case of parallel geomagnetic fields at low altitudes. On the other hand, a part of ENAs precipitating into the atmosphere returns to the magnetosphere. These reflected ENAs act as a secondary source that cannot be neglected in ENA data measured in the magnetosphere.

A new-type analyzer for measurement of energetic neutral atoms (ENAs) in an energy range of 4-40 keV is described. Incoming ENAs are ionized by electron stripping at passage of an ultrathin carbon foil. After post-acceleration (by 3 kV), the particles are guided to a time-of-flight (TOF) section over a wide energy-per-charge bandwidth by means of electrostatic deflection without any potential sweeping for electrodes. Then, their velocity is measured by the TOF technique, with which species can also be identified, because the particle energies are limited to a certain range by the electrostatic deflector and acceleration upon entering the TOF section. A unique feature in the present analyzer is in the rejection method of extreme ultraviolet (EUV) contamination. In contrast to conventional usage of serrated electrodes for EUV attenuation, one of the electrostatic deflection plates is machined to be so flat that EUV photons are guided to a photon trap regardless of wavelength. The TOF device can also be used in a coincidence mode for noise suppression. The present instrument was flown on a sounding rocket, and has successfully measured ENAs precipitating into the low-latitude upper atmosphere from the magnetosphere. © 2000 American Institute of Physics.