浅村 和史

This paper describes the on-orbit results and lessons learned 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. 23^<rd>, 2005 (UT) from Baikonur, Kazakhstan by Dnepr rocket. REIMEI satellite functions satisfactorily on the orbit. Three axis control is achieved with accuracy of 0.05 deg. Multi-spectrum images of aurora are taken with 8Hz rate and 2 km spatial resolution to investigate the aurora physics. REIMEI is a small scientific satellite for aurora observation and advanced satellite technologies, and was launched into a nearly sun synchronous polar orbit on Aug. 23^<rd>, 2005 (UTC) from Baikonur, Kazakhstan by Dnepr rocket. REIMEI satellite functions satisfactorily on the orbit. The three-axis attitude control is achieved with accuracy of 0.05deg. REIMEI is performing the simultaneous observation of aurora images as well as particle measurements. REIMEI indicates that even a small satellite launched as a piggy-back can successfully perform the unique scientific mission purposes.

Aurora is caused by the precipitation of energetic particles into a planetary atmosphere, the light intensity being roughly proportional to the precipitating particle energy flux. From auroral research in the terrestrial magnetosphere it is known that bright auroral displays, discrete aurora, result from an enhanced energy deposition caused by downward accelerated electrons. The process is commonly referred to as the auroral acceleration process. Discrete aurora is the visual manifestation of the structuring inherent in a highly magnetized plasma. A strong magnetic field limits the transverse (to the magnetic field) mobility of charged particles, effectively guiding the particle energy flux along magnetic field lines. The typical, slanted arc structure of the Earth's discrete aurora not only visualizes the inclination of the Earth's magnetic field, but also illustrates the confinement of the auroral acceleration process. The terrestrial magnetic field guides and confines the acceleration processes such that the preferred acceleration of particles is frequently along the magnetic field lines. Field-aligned plasma acceleration is therefore also the signature of strongly magnetized plasma. This paper discusses plasma acceleration characteristics in the night-side cavity of Mars. The acceleration is typical for strongly magnetized plasmas - field-aligned acceleration of ions and electrons. The observations map to regions at Mars of what appears to be sufficient magnetization to support magnetic field-aligned plasma acceleration - the localized crustal magnetizations at Mars (Acuna et al., 1999). Our findings are based on data from the ASPERA-3 experiment on ESA's Mars Express, covering 57 orbits traversing the night-side/eclipse of Mars. There are indeed strong similarities between Mars and the Earth regarding the accelerated electron and ion distributions. Specifically acceleration above Mars near local midnight and acceleration above discrete aurora at the Earth - characterized by nearly monoenergetic downgoing electrons in conjunction with nearly monoenergetic upgoing ions. We describe a number of characteristic features in the accelerated plasma: The "inverted V" energy-time distribution, beam vs temperature distribution, altitude distribution, local time distribution and connection with magnetic anomalies. We also compute the electron energy flux and find that the energy flux is sufficient to cause weak to medium strong (up to several tens of kR 557.7 nm emissions) aurora at Mars. Monoenergetic counterstreaming accelerated ions and electrons is the signature of field-aligned electric currents and electric field acceleration. The topic is reasonably well understood in terrestrial magnetospheric physics, although some controversy still remains on details and the cause-effect relationships. We present a potential cause-effect relationship leading to auroral plasma acceleration in the nightside cavity of Mars - the downward acceleration of electrons supposedly manifesting itself as discrete aurora above Mars.

Although the Mars Express (MEX) does not carry a magnetometer, it is in principle possible to derive the interplanetary magnetic field (IMF) orientation from the three dimensional velocity distribution of pick-up ions measured by the Ion Mass Analyser (IMA) on board MEX because pick-up ions' orbits, in velocity phase space, are expected to gyrate around the IMF when the IMF is relatively uniform on a scale larger than the proton gyroradius. During bow shock outbound crossings, MEX often observed cycloid distributions (two dimensional partial ring distributions in velocity phase space) of protons in a narrow channel of the IMA detector (only one azimuth for many polar angles). We show two such examples. Three different methods are used to derive the IMF orientation from the observed cycloid distributions. One method is intuitive (intuitive method), while the others derive the minimum variance direction of the velocity vectors for the observed ring ions. These velocity vectors are selected either manually (manual method) or automatically using simple filters (automatic method). While the intuitive method and the manual method provide similar IMF orientations by which the observed cycloid distribution is well arranged into a partial circle (representing gyration) and constant parallel velocity, the automatic method failed to arrange the data to the degree of the manual method, yielding about a 30 degrees offset in the estimated IMF direction. The uncertainty of the derived IMF orientation is strongly affected by the instrument resolution. The source population for these ring distributions is most likely newly ionized hydrogen atoms, which are picked up by the solar wind.

The general scientific objective of the ASPERA-3 experiment is to study the solar wind atmosphere interaction and to characterize the plasma and neutral gas environment within the space near Mars through the use of energetic neutral atom (ENA) imaging and measuring local ion and electron plasma. The ASPERA-3 instrument comprises four sensors: two ENA sensors, one electron spectrometer, and one ion spectrometer. The Neutral Particle Imager (NPI) provides measurements of the integral ENA flux (0.1 - 60 keV) with no mass and energy resolution, but high angular resolution. The measurement principle is based on registering products ( secondary ions, sputtered neutrals, reflected neutrals) of the ENA interaction with a graphite-coated surface. The Neutral Particle Detector (NPD) provides measurements of the ENA flux, resolving velocity ( the hydrogen energy range is 0.1 10 keV) and mass ( H and O) with a coarse angular resolution. The measurement principle is based on the surface reflection technique. The Electron Spectrometer (ELS) is a standard top-hat electrostatic analyzer in a very compact design which covers the energy range 0.01 - 20 keV. These three sensors are located on a scanning platform which provides scanning through 180 degrees of rotation. The instrument also contains an ion mass analyzer (IMA). Mechanically IMA is a separate unit connected by a cable to the ASPERA-3 main unit. IMA provides ion measurements in the energy range 0.01 - 36 keV/charge for the main ion components H+, He++, He+, O+, and the group of molecular ions 20 - 80 amu/q. ASPERA-3 also was its own DC/DC converters and digital processing unit (DPU).

小型科学衛星INDEX(れいめい)の打上げと初期成果

The ASPERA-3 (Analyser of Space Plasma and Energetic Atoms) instrument of Mars Express is designed to study the solar wind-Mars atmosphere interaction and to characterise the plasma and neutral gas environment in near-Mars space through energetic neutral atom (ENA) imaging and local charged-particle measurements. The studies address the fundamental question: how strongly do the interplanetary plasma and electromagnetic fields affect the martian atmosphere? This question is directly related to the problem of martian dehydration. The instrument comprises four sensors two ENA sensors, and electron and ion spectrometers. The Neutral Particle Imager (NPI) measures the integral ENA flux (0.1-60 keV) with no mass and energy resolution but with high angular resolution. The Neutral Particle Detector (NPD) measures the ENA flux, resolving energy (0.1-10 keV) and mass (H and O) with a coarse angular resolution. The electron spectrometer (ELS) is a standard top-hat electrostatic analyser of a very compact design. These three sensors are mounted on a scanning platform providing 4π coverage. The instrument includes an ion mass composition sensor, IMA (Ion Mass Analyser). Mechanically, IMA is a separate unit connected by a cable to the ASPERA-3 main unit. IMA provides ion measurements in the energy range 0.01-40 keV/q for the main ion components H+, He 2+, He+O+ with 20-80 amu/g.

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. (C) 2000 American Institute of Physics. [S0034- 6748(00)02008-6].

Energetic neutral atoms (ENAs) with energies of 4 to 35 keV were measured at altitudes of 170 to 570 km by a new ENA instrument on board a sounding rocket. The instrument measured particles precipitating into the ionosphere from the equatorial region of the magnetosphere at a magnetic local time of similar to 1830. The geomagnetic activity was quiet for a prolonged period before the launch. The measured ENA flux was similar to 10(2) (cm(2) s str keV)(-1) at energies of similar to 10 keV. The energy spectrum is in a good agreement with an expected spectrum of hydrogen atoms originating from the ring current region as reported by Milillo et al. [1996]. The altitude profile is also discussed in terms of collisional interaction of ENAs with upper-atmospheric constituents.

The ionospheric neutral wind is induced by the ion drag forcing under the sufficiently continuous southward IMF conditions in the polar region. If the IMF turns northward sharply after prolonged southward interval, the neutral wind gives its own momentum to the charged particles and makes the ionospheric currents. This phenomenon is one of the forms of ''flywheel'' effect. To ascertain the existence of the flywheel effect and to obtain its global pattern in the polar region we analyzed the ionospheric equivalent current system derived from ground-based geomagnetic observation by superposed epoch method. The results show (1) appearance of dawnward and antisunward currents after IMF northward turning, (2) its attenuation with a time constant of several hours, (3) seasonal dependence in the attenuation time constant, (4) small day-night difference in the current intensity after the turning. The antisunward current is stronger for the case with prolonged southward IMF interval before the northward turning than that for short and weak southward IMF case. These results are consistent with the theoretical expectations of the flywheel effect, though there are some difference with the prediction by computer simulations in the global current pattern such as the dawnward rotation of the current vector.