[P-CG38_1PM1] 惑星大気圏・電磁圏

[PCG38-16] 太陽風侵入イベント時のマグネトシース-火星電離圏間の境界層の特徴の研究

*松永 和成¹、関 華奈子¹、原 拓也¹、Brain David A.²、Lundin Rickard³、二穴 喜文⁴、Barabash Stas⁴ (1.名古屋大学　太陽地球環境研究所、2.Laboratory for Atmospheric and Space Physics (LASP), University of Colorado at Boulder、3.Space Physics Swedish Institute of Space Physics (IRF), Umea, Sweden、4.Space Physics Swedish Institute of Space Physics (IRF), Kiruna, Sweden)

キーワード：火星, 電離圏, 誘導磁気圏, 太陽風, 非磁化惑星

Deceleration of the solar wind due to the mass loading by planetary heavy ions forms the magnetic pile-up region around unmagnetized planets such as Mars and Venus. The Martian magnetic pile-up region diverts shocked solar wind plasma around the planet at altitudes typically in excess of 800 km [e.g., Vignes et al., 2000]. Mars Global Surveyor (MGS) measurements have shown, on one hand, that shocked solar wind (magnetosheath) plasma occasionally penetrates into much lower altitudes (〜400km) [e.g., Brain et al., 2005; Crider et al., 2005]. Our previous statistical study of these solar wind penetration events using MGS magnetic field and electron observations revealed that both solar wind dynamic pressure (Psw) and the orientation of the interplanetary magnetic field (IMF) control the occurrence of the events. However, MGS cannot observe the solar wind regions due to its orbital design. In this study, we focused on the simultaneous observation of the penetration events by MGS and Mars Express (MEX). MEX possess the ion mass analyzer (IMA) and electron spectrometer (ELS), which are parts of plasma packages of ASPERA-3. MEX partly observed the solar wind region, since the orbit of MEX is elliptical orbit. We can thus obtain the solar wind density and velocity from MEX data. Among the simultaneous observation data by MEX and MGS, we identified 46 simultaneous observation events of the solar wind penetration. We divided the 46 events into the low Psw (≤〜4nPa) and high Psw (≥〜4nPa) events. The solar wind penetration event on January 20, 2005 is observed during the high Psw periods, while the event on February 20, 2005 is during the low Psw periods. We investigated characteristics of the boundary layers between the magnetosheath and the ionosphere. We found that the electron flux shows a gradual decrease in the boundary in the high Psw event. On the one hand, intermittent appearance of both the magnetosheath plasma and the ionosphere plasma in the boundary is during the low Psw event. The signature of the boundary layer resembles with the K-H instability signature seen in LLBL (low-latitude boundary layer) in the Earth's magnetotail [e.g., Hasegawa et al., 2006]. We also report the results of statistical analysis of 46 simultaneous observation events.