[P-CG21] 惑星大気圏・電磁圏

[PCG21-19] 水星磁気圏の大規模構造：太陽風依存性

*八木 学¹、関 華奈子²、松本 洋介³、Delcourt Dominique⁴、Leblanc Francois⁴ (1.東北大学大学院理学研究科、2.東京大学大学院理学系研究科、3.千葉大学大学院理学研究科、4.フランス国立科学研究センター)

キーワード：水星磁気圏、磁気流体

Based on observations by MESSENGER, Mercury's magnetosphere is thought to be a miniature of the Earth's magnetosphere. These two magnetospheres have several characteristics in common, however, some critical differences are also evident. First, there is no atmospheric layer, but only tenuous exosphere. Second, the kinetic effects of heavy ions might not be negligible because Mercury magnetosphere is relatively small compared to the large Larmor radii. Recent observation by MESSENGER also found that the center of dipole is shifted to northward about 485km from the center of Mercury. Trajectory tracings is one of the dominant methods to estimate the kinetic effect of heavy ions which originate the exosphere, though the results of the simulation are quite sensitive to the electric and magnetic field. Therefore, it is important to provide a realistic field model in the trajectory tracings. In order to construct a large scale structure, we developed a MHD simulation code, and adopted to the global simulation of Mercury magnetosphere. In this study, first we performed two cases of simulation, low and high solar wind density cases(35cm^{-3}, 70cm^{-3}, and 140cm^{-3}) with velocity for 400km/s and northward IMF condition. When solar wind density is low, magnetopause is formed at 1.4R_{M}, and the global structure has weak north-south asymmetry in the MHD simulation. One of the important characteristics is open field line from south pole even in the northward IMF condition without Bx and By components. When solar wind dynamic pressure is high, Mercury's magnetosphere is compressed to the scale of Mercury itself. In this case, planetary surface disturbs the magnetospheric convection, and the north-south symmetry as well as similarity to Earth's magnetosphere are strongly violated. Trajectory trancings in the MHD fields show that there are enough space for energetic (~ few keV) sodium ions which are the main component of 'sodium ring' at the vicinity of the planet to go through the dayside magnetosphere in the low density case. In the high density case, dayside is too compressed and there are no space for sodium ions to go through. As a result, 'sodium ring' became not isotropic ring but formed only at nightside. In the next step, we performed higher dynamic pressure of the solar wind condition, it is, density for 140cm^{-3} and velocity for 800km/s. This parameter is rarely occurred except for the extreme case such as CME events. The result of MHD simulation shows that most of magnetic filed lines are opened, and continuous tail reconnection occurred by extremely high dynamic pressure. These structure and phenomenon partly correspond to that of magnetosphere with southward IMF, while magnetospheric convections are largely different because no magnetic reconnection occurs at the dayside magnetosphere. Another characteristics is secondary compression region in the magnetosheath at flank side of the planet. First compression is occurred by planetary surface at the front side and formed what we call bow shock. Second compression is caused by magnetopause at the flank side which lies at the direction of sheath flow. In the presentation, we will also report the ongoing simulation result of trajectory tracings in this extreme case.