[P-EM10] Dynamics of Magnetosphere and Ionosphere

[PEM10-09] Multipoint measurements of the Earth's magnetosphere during low Alfvénic Mach number solar wind by Geotail and Cluster

*Masaki N Nishino¹、Yoshifumi Saito¹、Benoit Lavraud²、Yukinaga Miyashita^3,4、Tsugunobu Nagai¹、Motoharu Nowada⁵ (1.Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science、2.Institut de Recherche en Astrophysique et Planétologie、3.Korean Astronomy and Space Science Institute、4.Korea University of Science and Technology、5.Shandong University)

キーワード：Low density solar wind, Low Alfvénic Mach number, Solar wind–magnetosphere interaction, Magnetopause boundary, Plasma acceleration, Geotail, Cluster

The density of the solar wind around the Earth's magnetosphere sometimes decreases to only several per cent of the usual value, and such density extrema result in a significant reduction of dynamic pressure and Alfvénic Mach number (M_A) of the solar wind flow. While a simple expansion of the Earth's magnetosphere by the low dynamic pressure was assumed in previous studies, a global-MHD simulation study predicted a remarkable dawn-dusk asymmetry of the magnetosphere in shape under low-density solar wind and Parker-spiral interplanetary magnetic field (IMF) conditions. Here we present evidence of asymmetric deformation of the magnetosphere under low-M_A solar wind and Parker-spiral IMF conditions, focusing on the significant expansion of the dawn-flank magnetosphere detected by the Geotail spacecraft. A global-MHD simulation reproduced the dawnward expansion of the near-Earth magnetosphere, which was consistent with the observation by Geotail. The solar wind flow had a non-negligible dusk-to-dawn component and partly affected the dawnward expansion of the magnetosphere, but most of the dawnward expansion derived from the oblique IMF effect. The antisunward-going ion beams detected at the flank magnetopause suggest that magnetic reconnection frequently occurs due to the very low-beta environment of the magnetosheath. At the same time, Cluster 1 and 3 located around the southern polar cusp region occasionally detected antisunward-going ion jets just outside of the magnetosphere, which may show a magnetic slingshot effect reported by Lavraud et al. (2007). These observations suggest that altered and enhanced plasma acceleration around the dayside and dawnside magnetopause must work under the low M_A and Parker spiral IMF conditions.