Asymmetric deformation of the Earth’s magnetosphere under low Alfvén Mach number solar wind: Observations and MHD simulation

Abstract

Abstract The density of the solar wind plasma near 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 the dynamic pressure and Alfvén Mach number ( $$M_A$$ M A ) of the solar wind flow. While a symmetric expansion of the Earth’s magnetosphere by the low dynamic pressure was assumed in previous studies, a global magnetohydrodynamic (MHD) simulation study predicted a remarkable dawn-dusk asymmetry of the magnetospheric shape under low-density solar wind and Parker-spiral interplanetary magnetic field (IMF) configuration. Here, we present observations consistent with the asymmetric deformation of the magnetosphere under low- $$M_A$$ 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. Local, roughly Alfvénic sunward acceleration of magnetosheath ions at the dawn flank magnetopause suggests magnetosheath plasma entry into the magnetosphere through open field lines generated by magnetic reconnection at the dayside magnetopause. At the same time, Cluster 1 and 3, located near the southern polar cusp, also detected continuous antisunward ion jets and occasional sunward jets, which are consistent with the occurrence of magnetic reconnection near the southern cusp. These observations suggest that enhanced plasma acceleration at the dayside magnetopause operates under the low- $$M_A$$ M A solar wind and Parker spiral IMF conditions and that plasma influx across the dawnside magnetopause is at work under such a low- $$M_A$$ M A condition. These results can be helpful in understanding interactions between low- $$M_A$$ M A solar/stellar winds and celestial objects, such as inner planets and exoplanets. Graphic Abstract

Keywords

• Low density solar wind
• Earth’s magnetosphere
• Alfvén Mach number
• Solar wind–magnetosphere interaction