Strong deformation of magnetospheres under low Alfven Mach number solar wind

The density of the solar wind (SW) around the Earth's magnetosphere at times decreases to only several percent of the usual value, which is often observed during a coronal mass ejection (CME) passage, and such density extrema results in a significant reduction of dynamic pressure and Alfven Mach number (Ma) of the SW flow. While simple expansion of the Earth's magnetosphere by the low dynamic pressure was assumed in most of previous studies, a recent simulation study predicted a remarkable dawn-dusk asymmetry of the magnetotail in shape under low Ma SW and oblique IMF configuration (Nishino et al., Phys. Rev. Lett., 2008). Therefore, direct observations of the magnetopause under these extreme conditions have been awaited. Here we show evidence of strong deformation of the magnetotail under low Ma SW and Parker-spiral IMF conditions, based on Geotail observations on both the dawn and dusk sides. The magnetosphere on the dayside and on the dawnside expanded drastically, while the tail magnetopause on the duskside remained at the usual position despite extremely low dynamic pressure in the SW. The strong deformation of the magnetosphere can be universal phenomenon, because the extremely low Ma (low beta) SW environment takes place not only around the Earth's magnetosphere but also around Mercury and in the interstellar medium outside the heliopause.