A note on the location of the stagnation point in the magnetosheath flow

Observational and theoretical investigations indicate that the shape of the magnetosphere is nearly symmetric about the plane defined by the aberrated solar wind velocity and the earth's magnetic dipole axis. Nevertheless, many phenomena such as magnetic pulsations and geomagnetic activity seem to indicate that the effective solar wind arrival direction is perhaps 15° to the dawn side of noon. This apparent paradox may have a simple resolution since it can be shown that the location of the stagnation point in the magnetosheath flow is shifted towards dawn by MHD effects not included in the simple gasdynamic model of the solar wind interaction. The shape of the magnetosphere is little affected by the inclusion of these MHD effects. However, the size of the stagnation point shift is very sensitive to the Alfven Mach number and can only account for the largest reported shifts (∼15°) at low Mach numbers (≲3.5).