Tracking the Subsolar Bow Shock and Magnetopause

Global magnetohydrodynamic models predict that plasma velocities vary almost linearly from 0 km s^-1 at the stationary subsolar magnetopause to ~0.25 V_SW at the subsolar bow shock, where V_SW is the solar wind velocity.  We show how two-point measurements of the plasma velocity  within ~15° of the Sun-Earth line can be used to determine gradients in the plasma velocity and consequently the time-dependent location of both the subsolar magnetopause and the subsolar bow shock.  A case study employing multiple simultaneous THEMIS spacecraft observations confirms that velocity gradients in the subsolar magnetosheath are linear, except when spacecraft observe rapid fluctuations downstream from the quasi-parallel bow shock.  The method may be useful to those binning magnetosheath observations to develop empirical models, those seeking to determine whether reconnection and hence magnetopause erosion are steady or bursty, and those determining the stand-off distance of the bow shock (or equivalently the polytropic index in the solar wind).