Velocity gradient method applied to magnetosheath observations

In plasma physics, boundaries play a crucial role separating regions with different plasma regimes. The Earth’s magnetopause is the outermost boundary of the magnetospheric magnetic field, it is defined by the pressure equilibrium between the magnetosheath and the magnetosphere. Similar importance has the bow shock, separating the supersonic solar wind from the magnetosheath plasma. Even though there are satellite missions able to measure locations and other magnetopause/bow shock properties in-situ, most of the time they are somewhere else. Numerical models predict that after crossing the bow shock in the subsolar region the Vx component of the solar wind velocity decreases linearly until zero where it encounters the subsolar magnetopause. When this assumption is valid, it is possible to determine the boundary location using radial gradient measurements of the magnetosheath plasma velocity made deep in the magnetosheath, away from the boundaries. We will present cases where the bow shock and magnetopause stand-off locations are determined using remote multipoint THEMIS magnetosheath velocity observations.  We will define when and where the method is effective.  We will compare results with the predictions of global MHD simulations.