Nonlocal electron heating at the Earth's bow shock and the role of the magnetically tangent point

Abstract

The solar wind interaction with the terrestrial magnetosphere results in a standing bow shock. Due to the shock's collisionless nature, the plasma in the magnetosheath is far from equilibrium. The highly mobile magnetosheath electrons move quickly between different points on the shock surface. Consequently, portions of phase space are populated with electrons from distant locations. This distant influence renders the heating properties of the shock nonlocal. Previous work predicted that the electron temperature along magnetosheath magnetic field lines should be approximately constant and controlled by heating at the point where the field line first encounters the shock, that is, where the field line is tangential to the shock. This point sends shock‐heated electrons to other bow shock locations but does not receive such electrons from elsewhere. In this paper, we compare electron temperatures measured by spacecraft connected by a field line in the magnetosheath and find a close agreement as predicted, unless electron traversal of the magnetosheath becomes impeded due to geometry. We also study the relationship between shock heating and plasma conditions both locally and at the tangent point. We find that the observed electron heating correlates much better with conditions at the tangent point than with the local shock parameters. Therefore, the tangent point plays a critical role in the electron heating and hence the global nature of the shock's partition of incident solar wind energy.