Direct measurement of the cross-shock electric potential at low plasma $\beta$, quasi-perpendicular bow shocks

TL;DR

This study measures the electric potential across low plasma beta, quasi-perpendicular bow shocks using Cluster spacecraft data, revealing significant variability and nonstationarity in shock structure and energy transfer.

Contribution

It introduces a method for directly measuring the cross-shock electric potential and compares it with ion energy changes, highlighting shock variability.

Findings

Cross-shock potential ranges from 500 to 2500 volts.

Potential accounts for 23-236% of ion energy change.

Shock structure is highly variable and nonstationary.

Abstract

We use the Cluster EFW experiment to measure the cross-shock electric field at ten low $\beta$, quasi-perpendicular supercritical bow shock crossings on March 31, 2001. The electric field data are Lorentz-tranformed to a Normal Incidence frame (NIF), in which the incoming solar wind velocity is aligned with the shock normal. In a boundary normal coordinate system, the cross-shock (normal) electric field is integrated to obtain the cross shock potential. Using this technique, we measure the cross-shock potential at each of the four Cluster satellites and using an electric field profile averaged between the four satellites. Typical values are in the range 500-2500 volts. The cross-shock potential measurements are compared with the ion kinetic energy change across the shock. The cross-shock potential is measured to be from 23 to 236% of the ion energy change, with large variations between the four Cluster spacecraft at the same shock. These results indicate that solar wind flow through the shock is likely to be variable in time and space and resulting structure of the shock is therefore nonstationary.