On the noncoplanarity of the magnetic field within a fast collisionless shock

Abstract

Within the magnetic ramp of fast collisionless plasma shocks observed with spacecraft instruments and simulated numerically, the magnetic field undergoes an excursion out of the plane of coplanarity (the plane determined by the upstream magnetic field and the shock normal). This rotation is consistently in the direction such that the electrostatic potential jump across the shock, as measured in the de Hoffman‐Teller frame of reference (HTF), is ∼2–6 times smaller than the electrostatic potential jump measured in the normal incidence frame. The preferred direction is consistent with a basic whistler mode transition between the upstream and downstream orientations. The potential jump in the HTF is considerably smaller than the change in bulk flow energy across the shock, confirming the recent suggestion that magnetic forces contribute importantly to the slowing of the plasma in that frame. A further consequence is that suprathermal particles leaking back into the upstream region across the shock do not gain much energy from the cross‐shock electric field.