High‐speed flows of H+ and He++ ions at the magnetopause

Abstract

During a crossing of the subsolar magnetopause into the boundary layer the Hot Plasma Composition Experiment on AMPTE/CCE has provided the first observations of high‐speed flows of both H+ and He++ ions. Furthermore, the flow velocities of the two species showed good agreement. When considered as a single fluid, the ion flow obeys the MHD momentum balance relation for a rotational discontinuity, and thus is consistent with expectations for magnetic reconnection. This balance test does not, however, predict that the flow speeds of H+ and He++ should be equal. Thus our observations provide information not contained in the fluid description. The observed agreement between the flow velocities of different ion species follows if one considers the motion of individual ions across the magnetopause current sheet. Assuming that the motion conserves the particles' pitch‐angle, or changes it by an equal amount, as for adiabatic motion, all particles will gain the same amount in speed regardless of mass. The particles gain their energy by sliding along the magnetopause in the direction of the tangential electric field intrinsic to the reconnection process. To preserve equality of speed, the He++ ions must move twice the distance of the H+ ions.