Generation of Diamagnetic Cavities at the Bow Shock by Ion Kinetic Effects

Abstract

Two-dimensional global-scale hybrid simulations are carried out to study the generation of diamagnetic cavities at the bow shock due to accelerated ion beams in (1) intrinsic process of the shock and (2) the interaction of the shock with an interplanetary tangential discontinuity (TD). In the simulation of the bow shock alone, strong electromagnetic waves occur in the foreshock regions of the quasi-parallel shock due to the interaction between thin, hot backstreaming/reflected ions and the cold, dense solar wind plasma. Diamagnetic cavities form in these foreshock regions. The cavities are crater-like and have a width ∼1-2R E , with a low-density and low-magnetic field center bounded by a rim of high density and high magnetic field. The craters convect downstream with the solar wind. When the interplanetary magnetic field (IMF) lies nearly parallel to the solar wind flow, the craters develop into field-aligned structures in upstream and downstream. On the other hand, as a TD carrying IMF direction change interacts with the bow shock, a strong hot flow anomaly (HFA) can be generated by coherent, gyrating multiple beams of reflected ions that are accelerated toward the TD by motional electric field. In the center of the HFA, a low plasma density and a low magnetic field strength are present. The plasma is heated to a nearly isotropic temperature, different from that in the foreshock cavities generated by process (1). The hot HFA cavity and the magnetosheath behind it may bulge into the solar wind by several R E . These transient cavities generated by the bow shock itself and a simple variation in the IMF direction can produce pressure pulses on the magnetopause.