Ion dynamics in magnetosonic shock front

Abstract

I address the ion dynamics at the front of magnetosonic shocks moving at different angles θ to the magnetic field vector. I employ a shock discontinuity model in which the ramp potential difference is taken into account. The analysis conditionally separates all the ions incoming to the front of oblique magnetosonic shocks into the following categories: 1) transient, 2) reflected, 3) gyrating in front of the ramp, 4) pickup in the ramp. Both gyrating and pickup ions are shown to be present temporarily at the magnetosonic shock front at any angles θ. In the end, both the former and the latter appear to be transient in a strictly transversal magnetosonic shock; and either transient or reflected, in an oblique magnetosonic shock. I have found the critical angle θ* that separates ions into transient and reflected in an oblique magnetosonic shock. The critical angle θ* depends both on the velocity of the particles, incident on ramp, and on dimensions of the ramp potential difference. The most important results are that I have identified the physical cause of the production of the reflected ions having a significant energy and have revealed the mechanism for their acceleration in the ramp (surfing). In the near-Earth shock ion foreshock, these very ener-getic ions (from tens to hundreds of keV) escaping from the magnetosonic shock front at a small angle to the front plane manifest themselves in observations in the form of so-called field-aligned beams (FABs) and form the ion foreshock boundary.