Survey of coherent ion reflection at the quasi‐parallel bow shock

Abstract

Ions coherently reflected off the Earth's bow shock have previously been observed both when the upstream geometry is quasi‐perpendicular and when it is quasi‐parallel. In the case of quasi‐perpendicular geometry, the ions are reflected in a nearly specular manner and are quickly carried back into the shock by the convecting magnetic fleld. In the quasi‐parallel geometry, however, near‐specularly reflected ions' guiding center velocities would on the average be directed away from the shock, allowing the ions to escape into the upstream region. The conditions under which coherent reflection occurs and the subsequent coupling of the reflected ions to the incoming solar wind plasma are important factors when assessing the contribution of the reflected ions to the downstream temperature increase and the shock structure. The survey presented in this paper, along with previously reported observations, suggests that near‐specularly reflected ions are indeed an important aspect of energy dissipation at the Earth's quasi‐parallel bow shock. We find that (1) cool, coherent, near‐specularly reflected ion beams are detected over nearly the full range of upstream plasma parameters commonly found at the Earth's bow shock; (2) the beams are typically observed only near the shock ramp or some shock‐like feature; and (3) the observed beam velocities are almost always consistent with what one would expect for near‐specularly reflected ions after only a small fraction of a gyroperiod following reflection. The second and third points indicate that the beams spread very quickly in velocity space. This spread in velocities could be due either to interactions between the beam and incoming solar wind ions or to some initially small velocity spread in the beam. In addition we present an observation of two simultaneous beams, one outgoing from the shock and one incoming towards the shock. They were detected when the local geometry was quasi‐perpendicular, even though the averaged field geometry was quasi‐parallel. This observation indicates that local variations in the magnetic field are in some cases responsible for returning the specularly reflected ions to the downstream region, allowing them to contribute directly to the downstream thermalization.