Diffuse minor ions upstream of simulated quasi‐parallel shocks

Abstract

We have performed a number of one‐dimensional hybrid (particle ions, massless fluid electrons) simulations of quasi‐parallel collisionless shocks in order to elucidate the origin of diffuse upstream minor ions. Minor ions are treated self‐consistently by the hybrid code; however, a number of runs have also been performed wherein the minor ions are treated as test particles. We have investigated the dependence of the density ratio bias factor fnof; (the alpha to proton ratio of the diffuse ions normalized to the solar wind ratio) on shock Mach number, solar wind beta (thermal to magnetic pressure) and on angle ΘBno between shock normal and magnetic field and found strong dependencies on all parameters. An addition of 5% alpha particles changes the shock structure, resulting in a significant modification of the density ratio bias factor as compared to the test particle cases. In the case of a low beta solar wind (β ≃ 0.1) the solar wind alpha particles penetrate the shock ramp rather unaffected and gyrate in the downstream magnetic field as a whole. The alpha particle beam returns occasionally to the shock ramp and spatially localized clouds of alpha particles can be found in the upstream region. With increasing Mach number the gyroradius of the alpha particles increases and it becomes easier for the alpha particle beam to return from downstream to the shock; a high density of clouds is found upstream. In the case of a higher beta solar wind (β≳0.5) and moderate Mach numbers, no localized alpha particle clouds leave from downstream into the upstream region. This can be attributed to the smoother shock transition in the case of higher beta shocks. The effect of beta and Mach number is counteracting: high Mach number shocks can lead even at higher beta to minor ion clouds in the upstream region. Higher beta shock simulations result in an increase of fnof; with beta and ΘBno. The increase with beta is due to the fact that diffuse ions originate mainly from the high‐energy tail of the distribution in the upstream rest frame. It is suggested that large ΘBno implies a more tangential downstream field and the alpha particles reach the shock during their downstream gyration easier.