Interstellar pickup ions and quasi‐perpendicular shocks: Implications for the termination shock and interplanetary shocks

Abstract

A new mechanism for the acceleration of pickup ions by repeated reflections from the electrostatic cross shock potential of a quasi‐perpendicular shock is presented. The acceleration mechanism, multiply reflected ion (MRI) acceleration, offers a resolution to the issue of injecting pickup ions into an efficient particle energization scheme, and the injection efficiency for pickup ions is found to be inversely proportional to ion mass and proportional to charge. By studying the particle energy gain in the motional electric field (where a steady shock frame is assumed) the energized pickup ion spectrum can be computed. Extremely hard power law spectra (E−1.5, for example) emerge from the upstream pickup ion distribution. The maximum energy that a reflected pickup ion can gain is found to be proportional to the square of the product of the Alfvén speed and (r − 1), where r is the shock compression ratio. For solar wind conditions at either interplanetary shocks or the termination shock the upper energy limit is typically in excess of 0.5 MeV. It is suggested here that MRI acceleration provides an efficient mechanism for injecting low‐energy pickup ions into a subsequent acceleration process such as diffusive Fermi acceleration. Such a two‐step acceleration scheme alleviates many of the difficulties which plague ion energization models at perpendicular shocks. The structure of a quasi‐perpendicular shock modified by shock reflection of pickup ions is discussed in general terms. By way of application we present a detailed study of the MRI acceleration mechanism at the termination shock for a wide range of parameters and discuss the implications for the anomalous cosmic ray component. The acceleration of pickup ions by an interplanetary traveling shock is also discussed, and the observations made by Ulysses [Gloeckler et al., 1994] are addressed. The puzzling aspects of the Gloeckler et al. [1994] observations appear to be explained quite naturally by shock energization based on repeated pickup ion reflections. Observational tests of MRI acceleration may be possible by using pickup He+ at either the terrestrial or Jovian bow shock or by using cometary ions at a cometary bow shock.