INTERSTELLAR PICKUP IONS AND QUASI-PERPENDICULAR SHOCKS - IMPLICATIONS FOR THE TERMINATION SHOCK AND INTERPLANETARY SHOCKS

A new mechanism for the acceleration of pickup ions by repeated reflec tions from the electrostatic cross shock potential of a quasi-perpendi cular shock is presented. The acceleration mechanism, multiply reflect ed ion (MRI) acceleration, offers a resolution to the issue of injecti ng pickup ions into an efficient particle energization scheme, and the injection efficiency for pickup ions is found to be inversely proport ional to ion mass and proportional to charge. By studying the particle energy gain in the motional electric field (where a steady shock fram e is assumed) the energized pickup ion spectrum can be computed. Extre mely hard power law spectra (E(-1.5), for example) emerge from the ups tream pickup ion distribution. The maximum energy that a reflected pic kup ion can gain is found to be proportional to the square of the prod uct of the ALfven speed and (r - 1), where r is the shock compression ratio. For solar wind conditions at either interplanetary shocks or th e termination shock the upper energy Limit is typically in excess of 0 .5 MeV. It is suggested here that MRI acceleration provides an efficie nt mechanism for injecting low-energy pickup ions into a subsequent ac celeration process such as diffusive Fermi acceleration. Such a two-st ep acceleration scheme alleviates many of the difficulties which plagu e 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 deta iled study of the MRI acceleration mechanism at the termination shock for a wide range of parameters and discuss the implications for the an omalous cosmic ray component. The acceleration of pickup ions by an in terplanetary traveling shock is also discussed, and the observations m ade by Ulysses [Gloeckler et al., 1994] are addressed. The puzzling as pects of the Gloeckler et al. [1994] observations appear to be explain ed quite naturally by shock energization based on repeated pickup ion reflections. Observational tests of MRI acceleration may be possible b y using pickup He+ at either the terrestrial or Jovian bow shock or by using cometary ions at a cometary bow shock.