Suprathermal ions upstream from interplanetary shocks

Suprathermal ions with energies between solar wind thermal energies and ∼29 keV are occasionally observed ahead of outward propagating interplanetary shocks with the Los Alamos/Garching fast plasma experiment on ISEE 1 and 2. Compared with suprathermal ion velocity distributions observed upstream from the earth's bow shock, the upstream interplanetary shock ion velocity distributions are relatively structureless, and the particle fluxes are less intense. Typically the suprathermal ion distribution emerges smoothly from the solar wind thermal distribution and is nearly isotropic in the solar wind frame. Such distributions are observed with the fast plasma experiment only in association with interplanetary shocks. Field-aligned beams, kidney-bean-shaped distributions, shells of ions in velocity space, and bunches of gyrating ions—all common to the upstream region of the earth's bow shock—have not been observed ahead of interplanetary shocks. Highly structured ion velocity distributions observed upstream of the earth's bow shock apparently are caused directly or indirectly by the nearly specular reflection of solar wind ions at the shock, a consequence of the generally high Mach number of the solar wind flow at 1 AU. By way of contrast, most interplanetary shocks at 1 AU have low, subcritical Mach numbers, and solar wind ion reflection at these shocks does not appear to play a role in producing upstream suprathermal ion distributions at 1 AU. Nevertheless, solar wind ions are accelerated to high energies at interplanetary shocks. Leakage of shocked thermal plasma across these low-Mach-number shocks from the downstream region may play an important role in producing upstream suprathermal ion populations and may therefore constitute the first step in the acceleration of solar wind ions to high energies at these shocks.

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