The Upstream Escape of Energized Solar Wind Protons from the Bow Shock

Abstract

Backstreaming 2–7 keV protons coming from the Earth’s bow shock have been identified directly (Asbridge et al., 1968; Scarf et al., 1970), while still higher energy particles have been found upstream from interplanetary shocks (Armstrong et al., 1970) and from Jupiter’s bow shock (Simpson et al., 1974). In addition, it has been concluded that the longest-period hydromagnetic precursors could not have propagated upstream from the Earth’s bow shock, but must have been generated upstream by some other agency, presumably reflected particles, and swept downstream with the solar wind (Greenstadt et al., 1970; Fairfield, 1969). Accordingly, it has also been shown that protons reflected from the bow shock should be accelerated by the interplanetary electric field, seen in the shock frame, to energies comparable to those observed by plasma experiments (Sonnerup, 1969). These energies correspond to particles traveling along B at velocities comparable to the rate at which locally-excited hydromagnetic precursor waves appear to progress upstream (Greenstadt et al., 1970). Recently, however, there have been some systematic observations of back-streaming protons at the Earth’s bow shock with parallel velocity components and total energies much too high to be associated with the usual long-period upstream waves or to be produced by Sonnerup’s simple reflection process (Lin et al., 1974), and these protons (30–100 keV) were indeed attributed to some unknown acceleration mechanism in the upstream region.