Counterstreaming suprathermal electron events upstream of corotating shocks in the solar wind beyond ∼2 Au: Ulysses

Enhanced fluxes of suprathermal electrons are commonly observed upstream of corotating forward and reverse shocks in the solar wind at heliocentric distances beyond ∼2 AU by the Los Alamos plasma experiment on Ulysses. The average duration of these events, which are most intense immediately upstream from the shocks and which fade with increasing distance from them, is ∼2.4 days near 5 AU. These events are caused by the leakage of shock‐heated electrons into the upstream region. The upstream regions of these shocks face back toward the Sun along the interplanetary magnetic field, so these leaked electrons commonly counterstream relative to the normal solar wind electron heat flux. The observations suggest that conservation of magnetic moment and scattering typically limit the sunward propagation of these electrons as beams to field‐aligned distances of ∼15 AU. Although it seems unlikely that these shock‐associated events are an important source of counterstreaming events near 1 AU, remnants of the backstreaming beams may contribute importantly to the diffuse solar wind halo electron population there.