Acceleration of >47 keV Ions and >2 keV electrons by interplanetary shocks at 1 AU


  • B. T. Tsurutani,

  • R. P. Lin


We present initial results from a survey of the effects of interplanetary shocks on energetic ≥2 keV electrons and ≥47 keV ions, as observed by the field, plasma, and energetic particle experiments on the ISEE 3 spacecraft. Shock normals, velocities, Mach numbers, and upstream and downstream plasma parameters were determined for 37 forward shocks out of a total of 55 shocks observed between August 1978 and November 1979. We find that a minimum shock velocity along the upstream magnetic field of ∼250 km/s is required for an interplanetary shock to have a significant effect on acceleration of ≥2 keV electrons or ≥47 keV ions. Shocks with no effect on the energetic particle populations also had relatively small ratios of downstream to upstream magnetic field magnitudes. These results suggest that magnetostatic reflection off the shock itself is a significant mechanism in the acceleration process. Both energetic electron and ion flux variations associated with shocks can be classified into four general types: (1) no significant variation at all, (2) a spike of a few minutes duration at or near the shock, (3) a steplike postshock increase, and (4) a slow rise beginning several hours before the shock (energetic storm particle event). Essentially, every quasi-perpendicular shock with θBn ≳ 70° produced a shock spike in the proton fluxes, while every quasi-parallel shock (θBn ≲ 50°) produced a proton energetic storm particles event, provided the shock velocity was greater than the above stated threshold. Electron spikes were also observed for most, but not all, shocks with θBn ≳ 70°. The most common effect observed in the electron fluxes was a steplike postshock increase of a factor of ∼2. These had no obvious dependence on θBn, but were found for every shock with speed greater than ∼175 km/s. Shock effects in the electron fluxes were about as common as for protons, but were limited to the 2–10 keV energy range except for 3 events which extended up to ∼50 keV. We find that significant ambient populations of both ≥2 keV electrons and ≳47 keV ions are present in the interplanetary medium prior to every shock. These particles could be the “seed” particles for the shock acceleration.