Two Examples of High-Energy ESP Events

Energetic Storm Particle (ESP) events are intensity increases of energetic ions associated with the passage of interplanetary shocks. Models for ESP events include those that invoke either energetic particle acceleration by interplanetary shocks or trapping by ambient structures in the shock vicinity. Typical ESP events at 1 AU have significant proton intensity increases at energies from a few tens of keV to some tens of MeV; rare are those events with increases in the >100 MeV range. We present two ESP events, the October 20, 1989 and the February 21, 1994 ESP events, whose proton intensity increases were observed at energies >500 MeV. We use solar wind and magnetic field data from the IMP-8 spacecraft and energetic particle data from the IMP-8 and GOES-7 spacecraft. Both ESP events were generated by solar events close to central meridian (S27E10 and N09W02, respectively). The October'89 ESP event was superimposed on one of the most intense solar energetic particle (SEP) events of solar cycle 22. During the February'94 ESP event, high-energy (>39 MeV) proton fluxes rose above background level just a few hours before the arrival of the shock. High-energy (>39 MeV) proton intensities in these two events differ by two orders of magnitude, being higher in October'89. The weak interplanetary shock for the October'89 ESP event, which arrived at earth ~28.4 hours after the progenitor solar event, was preceded by a structure ~0.03 AU wide with depressed magnetic field and solar wind density. By contrast, the strong interplanetary shock of the February'94 ESP event, which arrived at earth ~31.5 hours after the solar event, was preceded by a well-ordered solar wind. We show that (1) the February'94 ESP event is consistent with particle acceleration at the shock, and (2) the high-energy (>39 MeV) proton population for the October'89 ESP event is not a locally shock-accelerated population, but rather a particle population confined within the structure of depressed field and plasma in front of the shock.