Observations of Particle Acceleration at Interplanetary Shocks

Abstract

Collisionless shocks accelerate charged particles throughout heliosphere, from low in the solar corona to the outer edge of the supersonic solar wind. Particles can gain energy while interacting with shocks in different ways, depending on conditions at the shock and on particle species and energy. Different acceleration processes generally produce distinct features in energetic particle anisotropies, energy spectra, and time intensity profiles. Thus, a study of in‐situ interplanetary shocks and particle distributions in their vicinity provides the only meaningful way to test our theoretical understanding of shock acceleration. During solar cycle 23, advanced instrumentation from the ACE spacecraft detected over 300 interplanetary shocks. More than half of these shocks produced signatures (so called energetic storm particle (ESP) events) in the low energy ion intensities. Of these ESP events, only one event did exhibit signatures that agree with steady‐state diffusive shock‐acceleration theory. Generally, however, ion spectra measured at shocks steepen with energy, indicative of rollover in the spectral slope in this type of events. We also found for those ESP events that have particle enhancements, a minimum shock speed of 250 km/s along the shock normal angle is needed in order to have any significant (>20%) enhancement over the ambient intensity.