Energetic charged particles associated with strong interplanetary shocks

We analyze observations of energetic charged particles associated with many strong interplanetary shocks seen by Advanced Composition Explorer. We focus primarily on 47-187 keV suprathermal protons and restrict our analysis to strong interplanetary shocks (Alfvén Mach number >3 and the shock density compression >2.5). Eighteen shocks meeting this criterion from 1998 to 2003 were analyzed. All 18 had enhancements of the 47-65 keV proton intensity above the intensity seen one day before the shock. In 17 events, the particle intensity either rose to a quasi-plateau or peaked within 10 minutes of the shock. Most had intensities at the shock exceeding 100 times more than that seen the day before the shock arrived. The time-intensity profiles of the energetic proton events in many cases reveal a rise before the shock passage reaching a quasi-plateau or local peak at the shock, followed by a gradual decline. This suggests that the shock itself is the source of energetic particles. Energy spectra behind the shock were fit to an assumed power law over the interval from 46 to 187 keV, and the resulting spectral index was compared to the plasma density jump across each shock. Most events agree with the prediction of diffusive shock acceleration theory to within the observational uncertainties. We also analyzed a few selected events to determine the particle spatial diffusion coefficients and acceleration timescales. We find that the time to accelerate protons to 50 keV is of the order of an hour.