High-Energy Solar Particle Events: Constraints on Diffusive Shock Acceleration Theory

The physics of the acceleration of solar-energetic particles (SEPs) to energies as high as several GeV are discussed in the context of diffusive shock acceleration. Superstorms that produce high-energy solar cosmic rays provide important constraints on the possible sources of these particles and on the acceleration time scale. The time scale for acceleration depends strongly on the topology of the magnetic field and plasma flow speed. On the one hand, theoretical predictions for this time scale can range over several orders of magnitude. On the other hand, it can be shown that, very generally, low-energy particles can be accelerated to several GeV in time scales shorter than a few minutes by nearly perpendicular shocks. This will be discussed in detail in this talk. The results from new simulations of test-particles encountering shocks moving through a medium containing large-scale (much larger than the particle gyroradii) magnetic field turbulence will be presented. Also, there have been new interpretations of observations of the charge state of SEPs and its dependence on energy that point to the importance of acceleration by nearly perpendicular shocks. These will also be discussed.