Particle acceleration in solar wind compression regions

We present the results of a theoretical investigation of the acceleration of charged particles in regions of gradual solar wind compression. The mechanism we describe is similar to diffusive shock acceleration, except that it invokes a gradual compression of the plasma over many gyroradii rather than a shock. Recent observations of energetic particles associated with corotating interaction regions (CIRs) at 1 AU suggest that the particles were accelerated within the compression region between the fast and slow solar wind rather than at the associated forward and reverse shocks, which are at larger heliocentric distances. We show that nondiffusive effects such as magnetic mirroring are important in the inner heliosphere, particularly the injection of low-energy particles (e.g., pickup ions and suprathermal solar wind ions). We integrate the trajectories of an ensemble of test particles moving in synthesized electromagnetic fields, which are similar to what is currently known about corotating interaction regions in the inner heliosphere, prior to the formation of the forward and reverse shocks. We show that compression regions associated with CIRs at 1 AU with widths ∼0.03 AU can accelerate particles up to ∼10 MeV and produce energy spectra which are remarkably similar to recent observations.