Particle Acceleration at the Sun (Invited)

Hard X-ray and gamma-ray measurements show that electrons from ~10s of keV to 100s of MeV and ions of a few MeV to ~GeV energies, respectively, are accelerated in large solar flares, with a significant fraction, ~10-50%, of the >~10^32 ergs of energy released going to the accelerated particles. Imaging measurements from RHESSI indicate that the acceleration of both ions and electrons is related to magnetic reconnection in the corona above the flare soft X-ray loops. In at least one example, observed in an over-the-solar limb flare, the flare electron acceleration region was identified. Furthermore, the fluence of gamma-ray lines produced by tens of MeV ions show a close linear correlation with the fluence of >0.3 MeV X-ray continuum produced by >0.3 MeV electrons, suggesting that energetic ions and relativistic electrons may be accelerated by a common process. RHESSI imaging suggest that the most common (>100s per month near solar maximum) solar particle acceleration, that produces impulsive solar energetic particle events seen in the interplanetary medium (IPM), are related to magnetic reconnection between open (connected to the IPM) and closed magnetic field lines. These events are dominated by ~0.1 -100 keV electrons and by ~10 keV to ~MeV/nucleon ions with enormous enrichments in the isotope 3He and in heavy and ultraheavy ions. The acceleration mechanisms are, at best, poorly understood, but new in situ observations from spacecraft going close to the Sun, plus new types of imaging observations can lead to significant advances.