Robert P. Lin
Physics Department and Space Sciences Laboratory, University of California, Berkeley
Particle Acceleration and Explosive Energy Release by the Sun
The Sun is the most energetic natural particle accelerator in the solar system, generating ions up to ~1-10 GeV and electrons to ~100s of MeV, in two distinct processes: one associated with magnetic reconnection in large solar flares, and the other with collisionless shock waves driven by fast coronal mass ejections (CME). Large solar flares release up to ~1032 ergs in 100-1000 s, remarkably with >~10-50% of this energy going to accelerating > tens of keV electrons and > several MeV ions. Imaging of the hard X-ray (HXR)/gamma-ray continuum and gamma-ray line emission produced by the accelerated electrons and ions, respectively, by the RHESSI (Ramaty High Resolution Solar Spectroscopic Imager) mission, show that magnetic reconnection underlies both the flare energy release and particle acceleration. Direct in situ measurements by the Wind and Cluster spacecraft also show that electron acceleration up to ~300 keV also occurs in the magnetic reconnection region of the Earth’s magnetotail. For flares, the >0.3 MeV electron bremsstrahlung continuum and 2.223 MeV neutron-capture line fluences exhibit a close linear correlation over >3 orders of magnitude in flare intensities, strongly suggesting a common flare acceleration process for relativistic electrons and >30 MeV ions. The inferred e/p ratios (0.5 MeV electrons to 10 MeV protons) for these gamma-ray line flares are comparable to those measured for the frequent but generally small impulsive solar energetic particle (SEP) events observed near 1 AU, that exhibit strong enhancements of 3He, and heavy ions. Recent results from theory and simulations of magnetic reconnection, have led to significant progress in understanding these particle acceleration phenomena.