25 August 2005

Speaker: Bob Lin (Space Sciences Laboratory, Berkeley)

Title: Particle Acceleration by the Sun

Abstract: The Sun is the most energetic particle accelerator in the solar system, producing ions up to 10s of GeV and electrons to 100s of MeV, in both large solar flares and fast coronal mass ejections (CMEs), but through different physical mechanisms. Solar flares are the most powerful explosions in the solar system, releasing up to 1032-1033 ergs in 100-1000 s, with up to ~10-50% of this energy in accelerated electrons and ions. The intense solar energetic particle (SEP) events observed in the interplanetary medium, however, appear to be accelerated by shock waves driven by fast CMEs. Transient energy releases by the Sun down to the smallest and most frequently occurring (microflares and type III radio bursts) also appear to involve particle acceleration, but the physics of these acceleration processes are not understood. The RHESSI mission launched in 2002 provides a new window on particle acceleration at the Sun, through imaging spectroscopy of the hard X-ray/gamma-ray continuum and gamma-ray lines emission produced by the accelerated electrons and ions, respectively. I will present the many new RHESSI results and discuss the implications for our understanding of the energy release and particle acceleration processes.