Abstract
The Sun is the most powerful particle accelerator in the solar system, accelerating ions up to tens of GeV and electrons to hundreds of MeV in both solar flares and fast Coronal Mass Ejections (CMEs). Solar flares are the most energetic explosions, releasing up to 10^32-10^33 ergs in 10-1000s. The accelerated ~10-100 keV electrons (and sometimes >~1 MeV ions) appear to contain >~10-50% of this energy, indicating that the particle acceleration and energy release processes are intimately linked. The RHESSI (Ramaty High Energy Solar Spectroscopic Imager) Small Explorer mission utilizes rotating modulation collimators and cooled germanium detectors to provide the first high resolution imaging (~2 arcsec) and spectroscopy (~keV FWHM) of flare hard X-ray/gamma-ray continuum and gamma-ray lines emitted by energetic electrons and ions, respectively, at the Sun. I will present RHESSI hard X-ray imaging spectroscopy of solar flares, and discuss the implications for particle acceleration and energy release. In addition, I will present the first high resolution spectroscopy and imaging of flare gamma-ray lines, the first detection of continous solar emission and microflares in the 3-10 keV energy range, and first detection of hard X-ray counterparts to solar type III radio bursts.