Professor Umran S. Inan

Winter 1999-00

Date: Wednesday, February 23, 2000
Time: 4:15-5:30 PM; Refreshments at 4:00 PM
Location: 380-380X

Particle Acceleration and Explosive Energy Release in Solar Flares

Dr. Robert Lin
Space Science Laboratory, UC Berkeley


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. Solar flares release up to 10^{32}-10^{33} ergs in 10^{2}-10^{3} s. The accelerated 10-100 keV electrons (and possibly >1 MeV ions) appear to contain a significant fraction, perhaps the bulk, of this energy, indicating that the particle acceleration and energy release processes are intimately linked. How the Sun releases this energy, presumably stored in the magnetic fields of the corona, and how it rapidly accelerates electrons and ions with such high efficiency, and to such high energies, is presently unknown.

High-energy emissions are the most direct signature of particle acceleration in solar flares. Hard X-ray/gamma-ray continuum is produced as bremsstrahlung by energetic electrons. Nuclear collisions of energetic protons and heavier ions with the ambient solar atmosphere result in a complex spectrum of narrow and broad gamma-ray lines. I will discuss the scientific objectives and technical aspects of the High Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission, planned for launch in mid-2000. HESSI will provide the first hard X-ray imaging spectroscopy, the first solar gamma-ray measurements able to resolve the nuclear lines produced by the energetic ions, and the first gamma-ray line and continuum imaging of solar flares.