Richard Mewaldt
Senior Research Associate
Caltech

It is now more than 50 years since the discovery that during large solar
flares particles can suddenly be accelerated to very high energies.
The Sun is presently at the maximum of its 11-year activity cycle and
the past six months have been marked by two solar-particle events that
are the most intense in more than a decade. Such events provide a
high-energy sample of solar material, and since 1997 instruments on
NASA's Advanced Composition Explorer (ACE) have been measuring the
elemental, isotopic, and ionic charge-state composition of energetic
nuclei from the Sun.  These measurements reveal a surprisingly variable
composition with evidence for fractionation in isotope abundances of a
factor of 3 and fractionation of elemental abundances by as much as a
factor of 100. These data are providing new challenges to models of the
solar composition and particle acceleration processes.