The acceleration of charged particles from thermal to suprathermal energies at strong shocks in the solar wind

Abstract:

The physics of particle acceleration at shocks in the solar
wind is discussed focusing primarily on the formation of
high-energy tails in particle distributions as thermal
plasma is heated and compressed across a strong shock. This is
important for our understanding of the origin and sources of
high-energy solar-energetic particles (SEP) which is a major
unsolved problem in solar and heliospheric physics. 
Unfortunately, our current understanding of this is not
complete despite much analysis of spacecraft observations
and numerical simulations. It is fairly well established
that SEPs, which are known to be accelerated by strong
interplanetary shocks, have an isotopic composition that
is similar to the quiet-time pre-existing ``sea'' of suprathermal
particles, and is not consistent with that of the thermal
solar wind. This suggests that thermal solar wind is not a
source of SEPs, and that it is not efficiently accelerated at
interplanetary shocks, despite the fact that numerical simulations
generally show that thermal plasma is accelerated quite efficiently
at shocks. A brief review of this topic will be given with an
emphasis on outlining what new spacecraft observations are required
to make progress.