Over the last decade we entered a new exploration phase of solar flare
physics, equipped with powerful spacecraft such as Yohkoh, SoHO, and
TRACE that pro- vide us detail-rich and high-resolution images of solar
flares in soft X-rays, hard X -rays, and extreme-ultraviolet
wavelengths. Moreover, the large-area and high- sensitivity detectors on
the Compton GRO spacecraft recorded an unprecedented number of
high-energy photons from solar flares that surpasses all detected high-
energy sources taken together from the rest of the universe, for which
CGRO was mainly designed to explore. However, morphological descriptions
of these beau- tiful pictures and statistical catalogs of these huge
archives of solar data would not convey us much understanding of the
underlying physics, if we would not set out to quantify physical
parameters from these data and would not subject these measurements to
theoretical models. Historically, there has always been an
unsatisfactory gap between traditional astronomy that dutifully
describes the mor- phology of observations, and the newer approach of
astrophysics, which starts with physical concepts from first principles
and analyzes astronomical data with the goal to confirm or disprove
theoretical models. In this review we attempt to bridge this yawning gap
and aim to present the recent developments in solar flare high-energy
physics from a physical point of view, structuring the observations and
analysis results according to physical processes, such as particle
acceleration, propagation, energy loss, kinematics, and radiation
signatures.
