Solar flares can release up to 10³² ergs of energy over a period of several minutes. Based on the ubiquitous presence of large fluxes of high-energy radiation (hard X-rays and gamma rays), it is believed that a substantial fraction of this energy is channeled into the acceleration of massive numbers of electrons, protons, and other ions. A deeper understanding of the processes that convert magnetic energy into accelerated particles would have implications for many areas of space physics and astrophysics.

The HESP mission concept has as its primary goal the determination of hard X-ray and gamma-ray spectra with an unprecedented combination of spatial, spectral, and temporal resolution—observations that for the first time address the fundamental acceleration and propagation of high-energy particles on scales of fundamental interest. For example, the variation of hard X-ray spectra from point to point in the flaring structure will determine the physical processes acting to accelerate and degrade the bremsstrahlung-producing electrons, while observations of gamma-ray line profiles with high spectral resolution (the first true gamma-ray spectroscopy of an astrophysical source) will reveal the processes by which energetic ions are accelerated. The imaging spectroscopy capabilities of HESP will permit the high-energy equivalent of "color movies" of the explosive energy release process in solar flares to be made--observations which will dramatically increase our understanding not only of the nature of flares, but also of the fundamental physical processes themselves. The mission concept also involves the use of several state-of-the-art technologies, such as the fabrication of grids of X-ray-absorbing material with pitches as fine as tens of microns and a uniformity of a small fraction of a percent, and the development of robust mechanical cooling systems for the high-resolution detectors.

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