  FOXSI is a direct-imaging, hard X-ray (HXR) telescope optimized for solar
flare observations. It detects hot plasma and energetic electrons in and near
energy release sites in the solar corona via bremsstrahlung emission, measuring
both spatial structure and particle energy distributions. It provides two
orders of magnitude faster imaging spectroscopy than previously available,
probing physically relevant timescales (<1s) never before accessible to address
fundamental questions of energy release and efficient particle acceleration
that have importance far beyond their solar application (e.g., planetary
magnetospheres, flaring stars, accretion disks). FOXSI measures not only the
bright chromospheric X-ray emission where electrons lose most of their energy,
but also simultaneous emission from electrons as they are accelerated in the
corona and propagate along magnetic field lines. FOXSI detects emission from
high in the tenuous corona, where previous instruments have been blinded by
nearby bright features and will fully characterizes the accelerated electrons
and hottest plasmas as they evolve in energy, space, and time to solve the
mystery of how impulsive energy release leads to solar eruptions, the primary
drivers of space weather at Earth, and how those eruptions are energized and
evolve.
