Despite decades of observation, the high-energy Sun still presents some puzzling mysteries. How do solar flares so efficiently accelerate particles to MeV and GeV scales? What processes heat the corona to millions of degrees? And in direct relation to both of these questions, how do properties of flares scale with the amount of energy released? Bremsstrahlung hard X-rays emitted by hot or nonthermal electrons are useful for probing these questions.
High-energy exploration of the Sun is currently undergoing a transformation with the use of directly focusing hard X-ray instruments, as opposed to the previously utilized indirect imagers. Compared with previous instruments, the use of direct focusing offers orders of magnitude better sensitivity as well as higher-fidelity imaging. The first generation of solar-dedicated hard X-ray focusing optics has recently flown on suborbital missions (rockets and balloons). And from low-Earth orbit, the NuSTAR spacecraft, a direct-focusing instrument designed to look at the faintest objects outside the solar system, has observed the Sun on nine separate occasions.
This colloquium will cover recent advances in high-energy solar flare physics and will present new results from these novel instruments, concentrating on what we are learning about small-scale energy release on the Sun . Prospects for advances in understanding solar flares and astrophysical particle acceleration will be discussed.