RHESSI

RHESSI (for Reuven Ramaty High Energy Solar Spectroscopic Imager) was a NASA Small Explorer Program (SMEX) satellite to study solar flares. It was launched in 2002 and operated in low Earth orbit until 2018, and re-entered Earth atmosphere in 2023. It consisted of a single-instrument telescope capable of sensing both X-rays and gamma rays, recording them from the solar limb, where flares are most easily distinguished. These are produced by bremsstrahlung in the solar atmosphere's plasma, and by nuclear spectral lines.