RHessi

RHessi (Reuven Ramaty High Energy Solar Spectroscopic Imager) was a NASA small explorer mission designed to study the physics of solar flares by performing high-resolution imaging spectroscopy of hard X-rays and gamma rays. Originally known as HESSI (High Energy Solar Spectroscopic Imager), the spacecraft was renamed in honor of the late solar physicist Reuven Ramaty. It was launched in 2002 and operated for more than a decade, providing a comprehensive dataset on energetic processes in the solar atmosphere.

The primary instrument suite centered on nine cryogenically cooled germanium detectors that delivered high spectral resolution across X-ray and gamma-ray energies, coupled with a set of rotating modulation collimators for imaging. The spacecraft was spin-stabilized, and the combination of rotation and collimators allowed RHESSI to produce arcsecond-scale imaging together with detailed energy spectra, enabling simultaneous spatial and spectral analyses of flare emissions.

RHESSI produced several influential results: it revealed spatial and temporal structure of accelerated electron and ion populations, imaged gamma-ray line and positron-annihilation emissions from flares, identified coronal hard X-ray sources distinct from chromospheric footpoints, and constrained flare energetics and particle-acceleration mechanisms. Its observations also advanced understanding of microflares and the role of small-scale energy release in coronal heating.

Mission data and software have been archived for community use and have supported extensive follow-up research. Operational contact was lost in 2018, but RHESSI’s datasets and scientific contributions continue to influence solar flare and high-energy solar physics.