Imaging with RHESSI

Abstract

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is a NASA SMall EXplorer (SMEX) mission to study the acceleration and transport of high-energy electrons and nuclei in solar flares. This is done using high spatial (2.3 arcsec) and high spectral (~1 keV) resolution imaging spectroscopy of X-rays and gamma rays between 3 keV and 17 MeV. Such an energy range includes ~10-30MK thermal emission, non-thermal hard X-ray bremsstrahlung from accelerated electrons, and gamma-ray lines from accelerated nuclei. RHESSI's imaging is based on a set of rotating modulation collimators. Each of the nine subcollimators uses a pair of widely separated (1.55m) grids, mounted on a rotating spacecraft. Each grid in turn consists of a large number of parallel, equispaced, X-ray-opaque slats. A corresponding set of nine high-resolution cooled germanium detectors determines the energy and arrival time of each detected photon. As the spacecraft rotates at ~15 rpm, the grid pairs time-modulate the detected X-ray flux in a manner sensitive to the morphology and location of the X-ray source(s). Post-analysis then reconstructs the image from the set of time-modulated light curves. This paper reviews the RHESSI imaging instrumentation, data analysis approach, imaging concept and early indications of in-flight performance.