Instruments and Data

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) plays an important role HESPE activity. Indeed RHESSI is the most advanced instrument to date in its ability to provide X-ray and gamma-ray observations from which images and spectra of solar flares at energies between a few keV and a few MeV can be reconstructed. The HESPE belief is that it is now time to put the potential of RHESSI data all at disposal of the European solar, helio and space weather scientists. In fact, to date, X-ray and gamma-ray sets have not been well included into the major effort to understand and predict solar eruptive events. This is due in no small reason to the fact that focusing optics with the required angular resolution and energy range are not currently feasible at X-ray and gamma-ray energies. As a result, indirect collimator-based techniques must be used. These Fourier transform the source to measure visibilities. Although such visibilities measurements are familiar in radio interferometry, X-ray visibilities like the ones provided by RHESSI represent a new development, especially in that their coverage of the spatial frequency plane is very sparse. These visibilities represent the purest, uncalibrated and readily processable data in this kind of astronomy but are very indirectly related to the quantities of physical interest for solar physics. Consequently, relatively complex processing software must be used to recover the images and spectra encoded in the information transmitted from the instrument before the data can be used by scientists. This means that special software has to be created to generate images and spectra required for scientific usage. To date, no well-established computational paradigm has been formulated and validated for the analysis of spatially integrated X-ray spectra, for the reconstruction of spatially integrated averaged electron flux spectra, for the synthesis of X-ray images from visibilities and the reconstruction of electron maps at different time intervals and energy channels.

RHESSI is the latest operating hard X-ray and gamma-ray solar mission, but new and exciting enterprises are in preparation. In the US, the Gamma Ray Imaging Polarimeter for Solar Flares (GRIPS) represents a next generation visibility-based solar X-ray imager that will have its first balloon launch in 2012. On the European side, the Spectrometer/Telescope for Imaging X-rays (STIX) has been proposed for Solar Orbiter which is up for selection in the Cosmic Vision programme of ESA. Like RHESSI, the STIX imaging techniques is based on the measurement of visibilities. Therefore RHESSI concepts will represent the inescapable scientific background for both GRIPS and STIX and the methodologies developed by HESPE will provide the starting point for the processing of their observations.