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last change 2007 March 23, H. Önel
Solar Radio Physics - Research Branch I de
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Space Missions - RHESSI

Reuven Ramaty High Energy Solar Spectroscopic Imager

RHESSI is a NASA Small Explorer Mission dedicated to the study of particle acceleration and explosive energy release in solar flares. The satellite was launched by a Pegasus rocket into a low earth orbit on February 5th, 2002. The single instrument on board is an imaging spectrometer operating in the hard X-ray and gamma ray regime. For the first time, simultaneous, high resolution imaging and spectroscopy of solar flares from 3 keV X-rays to 17 MeV gamma rays with high time and energy resolution can be obtained. The mission was designed and carried out by teams from the United States (UCB Berkeley and Goddard Space Flight Center) and Europe (Paul Scherrer Institut and ETH Zurich). All data are public (HEDC).

A solar flare is the rapid release of a large amount of energy stored in the solar atmosphere. In less than an hour, an amount of energy sufficient to cover the world's energy need for more than 10000 years is liberated. A significant fraction of this energy is contained in energetic electrons. These electrons generate hard X-ray radiation when interacting with the solar atmosphere. Thus, X-ray spectra and images yield crucial information on
particle acceleration processes in flares. Since energetic electrons are also creating radio radiation, radio observations such as those conducted by our group offer an important complementary approach to the problem of particle acceleration.

Germany is involved in the RHESSI mission in a threefold manner

  • the ground station Weillheim of the German Space Operation Center (GSOC), operated by the German Space Agency DLR, provides additional data downlink capacity for RHESSI whenever the need arises. This represents a service to NASA of 74000 € per year.

  • the radio data of the Observatory for Solar Radioastronomy in Tremsdorf are provided to the RHESSI data center.

  • the Group for Solar Radioastronomy at the AIP participates in the joint analysis of RHESSI- and radio daten.

The practical relevance of this area of research is the study of solar activity and its influence on Earth and mankind. Termed "space weather".

X-ray image of the large flare of October 28, 2003. The blue contour lines show the non-thermal chromospheric footpoints (100-300 keV), red contours show the thermal coronal X-ray source (12-25 keV). The green image in the backround is an extreme UV image of the flare (19.5 nm, TRACE).


X-ray spectrum of the flare of  October 28, 2003. The energy range reaches from 15 to 300 keV. The photon flux (blue) can be expressed by a thermal component at low energies (red) and a non-thermal power law (green). From these fits important physical parameters, e.g. the temperature of the thermal plasma or the flux of non-thermal electrons can be deduced.