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HESSI - The High Energy Solar Spectroscopic Imager

A solar flare is the rapid release of a large amount of energy stored in the solar atmosphere. During a flare, gas is heated to 10 to 20 million degrees Kelvin (K) and radiates soft X-rays and longer-wavelength emission. Unable to penetrate the Earth’s atmosphere, the X-rays can only be detected from space.

Instruments on Skylab, SMM, the Japanese / US Yohkoh mission and other spacecraft have recorded many flares in X-rays over the last twenty years or so. Ground-based observatories have recorded the visible and radio outputs. These data form the basis of our current understanding of a solar flare. But there are many possible mechanisms for heating the gas, and observations to date have not been able to differentiate between them.

Researchers believe that much of the energy released during a flare is used to accelerate, to very high energies, electrons (emitting primarily X-rays) and protons and other ions (emitting primarily gamma rays). The new approach of the HESSI mission is to combine, for the first time, high-resolution imaging in hard X-rays and gamma rays with high-resolution spectroscopy, so that a detailed energy spectrum can be obtained at each point of the image. This new approach will enable researchers to find out where these particles are accelerated and to what energies. Such information will advance understanding of the fundamental high-energy processes at the core of the solar flare problem.


Gordon D. Holman
Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center


NASA Goddard Space Flight Center, University of Berkeley Space Science Laboratory, Montana State University, University of Alabama at Huntsville, Lawrence Berkeley National Laboratory, National Oceanic and Atmospheric Administration (NOAA)

Observatoire de Paris Meudon

ETHZ, Paul Scherrer Institute

University of Glasgow

The University of Tokyo

The Netherlands

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