3.4 Hard X-rays
During the flare flash phase, usually a few minutes after the soft X-ray burst, another kind of apparently
non-thermal radiation (as first noted by De Jager, 1965) is observed for many but not all flares: hard
X-rays with energies of tens of keV up to a few MeV in extreme cases (see, e.g. Garcia, 2004). This
radiation is due to electrons accelerated to very high energies right at flare onset which then hit the
atoms of the lower denser layers and produce a bremsstrahlung continuum in the form of hard
X-rays.
Since 2002, the new RHESSI mission (Ramaty and Mandzhavidze, 2000; Lin et al., 2003
; Hurford
et al., 2003), provides for the first time high-resolution imaging and spectroscopy from soft X-rays to
Gamma-rays. New results are being published in a fast pace (see the 
ADS for authors like Lin, Krucker,
Hudson, Hurford and others).
