4.1 Non-thermal electron energy
White light and soft X-rays probably do not represent primary forms of energy, but result from kinetic
energy, Ekin, transported to the chromosphere by particles:
where 
is the electron energy and F is the electron flux distribution per energy unit impinging on the
target. If the accelerated electrons have a power-law distribution with a spectral index of 
,
the emitted bremsstrahlung by a thick target is also approximately a power-law in photon
energy with index 
. As 
in all observations, the integral in Equation (3)
depends strongly on the low-energy cut-off 
. It is difficult to observe as the emission of the
non-thermal electrons usually outshone by the emission of the thermal plasma around 10 keV. Only
recently, with the 1 keV spectral resolution of RHESSI, it has become possible to reconstruct the
electrons’ energy distribution at low energies (
10 keV). Structure in the reconstructed
electron distribution was reported by Kontar et al. (2002, 2005), indicating that spectral features
may indeed be observed. It suggested that the inversion of photon spectra into electron energy
distributions is possible (Piana et al., 2003). However, several effects distort the photon spectrum
around 10 keV, in particular reflection of X-rays at the chromosphere, termed albedo effect
(Kontar et al., 2006; Kašparová et al., 2007), free-bound emission and pulse pile-up in the
detectors. Thus the low-energy turn-over of the electron distribution measured and reported to be
at 20 – 40 keV or below (Saint-Hilaire and Benz, 2005
) is unconfirmed and may be an upper
limit.
)
depends strongly on the low-energy cut-off 
