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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:
∫ ɛmax Ekin = F(ɛ)ɛdɛ , (3 ) ɛmin
where ɛ 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 δ, the emitted bremsstrahlung by a thick target is also approximately a power-law in photon energy with index γ = δ − 1. As δ > 2 in all observations, the integral in Equation (3View Equation) depends strongly on the low-energy cut-off ɛmin. 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. (20022005), 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., 2006Kašparová 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, 2005Jump To The Next Citation Point) is unconfirmed and may be an upper limit.
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