Global Energetics of Solar Flares. VIII. The Low-energy Cutoff

One of the key problems in solar flare physics is the determination of the low-energy cut-off: the value that determines the energy of nonthermal electrons and hence flare energetics. We discuss different approaches to determine the low-energy cut-off in the spectrum of accelerated electrons: (i) the total electron number model, (ii) the time-of-flight model (based on the equivalence of the time-of-flight and the collisional deflection time), (iii) the warm target model of Kontar et al., and (iv) the model of the spectral cross-over between thermal and nonthermal components. We find that the first three models are consistent with a low-energy cutoff with a mean value of ≈10 keV, while the cross-over model provides an upper limit for the low-energy cutoff with a mean value of ≈21 keV. Combining the first three models we find that the ratio of the nonthermal energy to the dissipated magnetic energy in solar flares has a mean value of q E = 0.57 ± 0.08, which is consistent with an earlier study based on the simplified approximation of the warm target model alone (q E = 0.51 ± 0.17). This study corroborates the self-consistency between three different low-energy cutoff models in the calculation of nonthermal flare energies.