Global Energetics of Solar Flares. Iii. Nonthermal Energies

This study entails the third part of a global flare energetics project, in which Ramaty High-Energy Solar Spectroscopic Imager ( RHESSI ) data of 191 M and X-class flare events from the first 3.5 years of the Solar Dynamics Observatory mission are analyzed. We fit a thermal and a nonthermal component to RHESSI spectra, yielding the temperature of the differential emission measure (DEM) tail, the nonthermal power-law slope and flux, and the thermal/nonthermal cross-over energy e co . From these parameters, we calculate the total nonthermal energy E nt in electrons with two different methods: (1) using the observed cross-over energy e co as low-energy cutoff, and (2) using the low-energy cutoff e wt predicted by the warm thick-target bremsstrahlung model of Kontar et al. Based on a mean temperature of T e = 8.6 MK in active regions, we find low-energy cutoff energies of for the warm-target model, which is significantly lower than the cross-over energies . Comparing with the statistics of magnetically dissipated energies E mag and thermal energies E th from the two previous studies, we find the following mean (logarithmic) energy ratios with the warm-target model: , , and . The total dissipated magnetic energy exceeds the thermal energy in 95% and the nonthermal energy in 71% of the flare events, which confirms that magnetic reconnection processes are sufficient to explain flare energies. The nonthermal energy exceeds the thermal energy in 85% of the events, which largely confirms the warm thick-target model.