Polytropic indices of solar flares

The polytropic indices of solar flares are studied by means of a semiempiri cal model of flare thermodynamic structure that utilizes soft X-ray measure ments of the Sun and two scaling laws: (1) relating the product of plasma p ressure and temperature to spatial scale; and (2) relating spatial scale to flare duration. The model and the main summary results of a survey of appr oximate to 1100 flares were described in two previous papers, cited herein. The principal result of the present study is that the polytropic indices, derived from the computed macro-thermodynamic structure of individual flare s, are distributed about a value of 1.66 +/- 0.02 for moderate flares and t rend uniquely toward the value of the ratio of specific heats of a non-rela tivistic ideal gas, 5/3, for the most intense flares. The main conclusion i s that magnetically contained, mostly thermalized plasma manifested in soft X-rays responds quasi-adiabatically to large energy fluxes that are absorb ed and released by the flare in a few tens of minutes. This finding is cons istent with the hypothesis that energy deposition is exactly balanced by co nductive and (mostly) radiative losses over relatively short time intervals .