On the Thermal Structure of the Flare-Produced Plasma

Summary

Recent flare studies have shown that soft X-ray data are not compatible with simple isothermal models of the source (Herring and Craig, 1973; Craig, 1973; Neupert et al., 1973). With this in mind, the emitting flare plasma has been represented by the temperature-emission measure distribution function, where ζ(T) is the differential emission measure (cm^–3 per 10⁶ K), T is the electron temperature in units of 10⁶ K, T₀ is a low temperature cut-off for the distribution, α_i are real positive numbers, and A_i are positive coefficients determined from data (for appropriate values of T₀ and α_i) by a least squares fitting procedure. Such a distribution is suggested by results obtained by the present author using simple delta-function representations for ζ(T) (with n ≤ 4); these discreet multi-temperature models usually indicate that the emission measure decreases with increasing temperature. Also, as discussed by Brown (1974), a power law distribution for ζ(T) is consistent with the observed bremsstrahlung emission in the hard X-ray (> 10 keV) domain. In attempting to find a suitable form for the differential emission measure, a simple empirical function of the type assumed by Chambe (1971) for active regions was also tried, but the fit, as evidenced by the χ² test was unsatisfactory.