SummaryRecent 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, αi are real positive numbers, and Ai are positive coefficients determined from data (for appropriate values of T0 and αi) 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.
Estimate of the Upper Limit on Hot Plasma Differential Emission Measure (DEM) in Non-Flaring Active Regions and Nanoflare Frequency Based on the Mg xii Spectroheliograph Data from CORONAS-F/SPIRIT
