The structure of the temperature minimum region in solar flares and its significance for flare heating mechanisms

We analyze Ca ii K-line profiles of one flare and EUV continuum observations of two other flares in order to infer values for the temperature enhancements (over active region values) produced in the upper photosphere around and above the temperature minimum region. The results, obtained through a partial redistribution calculation of the Ca ii K-line profiles and an LTE approach to the continuum observations, show that the flare temperature minimum is depressed some two scale heights below its preflare level, and that substantial temperature enhancements are produced even at this depth. Estimates for the energy release in these photospheric layers are given, and are found to be comparable with that released in chromospheric H and L emission.We then turn our attention to the investigation of possible heating mechanisms which might be responsible for the observed enhancements. Bombardment by both electrons and protons, and irradiation by soft X-rays, are each considered and found to be largely ineffective, due to the large attenuation of flux by photospheric depths, unless new ideas on the precise nature of these mechanisms are invoked, particularly if the same mechanism is also to explain the observed chromospheric emissions. We therefore conclude that it is most likely that some other mechanism must be advocated in order to explain the observed heating. Possibilities for this are (a) heating by EUV radiation, (b) proton beams with low dispersion energy spectra centered around 10–20 MeV, and (c) localized heating at temperature minimum levels.On leave from: Department of Astronomy, The University, Glasgow G12 8QQ, Scotland, United Kingdom.