A Discussion on the physics of the solar atmosphere - A.t.m. observations on the X u.v. emission from solar flares

Abstract

Far ultraviolet spectroheliograms (190-465 A) and ultraviolet spectra (900-1900 A) of selected flares obtained on board Skylab by two instruments of the Naval Research Laboratory are presented. Time sequences of three events are discussed in detail; they are compared with simultaneous X-ray measurements from the N.R.L.’s Solrad 9 and from the OSO-7 X-ray spectrometer of the University of California, San Diego. In all events, where proper observational coverage was obtained, a small kernel seems to be the source of the prime energy release of a flare. The size of this kernel is determined by the spatial resolution of the spectroheliograph, which is approximately 2". Densities in this kernel exceed 3 x 1012 cm-3. Ion temperatures in a subflare kernel are determined to be larger than 20 MK. It is very likely, that these temperatures are much higher in the flare kernel of more energetic events. Hot clouds of coronal gas at 20 MK surround the kernel. These clouds are trapped below high magnetic structures, and subsequently seem to be the energy reservoir for heating the post flare loops. During the explosive phase, strong Doppler broadening can be seen, especially in transition zone lines of the brightened plages. These can be interpreted as impact of high energetic particles guided down to the solar surface along the loops. The observations suggest that high speed mass motions have their origin in the exploding flare kernel.