Skylab flare associated with a hard x-ray burst

We have studied the Skylab flare of l974 January 21 as a clear example of impulsive EUV emission associated with a hard X-ray burst. The flare images in He II and transition-zone lines show two bright patches of emission (A and B)= which appear to be the footpoints of a large, cool loop. A third source of high temperature emission is present in Fe XXVand Ca XVIII images. This source lies outside the cool loop defined by the He II emission patches, but adjacent to one of them. It is possible that two interacting loops are involved, such as a small, hot loop beside a larger one. The temporal behavior of emission patches A and B suggest that they are the impulsive EUV source regions associated with the hard X-ray burst, while the hot plasma represents the soft X-ray, gradual component of the flare. The emission measure distributions of the hot plasma and of the brightest He II patch are derived and compared. Considered together, these plots do not appear to represent the conductive interface between a hot region and a cool foot point region which is impulsively heated by the hot region. The radiative loss rate from emission patch B is within a factor of 4 of the estimated thick target energy flux injected at electron energies greater than 25 keV. This suggests that collisional losses of fast electrons are the main energy source for the impulsive He II emission regions