A survey of soft X-ray limb flare images: the relation between their structure in the corona and other physical parameters.

A survey of soft X-ray limb flare images obtained by the S-054 experiment on board Skylab is presented. From a morphological point of view, limb flares have been subdivided into three groups: (A) flares characterized by compact loop structures; (B) flares with a pointlike appearance; (C)flares with large and diffuse systems of loops. The significance of this subdivision is investigated with the aid of plasma parameters determined by combining the S-054 spatially resolved observations with full-disk data obtained by the Solrad 9 satellite. From a comparison of the spatial structure with physical parameters such as height, volume, energy density, and characteristic times, and from the correlation with white-light coronal transients and H..cap alpha.. active prominences, the existence of two physically distinct classes of flares is established: class I, which consists of both morphological groups A and B, and class II, which comprises only events of group C.