Spatial distribution of soft X-ray and EUV emission associated with a chromospheric flare of importance 1B on August 2, 1972

Abstract

Soft-X-ray and extreme ultraviolet spectroheliographs carried by the OSO-7 (Orbiting Solar Observatory) have been used to record the development of XUV emission associated with a flare of importance 1b on August 2, 1972. Spatial resolution was 20″ and spectral resolution was adequate to select emission lines originating within well-defined ranges of electron temperature between 5 × 104 and 30 × 106K. The data show that heating only the pre-existing coronal material adjacent to the flare site cannot account for the soft X-ray emission measure observed during the event. The flare emission originating at Te ⩽ 2.3 × 106K exhibits an impulsive component coincident with an impulsive microwave event. This radiation appears to coincide spatially with Hα radiation emitted at that time and is centered on the neutral line separating magnetic fields of opposite polarity. One soft X-ray-emitting feature, estimated to have an initial electron temperature of 2–10 × 106K, forms during the impulsive phase immediately over the Hα flare. A second, arch-like feature observed at wavelengths near 1.9 A and estimated to have Te≅30 × 106K is located approximately 35000 km above the Hα event. Both regions have lengths of about 27000 km but transverse dimensions small compared to the spatial resolution of the spectroheliograph, i.e., less than 14000 km. The region with highest electron temperature exhibits the greatest stability in position. This region cools to approximately 10 × 106K in 6 to 12 min which is compatible with cooling by conduction to the chromosphere. The best association with a dark surge is found in an emission line of Fe xiv.