Solar Activity Observed in X-Rays and the EUV from OSO 7

Abstract

Since 1971 the Goddard X-ray and EUV spectroheliograph aboard OSO 7 has been measuring the spatial distribution and time-variations of localized temperature and density features in solar active centers and flares. In some cases the sizes, shapes, orientations and locations of emitting plasmas at temperatures ranging from 104 K (Hα) to as high as 2 × 107 K (Fe xxv) have now been measured simultaneously. Our observations of active regions are consistent with the coronal structure being made up of nested systems of arches with footpoints in areas of opposite magnetic polarity. Temperatures seem to increase for arches nearer the center and also towards the top of any given magnetic arch, the innermost loops having the highest temperature gradients. There is also some evidence for electric current flow along such loops. Radiative cooling is significant for the region's hot central core which therefore must be maintained by a more or less continuous injection of energy.A nested arch structure is also indicated for XUV flares of the two-component type, which likewise may require continuous energy input since conduction cooling should otherwise be very rapid. Multiple spikes during the impulsive phase seem to represent the consecutive triggering of different sources within the region and may occur outside of any detectable pre-existing coronal feature. Comparison of spatial distributions at several wavelengths during various stages of flare events provides information on interactions between the wide range of atmospheric levels involved. We have evidence for polarization of about 20% in a number of X-ray bursts, continuing throughout the decay phase. At least for some flares, our measurements seem to contradict the model of electron beams being radially injected into the chromosphere.