The Physical Nature of the Loop‐Top X‐Ray Sources in the Gradual Phase of Solar Flares

Abstract

We have analyzed Yohkoh soft and hard X-ray images of 36 flares, primarily to study the loop-top source that often prevails in these wavelengths during and following the impulsive phase. There are typically two patterns for the location of the low-energy (15-30 keV) hard X-ray (HXR) source with respect to the soft X-ray (SXR) loop. In a quarter of the flares, the HXR source lies in an extended structure separate from the brightest SXR loop. In other flares, the HXR source appears to be part of the same bipolar structure as the SXR loop, but its centroid is often displaced from the SXR loop-top source. The fact that the HXR source is not cospatial with the SXR source may reflect the presence of a distinct hotter structure. According to Yohkoh X-ray emission-line spectroscopy, the ~20 MK plasma accounts for only a fraction of the HXR counts. The temperature maps obtained from the SXR broadband photometry occasionally reveal high-temperature areas outside the bright loop, but they also tend to be displaced from the HXR source, indicating that they do not represent the superhot (30 MK) plasma. We discuss possible distributions of plasma of different temperatures that could be consistent with the data.