The loop-top region in a solar flare has interesting properties revealed
by Yohkoh soft X-ray imaging: there is often an apparently
over-pressure blob there; and a celebrated "Masuda
source" with an improbably high temperature located distinctly above
the SXT soft X-ray source may also occur. What do we see at lower temperatures?
The EIT instrument on Soho, while not optimized for flare observations
(usually with very poor cadence), can't help seeing some of them because
of its whole-Sun field of view.
These images were taken during the impulsive phase of an M-class flare at the NW limb of the sun. The top left image is from SOHO-EIT and the top right from YOHKOH-SXT. The white pixels in the EIT image are saturated, but we can still derive a limit on the flux. The EIT image shows two bright sources. The source on the left is located just inside the limb at the foot of a loop system. The source on the right is located at the top of the loops.
This loop-top source is very localized and consists of relatively cool (1-2 MK) plasma. The SXT instrument with its higher temperature response shows only weak and diffuse emission at the same location. The EIT intensity contours on the SXT image in the bottom left panel further illustrate this point. The high energy (13-23 keV) HXT L-channel image shows the foot-point source only (black contours in the bottom right panel).
Most interesting are not only the compactness of this source, but its appearance during the early impulsive phase, and its low temperature compared to the "superhot" (~100 MK) loop-top sources reported by Masuda.