Detection of cooler loop-tops in a coronal structure

Abstract

We have obtained off the limb' spectroscopic observations covering a coronal region of 200" x 500" in [Fe x] 6374 A and [Fe xi) 7892 A emission lines simultaneously. Yohkoh soft X-ray images of the sun showed two independent coronal structures in the observed coronal region on the North-West limb. The structure at 20° N, overlying a sunspot group showed an increase in FWHM of both the lines and increase in the intensity ratio of 7892• A to 6374 A line with height above the limb, implying an increase in temperature with height in the coronal loops. Whereas the coronal structure at 32° N showed decrease in the FWHM of both the lines and also decrease in the intensity ratio with height. These variations with height imply that coronal loop-tops were cooler with respect to the foot points in the coronal structure at 32° during the observing period of about 11 hours. The reason for such a behaviour of this structure may be absence of an associated sunspot group, or the magnetic field configuration of the underlying region or coronal structure itself, or the beginning of formation of the structure or the decay phase of the structure.