Observational Constraints on Coronal Heating Models Using Coronal Diagnostics Spectrometer and Soft X‐Ray Telescope Data

Abstract

We have constructed a multithermal di †erential emission measure distribution for several pixels along a relatively isolated coronal loop on the southwest limb of the solar disk using spectral line data from the Solar and Heliospheric Observatory coronal diagnostics spectrometer (CDS) and broadband data from the Y ohkoh soft X-ray telescope. The temperature distributions are clearly inconsistent with isother-mal plasma along either the line of sight or the length of the loop. These conclusions disagree with some recent results that used an isothermal approximation derived from narrowband Ðlter ratios to calculate loop temperature proÐles. The di †erences between their results and ours could be attributed to pixel sizeÈCDS pixels are larger, and more "" contaminating ÏÏ material would be expected along the line of sight. To test this idea, we used CDS iron line ratios from our data set to mimic the isothermal results from the narrowband Ðlter instruments. The results gave temperature gradients that were almost Ñat, indicating that the larger CDS pixel size is not the cause of the discrepancy. A signiÐcant intensity was measured for the O V line about 8 scale heights above the limb. In order to account for these observed values, the cool end (below 1 mK) of the emission measure curves must turn up again, even for the pixels at the top of the loop. Plasma densities fell o † with loop height producing a relatively constant pressure, and radiative losses were greater than, but did not overwhelm, conductive losses.