Observational constraints on coronal heating models using coronal diagnostics spectrometer and soft X-ray telescope data

We have constructed a multithermal differential 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 Yohkoh soft X-ray telescope. The temperature distributions are clearly inconsistent with isothermal 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 filter ratios to calculate loop temperature profiles. The differences between their results and ours could be attributed to pixel size-CDS pixels are larger, and more "contaminating" 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 filter instruments. The results gave temperature gradients that were almost flat, indicating that the larger CDS pixel size is not the cause of the discrepancy. A significant 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 off with loop height producing a relatively constant pressure, and radiative losses were greater than, but did not overwhelm, conductive losses.