Multithermal Analysis of a SOHO/CDS Coronal Loop

The observations from 1998 April 20 taken with the Coronal Diagnostics Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) of a coronal loop on the limb have shown that the plasma was multithermal along each line of sight investigated, both before and after background subtraction. The latter result relied on emission measure (EM) loci plots, but in this Letter, we used a forward-folding technique to produce differential emission measure (DEM) curves. We also calculate DEM-weighted temperatures for the chosen pixels and find a gradient in temperature along the loop as a function of height that is not compatible with the flat profiles reported by numerous authors for loops observed with the EUV Imaging Telescope (EIT) on SOHO and the Transition Region and Coronal Explorer (TRACE). We also find discrepancies in excess of the mathematical expectation between some of the observed and predicted CDS line intensities. We demonstrate that these differences result from well-known limitations in our knowledge of the atomic data and are to be expected. We further show that the precision of the DEM is limited by the intrinsic width of the ion emissivity functions that are used to calculate the DEM, which for the EUV lines considered is of the order d log T = 0.2-0.3. Hence, we conclude that peaks and valleys in the DEM, while in principle not impossible, cannot be confirmed from the data.