34th Solar Physics Division Meeting, June 2003
Session 17 Corona III
Poster, Wednesday, June 18, 2003, 3:30-5:00pm, Mezzanine

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[17.09] SOHO-CDS Coronal Loops: Multi-thermal Analysis and Background Subtraction

K. Nasraoui, J.T. Schmelz, C.R. Nevels (University of Memphis)

Our original analysis of the 20 April 1998 and 30 June 1999 SOHO-CDS spectral line data for two loops on the solar limb showed that the plasma was multi-thermal, both along the length of the loop as well as along the line of sight. But this analysis did not consider background subtraction. Here, we add this additional step to our analysis and compare the background-subtracted results with our original results. First, we selected a `background pixel' for each loop. This pixel was inside the loop at a location where no structures contaminated the emission - it was as close to pure diffuse background corona as we could get given the CDS resolution. We then fit the spectral lines in this pixel with Gaussian profiles and determined the intensities. These were then subtracted from the intensities of the 'loop pixels' and the differential emission measure (DEM) curves were reevaluated with these adjusted values. Second, a pair of background pixels was selected for each loop pixel, one inside the loop and one outside the loop. The line intensities were measured at each of these locations, averaged, and subtracted from the associated loop pixel intensity. A third set of DEM curves was constructed. These results will be compared and contrasted with both the original results as well as those from phase one. Does background subtraction make a difference? Of course: the intensities of all lines are smaller so the DEMs are lower in every temperature bin. But does the multi-thermal distribution from the original result simplify to an isothermal one reminiscent of the results seen in TRACE and EIT loops? We're still working on the answer to that question. Solar physics research at the University of Memphis is supported by NASA grants NAG5-9783 and NAG5-12096.


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