AAS 204th Meeting, June 2004
Session 56 Solar Active Regions and Coronal Heating
SPD Poster, Wednesday, June 2, 2004, 10:00am-7:00pm, Ballroom

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[56.07] SOHO-CDS Coronal Loops: More deeply into Background Subtraction

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

Analyzing two CDS data sets from 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. Background subtraction is the latest step in our analysis. We chose three loop pixels: one at the south footpoint, one on the south leg, and one at the top of the loop. We also selected a pair of background pixels associated with each loop pixel: one inside the loop and one outside. At these locations there were no structures contaminating the emission, and it was as close to pure diffuse background corona as we could get given the CDS resolution. We then fit the spectral lines in these pixels with Gaussian profiles and determined the intensities. Both background intensities were averaged and subtracted from the associated loop pixel intensity, and the differential emission measure curves were reevaluated with these adjusted values. These two loops have several things in common – an event occurred several hours earlier, triggering activity in the general area. Both loops are relatively isolated at the time of the CDS observations, but a companion loop emerges near the primary target. There are also properties that are quite different. Our analysis indicates that the loop observed on 1998 April 20 is hotter and stable; it formed in place as hot plasma filled the magnetic flux tube from the southern footpoint. The 1999 June 30 loop is cooler and dynamic; it emerges from an unresolved knot of activity and grows substantially over the course of the next several hours. It does not appear that these two loops are simply different `snapshots’ of the same overall time evolutionary process, i.e., a cool loop evolving to a hotter phase of vise versa. Solar physics research at the University of Memphis is supported by NASA grants NAG5-9783 and NAG5-12096.

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