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Abstract: The temperature distribution along coronal loops provides an important clue for solving the coronal heating problem. Recent analysis, however, has produced conflicting results. Here, we analyze in detail one component of this analysis – the effect of background subtraction on the temperature of loops observed with the Transition Region and Coronal Explorer (TRACE). Specifically, we selected 10 coronal loops that were visible in the TRACE 171Å and 195Å passbands. We chose between 20 and 30pixel along each loop and background pixels to correspond with the loop pixels. Temperature analysis was done in three different ways: (1) standard TRACE analysis of the loop pixels with no background subtraction; (2) constant background subtraction for each TRACE image; (3) pixel pair background subtraction. Each method produced a temperature estimate for the selected pixels. We find that a flat line is an excellent fit to the temperature results – the analysis indicates that the temperature of the loop is uniform along the length visible by TRACE. However, if we select random pixels and plot the temperature results in the same way, these pixels indicate that the temperature of this “structure” is also uniform. We conclude therefore, that in the cases considered here, the image ratio analysis does not produce a physically meaningful value of plasma temperature; in addition, background subtraction makes no significant difference to the temperatures results. [Copyright &y& Elsevier]

Abstract: The solar loop that formed off the northeast limb of the Sun on 1999 November 6 (a.k.a. the Cinderella loop) is one of the few examples of a loop on the limb observed with all three of the following imaging instruments: the Transition Region and Coronal Explorer (TRACE), the SOHO Extreme-ultraviolet Imaging Telescope (EIT), and the Yohkoh Soft X-ray Telescope (SXT). In this project we investigate the temperature differences that result when examining the Cinderella loop with one instrument compared with another. For example, what temperature differences result from the increased spatial resolution between the two EUV imagers? More specifically, given that TRACE and EIT have almost identical temperature response to coronal plasma, does the different spatial resolution of TRACE (with 0.5″ pixels) and EIT (with 2.6″ pixels) produce statistically different results? We find that the answer is no, and that our results do not change after background subtraction. In addition, the spatial resolution of EIT and SXT is similar, but the temperature responses of the two instruments are quite different. The two instruments do not seem to be viewing the same loop strands, and the plasma temperature differences are significant. [Copyright &y& Elsevier]