Analysis of a multi-wavelength time-resolved observation of a coronal loop

Aims.We analyze a well-defined loop system detected in a time-resolved observation in several spectral bands to characterize the structure and evolution of coronal loops. Methods.The dataset includes simultaneous sequences of images in the 171 Å, 195 Å and 284 Å filter bands of TRACE, and in one filter of Yohkoh/SXT, with a time coverage of about 2.5 h, and two rasters taken with SoHO/CDS in twelve relevant lines, forming between log T ≈ 5.4 (O V 629 Å) and log T ≈ 6.4 (Fe XVI 360 Å). The loop is initially best seen in the TRACE 195 Å filter band, with some correspondence to the simultaneous SXT images, and later in the 171 Å filter band, with good correspondence to the CDS raster images in the lines with formation temperature around log T ≈ 6.0-6.1. We have taken as pixel-by-pixel background the latest TRACE, Yohkoh and CDS images where the loop has faded out. We examine the loop morphology evolution, the light curves, the TRACE filter ratio distribution and evolution, the images and emission measure from the CDS spectral lines.Results.Our analysis detects that, after background subtraction, the emission along the loop and its evolution are non-uniform, especially in the 171 Å filter band, and that the TRACE 195/171 filter ratio has a moderately non-uniform distribution along the loop and evolves over time. Both the light curves and the filter ratio evolution indicate a globally cooling loop. Relatively hot plasma may be present at the beginning while, during the first CDS raster, the data indicate a rather moderate thermal structuring of the loop.Conclusions.Our analysis supports a coherent scenario across the different bands and instruments, points out difficulties in diagnostic methods and establishes quantitative basis for detailed forward modeling.