The Effect of Radiative Cooling on Coronal Loop Oscillations

Coronal loops that exhibit kink-mode oscillations have generally been assumed to have a constant density and temperature during the observed time interval. Analyzing their intensities in an EUV wave band, however, clearly shows that their brightness varies in a way that is consistent with a temperature cooling through the EUV passband, which limits their detection time, observed damping time, and number of observable periods. We study kink-mode oscillations of eight loops observed during the so-called harmonica event on 2001 April 15, 21:58-22:27 UT in the 171 Å band. We find loop densities of n e = (1.4 ± 0.6) × 10 9 cm −3 , loop widths of w = 2.0 ± 2.6 Mm, and e -folding cooling times of τ cool = 17 ± 7 minutes, when they cool through the peak temperature T = 0.95 MK of the 171 Å band. We conclude that oscillations of a single loop cannot be detected longer than 10-20 minutes in one single filter and appropriate light curve modeling is necessary to disentangle the subsequent oscillation phases of multiple near-cospatial loops.