Coronal oscillations and internal loop structure

Abstract

The transverse oscillations of a coronal magnetic loop whose ends are rigidly fixed in the photosphere are investigated. The loop is assumed to be inhomogeneous and to comprise two internal structural components: a central dense hot filament and a coaxial rarefied shell around it, in which the plasma density is lower than the density of the surrounding coronal plasma. The Alfven speed in the shell, VAm, is higher than that in the central filament and in the corona: VAm>VAe>VAi. It is shown that, in the perfectly conducting plasma approximation, such a loop can generate two fast magnetosonic waves. The higher velocity wave is emitted in a radial direction, thereby ensuring the effect of the radiative damping of oscillations at the frequency of the m=1 cylindrical mode. The results of calculating the effect of radiative losses show that, for typical loop parameters (corresponding to those of the loops observed in the solar corona), the quality factor of oscillations may be fairly low (Q≈40). Under the conditions in question, the second (lower velocity) fast magnetosonic wave is not emitted (in contrast to the first) but rather turns out to be trapped in the magnetic flux tube.