Strong and weak damping of slow MHD standing waves in hot coronal loops

Abstract

Slow magnetohydrodynamic (MHD) standing wave oscillations in hot coronal loops for both strong (i.e. τd/P ∼ 1) and weak (i.e. τd/P ≥ 2) damping are investigated taking account of viscosity, thermal conductivity and optically thin radiation. The individual effect of the dissipative terms is not sufficient to explain the observed damping. However, the combined effect of these dissipative terms is sufficient to explain the observed strong damping, as well as weak damping seen by SUMER. We find that, the ratio of decay time (τd) and period (P) of wave, i.e., τd/P (which defines the modes of damping, whether it is strong or weak) is density dependent. By varying density from 108 to 1010 cm-3 at a fixed temperature in the temperature range 6 - 10 MK, observed by SUMER, we get two sets of damping: one for which τ d/P ∼ 1 corresponds to strong damping that occurs at lower density and another that occurs at higher density for which τd/P ≥ 2 corresponds to weak damping. Contrary to strong-damped oscillations, the effect of optically thin radiation provides some additional dissipation apart from thermal conductivity and viscosity in weak-damped oscillations. We have, therefore, derived a resultant dispersion relation including the effect of optically thin radiation. Solutions of this dispersion relation illustrate how damping time varies with physical parameters of loops in both strong and weak damping cases. © Springer 2006.