The Effects of Stratification on Oscillating Coronal Loops

Abstract

Recent observations by the Solar and Heliospheric Observatory (SOHO) and the Transition Region and Coronal Explorer (TRACE) have confirmed previous theoretical predictions that coronal loops may oscillate. These oscillations and their damping are of fundamental importance, because they can provide diagnostics of the coronal plasma. In the present paper, we perform numerical hydrodynamic calculations of a one-dimensional loop model to investigate the effects of stratification on damping of longitudinal waves in the hot coronal loops observed by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) on board the SOHO satellite. In particular, we study the dissipation by thermal conduction and by compressive viscosity of standing slow magnetosonic disturbances in loops of semicircular shape. For the parameter regime that characterizes the SUMER hot loops, we find that stratification results in a similar to10%-20% reduction of the wave-damping time compared to the nonstratified loop models because of increased dissipation by compressive viscosity due to gravity. We show that temperature oscillations are more strongly dissipated by thermal conduction, while density and velocity waves are mostly damped by compressive viscosity. However, the decay time of the oscillations is always governed by the thermal conduction timescale. The scalings of the decay time with wave period, temperature, and loop length all point toward higher dissipation rates in the stratified, hotter loops because of the increased effects of thermal conduction and compressive viscosity.