OSCILLATIONS IN THERMALLY-CONDUCTING POLYTROPIC ATMOSPHERES IN THE PRESENCE OF A VERTICAL MAGNETIC FIELD

Abstract

We have investigated the normal modes of subadiabatic and superadiabatic polytropic atmospheres with constant vertical magnetic field and constant thermal conductivity. In the subadiabatic case, we found the lowest, third and fifth modes were always damped, however overstability was detected in the second and fourth modes at low values of the background magnetic field. In the superadiabatic case, instability was detected in several modes, however the effect of the change from sub to superadiabatic had little effect on the frequencies of the modes. The introduction of a variety of boundary conditions varying the degree of thermal and mechanical isolation altered the decay rates of the modes from moderately damped with rigid boundary conditions toward zero decay with the less restrictive conditions, again with essentially no effect on the oscillation frequencies. In both types of atmosphere, modes with periods in the 3-min and 5-min bands were present with magnetic fields around 3000 G, whereas only the 5-min modes were present at 2000 G.