On the subadiabatic magnetohydrodynamic overstability in background polytropes

Abstract

The instability properties are studied of nonadiabatic MHD oscillations in background polytropes permeated by a uniform vertical magnetic field with the effects of thermal conduction, viscosity, and resistivity included. In particular, the very existence of subadiabatic MHD overstabilities for the quasi-fast modes over a wide range of field strengths are shown, and it is demonstrated that their presence originates from the subadiabatic acoustic overstability. The magnetic Lamb mode based on the previous results is identified. It is also shown that, in the strong-field limit, isothermal oscillations are neutral. Since the convective stability is enhanced by the presence of vertical magnetic field, MHD overstabilities can thus occur in a wide range of temperature gradients (subadiabatic as well as superadiabatic) while the system retains convective stability. The growth rates of oscillations are larger for steeper adverse temperature gradients. These results are important for understanding umbral oscillations and the heat transport process in sunspots, and for determining possible mechanisms to excite rapid oscillations of magnetic Ap stars.