The effect of magnetic field on the damping of slow waves in the solar corona 

Timothy James Duckenfield Submitted: 20201123 05:24
Context: Slow magnetoacoustic waves are routinely observed in astrophysical plasma systems such as the solar corona, and are usually seen to damp rapidly. As a slow wave propagates through a plasma, it modifies the equilibrium quantities of density, temperature, and magnetic field. In the corona and other plasma systems, the thermal equilibrium is comprised of a balance between continuous heating and cooling processes, the magnitudes of which vary with density, temperature and magnetic field. Thus the wave may induce a misbalance between these competing processes. Its back reaction on the wave has been shown to lead to dispersion, and amplification or damping, of the wave.
Aims: This effect of heating/cooling misbalance has previously been studied in the infinite magnetic field approximation, in a plasma whose thermal equilibrium comprises of optically thin radiative losses and fieldaligned thermal conduction, balanced by an (unspecified) heating process. In this work we extend this analysis by considering a nonzero beta plasma. The importance of the effect of magnetic field in the rapid damping of slow waves in the solar corona is evaluated, and compared to the effects of thermal conduction.
Methods: A linear perturbation under the thin flux tube approximation is considered, and a dispersion relation describing the slow magnetoacoustic modes is found. The dispersion relation’s limits of strong nonadiabaticity and weak nonadiabaticity are studied. The characteristic timescales are calculated for plasma systems with a range of typical coronal densities, temperatures and magnetic field strengths.
Results: The number of timescales characterising the effect of misbalance is found to remain at two, as with the infinite magnetic field case. In the nonzero beta case, these two timescales correspond to the partial derivatives of the combined heating/cooling function with respect to constant gas pressure and with respect to constant magnetic pressure. The predicted damping times of slow waves from thermal misbalance in the solar corona are found to be of the order of 10–100 minutes, coinciding with the wave periods and damping times observed. Moreover the slow wave damping by thermal misbalance is found to be comparable to the damping by fieldaligned thermal conduction. The change in damping with plasmabeta is complex and depends on the coronal heating function’s dependence upon the magnetic field in particular. Nonetheless we show that in the infinite field limit, the wave dynamics is insensitive to the dependence of the heating function on the magnetic field, and this approximation is found to be valid in the corona so long as the magnetic field strength is greater than approximately 10G for quiescent loops and plumes, and 100G for hot and dense loops.
Conclusions: Thermal misbalance may damp slow magnetoacoustic waves rapidly in much of the corona, and its inclusion in our understanding of slow mode damping may resolve discrepancies between observations and theory relying on compressive viscosity and thermal conduction alone.
Authors: T. J. Duckenfield, D. Y. Kolotkov, and V. M. Nakariakov
Projects: None

Publication Status: A&A (accepted)
Last Modified: 20201125 12:04




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