Acoustic Waves in a High-Temperature Plasma II. Damping and Instability

Abstract

In this article we study the properties of acoustic waves in the rarefied high-temperature plasma of the solar corona, assuming that the heating and cooling of the plasma has a well-defined description. We consider a constant heating function supposing that the heating processes are generally established. For the radiative-loss function, a number of values are taken, which have been found using the CHIANTI code. On their basis, an analytical expression of the function in the form of a cubic interpolation has been worked out. We analyze the dispersion relation for linear acoustic waves. The heating and cooling function, introduced along with the classical expression of the thermal conductivity, allows us to obtain some specific results about their properties. In other words, a model of non-adiabatic acoustic waves with field-aligned thermal conduction, CHIANTI-based radiative cooling and constant heating function is constructed. Using the available observational data on compression waves, we can set the problem of finding the parameters of the coronal plasma. The model allows to specify the temperature range at which the thermal instability of waves is possible and to draw some conclusions about their damping. The coronal temperatures considered can be divided into intervals from 0.5 to 0.98 MK and from 4.57 to 8.38 MK, where the radiation function increases, and intervals from 0.98 to 4.57 MK and from 8.38 to 10 MK, where the radiation function decreases. With constant heating, at large wavelengths, acoustic waves can be unstable in the decreasing interval from 1.38 to 3.15 MK. In the increasing intervals, they may have a zero real part of the oscillation frequency and thus become non-propagating, also subject to a large wavelength. In some cases, the plasma density has a significant effect on the damping of acoustic oscillations due to heating and cooling. A change in density within the same order can lead to the fact that the heating and cooling effects prevail over the effect of thermal conductivity on long-wave perturbations.