Acoustic Waves in a High-Temperature Plasma III. Two-Periodic Disturbances

Abstract

The aim of this work is to study the dispersion of acoustic waves in the rarefied high-temperature plasma of the solar corona and its role in propagating intensity disturbances (PDs) occurring in this region. We believe that a multi-periodicity in wavelet spectra, recorded when observing PDs in coronal holes and loops, is due to a result of the combined effect of dispersion and damping of compression waves. Observations show the presence of continuous spectra, where periods are distinguished by suitable maxima. The shape of the spectra is characteristic of localized disturbances. This study is based on our previously proposed clear model of nonadiabatic waves in high-temperature plasma, which takes into account the properties of thermal conduction, radiative cooling, and constant heating. Thermal conduction forms a local minimum of group speed, separating groups of waves with short and long periods. Waves of the first group have strong dispersion and weak damping while waves of the second group have the opposite properties. This effect leads to the fact that the initial pulse disturbance eventually acquires a form in which the indicated groups are clearly separated. Two maxima appear in the wavelet spectrum which determine the short period \(P_{s}\) and the long period \(P_{l}\). Two groups of waves with dominant periods propagate at the same speed, which is less than the sound speed. We assume that the form of PDs can indeed arise in the corona under the influence of small-scale disturbances in the lower atmosphere. The speed of the observed disturbances is also less than the sound speed. This is usually explained by the projection effect, but can also be explained by the fact that PDs propagate with a group speed. The time signals in the corona recorded by observing PDs bear little resemblance to the superposition of just a few harmonic components. The observed periods are not regular, they can change several times during the entire observation time (often 2 – 3 hours). We propose to consider PDs as a sequence of independent pulse disturbances. Two periods occur only in a given pulse, the duration of which is on the order of a long period \(P_{l}\), often 20 – 30 minutes. They may change in the next pulse, since they depend on the length of the initial pulse formed at the boundary of the corona and the lower atmosphere.