Special features of the ion-acoustic oscillation instability in the transition region of a quiet solar atmosphere are examined. A model of electron distribution function, which corresponds to the heat flux, under condition of absence of a particular beam, is analyzed. It is shown that the heat flux-related anisotropy of the distribution function is sufficient for the achievement of the threshold of ion-acoustic instability in the transition region. A characteristic value of the electric field in ion-acoustic turbulence is estimated.
Natural oscillations of the entire nonisothermal solar atmosphere are analysed. Such oscillations are probably related to the acoustic gravity wave. Analytical and numerical solutions describing acoustic gravity wave perturbations in the entire solar atmosphere are studied. Based on the model temperature profile, we find the spatio-temporal dependence for the linear acoustic gravity wave characteristics. The performed analysis using the approximate local method showed a possibility for the existence of instability of the acoustic gravity wave in the nonisothermal atmosphere. Such an instability develops at frequencies and spatial scales typical for the vertical five-minute oscillation of the solar atmosphere.
Within the framework of model calculations the possibility of occurrence of the ion-acoustic oscillation instability in a plasma without current and particle fluxes, but with an anisotropic distribution function, which corresponds to heat flux is shown. The model distribution function was selected taking into account the medium conditions. The increment of ion-acoustic oscillation is investigated as functional of the distribution function parameters. The threshold condition for the anisotropic part of the distribution function, under which the build-up of ion-acoustic oscillation with the wave vector opposite to the heat flux begins is studied. The critical heat flux, which corresponds to the threshold of ion-acoustic instability, is determined. For the solar conditions, the critical heat flux proved to be close to the heat flux from the corona into the chromosphere on the boundary of the transition region. The estimations show that outside of active regions and even in active regions with weaker magnetic fields ion-acoustic turbulence can be responsible for the formation of the sharp temperature jump. The generalized Wiedemann-Franz law for a non-isothermic quasi-neutral plasma with developed ion-acoustic turbulence is discussed. This law determines the relationship between electrical and thermal conductivities in a plasma with well-developed ion-acoustic turbulence. The anomalously low thermal conductivity responsible to the formation of high temperature gradients in the zone of the temperature jump is explained. The results are used to explain some properties of stellar atmosphere transition regions.
Peculiarities of acoustic-gravity wave near the solar atmosphere transition region are analysed. An investigation is based on an original characteristic relation of waves in a two layers model with a temperature jump. Special attention is paid to an analysis of the properties of the surface waves, generated by the source of mass, which crosses the solar atmosphere transition region. An exact analytical solution of this problem, which involves several modes propagating along the boundary, is found. It is shown on the basis of the obtained results that the wave front from the local instantaneous source moves in radial directions with acceleration. The obtained results are important for explanation of observed properties of wave perturbations near the solar atmosphere transition region, whose appearance correlates with coronas mass injection.
Some new features of the acoustic-gravity wave propagation from the sources at the photospheric heights through a nonisothermic solar chromosphere are examined. Within the framework of the plane-layered model of the atmosphere some properties of the wave perturbations near the height at which the horizontal phase wave velocity coincides with the local sound velocity are studied. At this height, a resonance singularity in the pressure disturbance occurs and above the resonance level the wave field is absent. The results of numerical calculation of the wave field by means of the full-wave numerical model are given for the experimentally known altitude temperature profile. The conclusion is made that a relatively high-frequency acoustic branch of acoustic-gravity waves with periods less than two minutes does not contribute to the vertical energy flux through the chromosphere.