Plasma theory of radio echoes from the Sun and its implications for the problem of the solar wind

Abstract

It is demonstrated that the solar radio echo data cannot be understood in the framework of conventional theories of reflections from those coronal layers, where the refraction index n = 0, from moving inhomogeneities or shock fronts. It was found that all peculiar and unexpected features of radio echo spectra can be explained by the theory of its formation and amplification by induced processes of scattering from turbulent pulsations of the coronal plasma. So, the structure of the radio echo spectra brings information, not only about the systematic large-scale motions of coronal plasma, but also about the plasma turbulent pulsation spectrum, its temperature and density as well. It was found also that the reflected signal originates from the disturbed coronal zones situated above active regions while reflections from the quiet corona are weak or completely absent. The analysis of the radio echo spectra shows that in comparatively thin layers of the corona situated above the active regions, rapid acceleration and heating of the solar wind sometimes take place. The velocity gradient is found to be changing in fairly wide limits. Solar wind velocities as high as 120 km/s were sometimes observed at a distance of only ≈ 1.5 R ⊙ from the Sun's centre. The source of energy for the acceleration and heating of solar wind is the dissipation of turbulent plasma pulsations. These regions where the high temperature gradient precludes the possibility of a continuous flow of coronal plasma, possess the character of weak detonation waves. It can be anticipated that the non-thermal heating of the coronal plasma which was postulated to remove discrepancies between the existing models and the observations of the solar wind, is localized mainly in these regions, thus playing an important role in the formation of fundamental properties of the interplanetary medium. So, for the further development of the theory of the solar wind, it is of prime importance to obtain complete information about the actual time-space distribution of these additional non-thermal energy sources acting in the ‘active’ corona. This information can be gained by radar probing of the Sun in the frequency range from 40 to 10 MHz with the scanning beam such that it would enable the continuous monitoring of the Sun. Combined radar experiments and simultaneous observations of the sporadic solar radio-emission may also be of a high value, since as a result of a comparison of the information about the state of the turbulent coronal plasma obtained through these two different channels, a full description of its state in a local region of rapid acceleration of the solar wind could be achieved.