A. Kuznetsov
Institute of Solar-Terrestrial Physics Siberian Branch of
the Russian Academy of Sciences, Lermontov St. 126, Irkutsk 664033,
Russia
Solar microwave bursts observed by the Siberian Solar Radio Telescope (at the frequency of 5.7 GHz) sometimes have high degree of circurar polarization (up to 100%). Comparison with magnetograms and soft X-ray images shows that these bursts are generated, most probably, at the second harmonic of plasma frequency in a relatively weak magnetic field. However, according to the existing models, the harmonic emission should be low-polarized.
In this work the processes of generation and nonlinear interaction of plasma waves are modeled numerically. It is assumed that an electron beam generates upper-hybrid waves which are transformed then into electromagnetic ones at the doubled frequency due to coalescence. The purpose of the investigation was to find the conditions when the considered process can produce a highly-polarized emission.
It was found that the required conditions can be realized in the tops of magnetic loops where electron distribution of Dory-Guest-Harris or symmetric two-sided loss-cone type is formed. In this case the maximum of the emission intensity corresponds to the transversal (with respect to the magnetic field) propagation direction. The emission directed strictly across the magnetic field is almost fully polarized, that is caused by the difference of dispersion properties of X- and O-modes. With a deviation from the transversal propagation both the emission intensity and the polarization degree decrease; the interval where the polarization degree exceeds 60% is about 10 degrees (for the ratio of plasma and cyclotron frequencies about five). The efficience of the plasma mechanism of emission generation is sufficient to provide the observed intensity of bursts.