17 GHz Mode Coupling in the Solar Corona

Details 17 GHz Mode Coupling in the Solar Corona 17 GHz Mode Coupling in the Solar Corona

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999spro.proc...83l&link_type=abstract

Solar Physics with Radio Observations, Proceedings of the Nobeyama Symposium, held in Kiyosato, Japan, Oct. 27-30, 1998, Edited

Physics

Scientific paper

We studied the development of microwave polarization of a group of active regions for a period of 10 days during April, 1993 using data obtained by the Nobeyama radioheliograph. The observed sense of polarization at 17 GHz changed with the active region position on the solar disk. This change of polarization can be explained by the mode coupling theory according to which a weak coupling between the ordinary and extraordinary electromagnetic modes takes place when the radiation crosses a region of transverse magnetic field and results in a polarization reversal. Since the strength of the mode coupling depends on the physical parameters (and their gradients) of the quasi-transverse region, observations of polarization changes can be used to obtain key values of the magnetic field and field gradient in the active region corona. Using the intensity and polarization images of active regions, we found that the coupling constant is typically > 103 corresponding to a weak coupling regime. We determined the mean value of the transition frequency to be ~ 5.3 × 1011 Hz, below which the weak coupling effect is important. For all the active regions studied in this paper, there seems to be a similarity in the position on the solar disk where the mode coupling effects become important. The polarization reversal always occurred when the active regions were farther than the 500 arc sec mark from the disk center. Using this fact and extrapolated photospheric magnetic field we are able to estimate heights of both the quasi-transverse layer and the source region. Assuming a value of ~ 70 G, we obtain a value of 2.2 × 104 km for the Q-T layer height.

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