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Session 36 - Solar Activity.
Display session, Tuesday, June 11
Tripp Commons,
High resolution microwave maps of a complex bipolar active region, AR6615, were obtained using the VLA on 1991 May 7 at three frequencies, 4.9 GHz, 8.4 GHz, and 15 GHz. Comparison of this microwave observation with Big Bear magnetogram suggests that inversion and depolarization of microwave emission must have occurred at different sites of the active region depending on frequency.
For quantitative interpretation of the polarization data, we constructed the coronal magnetic fields above the active region using the potential field extrapolation. In the model, we identified the quasi-transverse (QT) surface across which change of the polarization may occur. It is found that the required topology of the QT surface to explain the observed polarization is correctly predicted by the potential field model, although the locations of the 15 GHz gyroresonance sources required a nonlinear force-free field extrapolation in part. With the calculation of the mode coupling coefficient along the QT surface, we were able to locate the region of depolarization above a strong sunspot, consistent with the observation. We also discuss the appropriate theoretical gyroresonant opacity for waves propagating perpendicular to the magnetic fields, as needed to understand the observed polarization across the magnetic neutral line. Applicability of the present results to study of the coronal magnetic structure above complex bipolar regions, in general, is briefly discussed.
