Constarining kappa distribution in active regions at the sun using microwave gyroresonant radiation

Typically, the distribution function of an astrophysical plasma in a steady state is assumed to be maxwellian. However, for many observations the maxwellian distribution is hardly distinguishable from the kappa distribution with a reasonably large index, say kappa=10. Interestingly, in case of plasma with the kappa distribution, the microwave gyroresonance emission with its large optical depth is extraordinarily sensitive to the kappa index value, because the optically thick emission is highly sensitive to small variations at the tail of the distribution. Here, to constrain possible values of the kappa index in the thermal plasmas of solar corona we consider gyrosynchrotron emission from solar active regions using 3D modeling enabled by GX Simulator modeling tool. The 3D magnetic structure is modeled using nonlinear force-free field extrapolation from photospheric vector magnetic field data available from SDO/HMI. Then, this magnetic skeleton is filled with a thermal plasma (n and T values in each voxel of the 3D model) in such a way that the EUV emission computed from the model agrees quantitatively (on average) with the EUV emission observed with SDO/AIA. At the next step we adopt these n and T pair obtained from the comparison with the optically thin EUV data, but allow a kappa distribution with unknown index rather than the maxwellian distribution. In our study we vary the kappa index and compute the radio brightness and polarization maps from the same 3D model using theory developed by Fleishman & Kuznetsov (2014). These synthetic maps are convolved with the point spread function of a given radio interferometer and compared with the observed radio brightness maps. The syntetic radio brightness and the degree of polarization both increase rapidly as the kappa index decreases, which allows, perhaps, the ever most stringent constraints on the kappa-indices consistent with the data. For this study we selected two solar active regions, one bipolar and one unipolar, and found that the allowable kappa indices are in both cases well above the value around 15. We discuss implications of this finding and potential ways to further constraint the kappa distribution shape in the solar plasma.