Estimating the coronal and chromospheric magnetic fields of solar active regions as observed with the Nobeyama Radioheliograph Compared with the Extrapolated Linear Force-Free Field
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Adopting the thermal free-free emission mechanism, the coronal and chromospheric magnetic fields are derived from the polarization and spectral observations with the Nobeyama Radioheliograph at 1.76 cm. The solar active regions (AR) located near the disk center observed on January 8, 2015 (AR 12257) and December 4, 2016 (AR 12615) are used for the estimate of the chromospheric and coronal magnetic fields with the microwave radio observations. We compare solar radio maps of active regions for both intensity and circularly polarized component with the optical maps from observations with the Helioseismic and Magnetic Imager and the chromosphere and corona transition region images obtained with the Atmospheric Imaging Assembly instrument, on board the Solar Dynamic Observatory. We notice from the comparison between radio maps of both AR that the circular polarization degree in the AR 12257 is about 2 percent, but the AR 12615 has a higher existent value by 3 percent. Radio observations provide us for direct measurements of magnetic fields in the chromospheric and coronal layers. We estimate the coronal magnetic fields using the Atmospheric Imaging Assembly observations by adopting magnetic loops in the corona over some patches with weak photospheric magnetic fields. The coronal magnetic field derived from the Atmospheric Imaging Assembly data was from 90 to 240 Gauss. We also study the coronal magnetic fields based on the structure of the extrapolated field, where the result of the magnetic fields was in the range from 35 to 145 Gauss, showing that the difference in the coronal magnetic fields between both results is attributed to the assumption of the force-free approximation.
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