Coronal magnetography using multiwavelength observations of the Sun by Siberian Radioheliograph

Sergey Anfinogentov, Institute of Solar-Terrestrial physics SB RAS, Irkutsk

The magnetic field is a key element in solar activity that controls most processes in the solar corona including energy accumulation and its explosive release during solar flares and coronal mass ejections. Thus, the information about the magnetic field in the solar corona is crucial for understanding and forecasting of geoeffective events such as solar flares and coronal mass ejections. At the level of the photosphere, the magnetic field can be measured directly using the Zeeman effect. Unfortunately, in the corona where plasma is extremely hot and rarefied, such measurements are not possible. Nowadays, the most valuable information about the magnetic field in the corona is obtained by its reconstruction from photospheric magnetograms in potential or force-free approximation. However, this approach has a number of difficulties connected with the underlying assumptions and available boundary conditions. Hence, the reconstruction methods are a subject of improvement. Here, observations in the microwave range can essentially help. Allowing us to measure the magnetic field in the corona directly, they can test the results of the extrapolation and provide additional constraints for its improving. We present the results of mapping the absolute value of the magnetic field at the transition region by observing gyroresonant emission at 32 frequencies in 4-8 GHz range with Siberian Radioheliograph (SRH). The absolute value of the magnetic field is estimated by determining the frequency where the brightness temperature of the microwave emission drops down to the chromospheric values. Also, we discuss a more rigorous approach of forward fitting synthetic microwave spectrum calculated from a 1D model of the solar atmosphere to the observed data. The parameters of the models are inferred using the Bayesian analysis and Markov chain Monte-Carlo sampling. The reliability of these methods has been tested on a realistic model of a solar active region created with the GX-simulator package and based on a 3D NLFF magnetic cube reconstructed from a photospheric magnetogram. The SRH data has been verified by comparing them with the RATAN-600 observations. Thus, mapping the magnetic field in solar active regions is now available on a regular basis thanks to the routine spatially resolved multiwavelength observations of the Sun carried out by SRH team since 2016. This work is supported by the RFBR grant18-32-20165 mol_a_ved.