Probing Twisted Magnetic Field Using Microwave Observations in an M Class Solar Flare on 11 February, 2014

TL;DR

This study demonstrates that microwave polarization observations can diagnose the magnetic topology of solar flares, specifically revealing twisted magnetic structures through polarization patterns, with implications for understanding flare energy release.

Contribution

The paper introduces a method to infer the magnetic field topology of solar flares using microwave polarization data combined with NLFFF magnetic field extrapolation.

Findings

Potential magnetic field models fail to reproduce observed polarization.

NLFFF models successfully explain the polarization patterns.

Radio polarization maps can diagnose magnetic twist in flare regions.

Abstract

This work demonstrates the possibility of magnetic field topology investigations using microwave polarimetric observations. We study a solar flare of GOES M1.7 class that occurred on 11 February, 2014. This flare revealed a clear signature of spatial inversion of the radio emission polarization sign. We show that the observed polarization pattern can be explained by nonthermal gyrosynchrotron emission from the twisted magnetic structure. Using observations of the Reuven Ramaty High Energy Solar Spectroscopic Imager, Nobeyama Radio Observatory, Radio Solar Telescope Network, and Solar Dynamics Observatory, we have determined the parameters of nonthermal electrons and thermal plasma and identified the magnetic structure where the flare energy release occurred. To reconstruct the coronal magnetic field, we use nonlinear force-free field (NLFFF) and potential magnetic field approaches. Radio emission of nonthermal electrons is simulated by the GX Simulator code using the extrapolated magnetic field and the parameters of nonthermal electrons and thermal plasma inferred from the observations; the model radio maps and spectra are compared with observations. We have found that the potential magnetic field approach fails to explain the observed circular polarization pattern; on the other hand, the Stokes $V$ map is successfully explained by assuming nonthermal electrons to be distributed along the twisted magnetic structure determined by the NLFFF extrapolation approach. Thus, we show that the radio polarization maps can be used for diagnosing the topology of the flare magnetic structures where nonthermal electrons are injected.