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Polarisation measurements of solar flares at millimetre-waves were used toinvestigate the magnetic field configuration of the emitting sources. Weanalyse two solar flares (SOL2013-02-17 and SOL2013-11-05) observed by thePOlarisation Emission of Millimetre Activity at the Sun (POEMAS) at 45 and 90GHz, at microwaves from 1 - 15 GHz by the Radio Solar Telescope Network (RSTN),and at high frequencies (212 GHz) by the Solar Submillimetre Telescope (SST).Also, hard X-rays from these flares were simultaneously detected by the ReuvenRamaty High-Energy Solar Spectroscopic Imager (RHESSI). The flux andpolarisation radio spectra were fit using a model that simulatesgyrosynchrotron emission in a spatially-varying 3D magnetic field loopstructure. For the {modelling}, the magnetic loop geometry was fixed and thefield strength was the only free parameter of the magnetic field. In addition,a uniform electron distribution was {assumed by} the model, with the numberdensity of energetic electrons and the electron spectral index as freeparameters. The fitted model reproduced reasonably well the observed degree ofpolarisation and radio flux spectra for each event yielding the physicalparameters of the loop and flaring sources. Our results indicate that the highdegree of polarisation during a solar flare can be explained by two sourceslocated at the {footpoints} of highly asymmetric magnetic loops whereas lowpolarisation degrees arise from footpoint sources of symmetric magnetic loops.

Inferring the magnetic field asymmetry of solar flares from the degree of polarisation at millimetre wavelengths