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The new generations of multiwavelength radioheliographs with high spatialresolution will employ microwave imaging spectropolarimetry to recover flaretopology and plasma parameters in the flare sources and along the wavepropagation paths. The recorded polarization depends on the emission mechanismand emission regime (optically thick or thin), the emitting particleproperties, and propagation effects. Here, we report an unusual flare,SOL2012-07-06T01:37, whose optically thin gyrosynchrotron emission of the mainsource displays an apparently ordinary mode sense of polarization in contrastto the classical theory that favors the extraordinary mode. This flare producedcopious nonthermal emission in hard X-rays and in high-frequency microwaves upto 80 GHz. It is found that the main flare source corresponds to an interactionsite of two loops with greatly different sizes. We have investigated the threepossible known reasons of the circular polarization sense reversal - modecoupling, positron contribution, and the effect of beamed angular distribution.We excluded polarization reversal due to contribution of positrons becausethere was no relevant response in the X-ray emission. We find that a beam-likeelectron distribution can produce the observed polarization behavior, but thesource thermal density must be much higher than the estimate from to the X-raydata. We conclude that the apparent ordinary wave emission in the opticallythin mode is due to radio wave propagation across the quasi-transverse (QT)layer. The abnormally high transition frequency (above 35 GHz) can be achievedreasonably low in the corona where the magnetic field value is high andtransverse to the line of sight. This places the microwave source below this QTlayer, i.e. very low in the corona.

Flare SOL2012-07-06: on the origin of the circular polarization reversal between 17 GHz and 34 GHz