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

TL;DR

This paper investigates an unusual solar flare with a polarization reversal in microwave emissions, attributing it to radio wave propagation effects across a quasi-transverse layer in the corona, challenging classical emission mode expectations.

Contribution

It identifies propagation effects as the cause of polarization reversal in a solar flare, providing new insights into flare source conditions and microwave polarization behavior.

Findings

Polarization reversal not caused by positrons or mode coupling.

Beam-like electron distribution can produce observed polarization.

Radio wave propagation across QT layer explains the polarization sense.

Abstract

The new generations of multiwavelength radioheliographs with high spatial resolution will employ microwave imaging spectropolarimetry to recover flare topology and plasma parameters in the flare sources and along the wave propagation paths. The recorded polarization depends on the emission mechanism and emission regime (optically thick or thin), the emitting particle properties, and propagation effects. Here, we report an unusual flare, SOL2012-07-06T01:37, whose optically thin gyrosynchrotron emission of the main source displays an apparently ordinary mode sense of polarization in contrast to the classical theory that favors the extraordinary mode. This flare produced copious nonthermal emission in hard X-rays and in high-frequency microwaves up to 80 GHz. It is found that the main flare source corresponds to an interaction site of two loops with greatly different sizes. We have investigated the three possible known reasons of the circular polarization sense reversal - mode coupling, positron contribution, and the effect of beamed angular distribution. We excluded polarization reversal due to contribution of positrons because there was no relevant response in the X-ray emission. We find that a beam-like electron distribution can produce the observed polarization behavior, but the source thermal density must be much higher than the estimate from to the X-ray data. We conclude that the apparent ordinary wave emission in the optically thin mode is due to radio wave propagation across the quasi-transverse (QT) layer. The abnormally high transition frequency (above 35 GHz) can be achieved reasonably low in the corona where the magnetic field value is high and transverse to the line of sight. This places the microwave source below this QT layer, i.e. very low in the corona.