Origin of high-frequency circular-polarization reversals in solar microwave bursts: quasi-transverse propagation versus intrinsic emission physics

About 1% of solar microwave bursts recorded by the Nobeyama instruments show a reversal of the circular-polarization sense between 17 GHz and 34/35 GHz, opposite to the extraordinary-mode polarization expected from optically thin gyrosynchrotron emission by isotropic electrons. A detailed case study of the disk-center flare SOL2012-07-06 (Altyntsev et al. 2017) concluded that in that event the reversal is most plausibly a propagation effect — a sign flip on crossing a quasi-transverse (QT) magnetic layer low in the corona with an abnormally high transition frequency above 35 GHz — rather than emission by relativistic positrons or a strongly beamed electron distribution. That conclusion rests on an unmeasured QT layer (density, field gradient, and angular scale inferred only from a force-free field extrapolation), single-event forward fits with simplified source geometry, and a positron non-detection established for one flare, while an earlier interpretation of this same reversal class as a positron signature remains contested. The open problem is to determine the origin of these reversals across the burst population: how often QT layers with transition frequencies of tens of GHz occur and under what active-region conditions, whether the QT-layer structure can be directly measured or tightly constrained, whether realistic gyrosynchrotron modeling discriminates the candidate mechanisms uniquely, and whether positron-dominated emission can be excluded generally.

Background

Solar flares accelerate electrons to mildly relativistic energies, and these electrons emit broadband microwave (centimeter-wavelength) continuum, called gyrosynchrotron radiation, as they spiral around coronal magnetic fields. The emission is circularly polarized, and its degree and sense — quantified by the Stokes parameter $V$ (the difference between right- and left-handed circular intensity) — depend on which magnetoionic mode carries the radiation: the ordinary (O) or the extraordinary (X) mode, the two eigenmodes of electromagnetic wave propagation in a magnetized plasma. In an optically thin source (a source transparent to its own radiation) with an isotropic distribution of electron pitch angles, gyrosynchrotron theory predicts that the X mode dominates, so the observed polarization sense tracks the sign of the line-of-sight magnetic field at the source (Dulk & Marsh 1982); early exceptions with ordinary-mode-like emission were reported by Alissandrakis, Nindos & Kundu (1993).

Three mechanisms are known to produce reversed (ordinary-mode-like) circular polarization at high microwave frequencies. First, a strongly beamed, anisotropic pitch-angle distribution of the emitting electrons can flip the dominant mode for certain viewing angles (Fleishman & Melnikov 2003). Second, relativistic positrons — created in nuclear reactions during flares — radiate gyrosynchrotron emission with the same spectrum as electrons but the opposite helicity, so a positron-dominated component emits in the opposite circular polarization (Lingenfelter & Ramaty 1967). Third, propagation effects: when radiation crosses a quasi-transverse (QT) region, where the magnetic field is nearly perpendicular to the line of sight and its longitudinal component typically reverses, the polarization sense reverses if the coupling between the O and X modes is weak, and is preserved if the coupling is strong. Cohen (1960) and Zheleznyakov & Zlotnik (1963) defined the transition frequency $f_t$ separating the weak-coupling regime ($f<f_t$, reversal occurs) from the strong-coupling regime ($f>f_t$, no reversal); $f_t$ is set by the thermal density, the magnetic-field strength, and the spatial scale over which the field direction changes in the QT layer, with the strongest sensitivity to the field strength. Because $f_t$ is usually well below 17 GHz, high-frequency reversals are rare, and polarization reversals have been exploited mainly as coronal density and magnetic-field diagnostics (Lee et al. 1998; Ryabov et al. 2005).

The observational puzzle comes from the Nobeyama instruments: the Nobeyama Radio Polarimeters (NoRP) measure total flux and circular polarization at fixed frequencies including 17 and 35 GHz, while the Nobeyama Radioheliograph (NoRH) provides imaging at 17 GHz in intensity and polarization and at 34 GHz in intensity only. Fleishman, Altyntsev & Meshalkina (2013) searched the entire Nobeyama database and found that about 1% of microwave bursts (6 of 600+) show opposite senses of circular polarization at 17 and 35 GHz; they interpreted these events as the microwave signature of relativistic positrons. Independently, Huang & Nakajima (2009) found that most flaring loops observed by NoRH show the same polarization sense at both footpoints, contrary to the expectation for intrinsic X-mode emission from opposite magnetic polarities, indicating that propagation effects strongly affect observed microwave polarization.

Altyntsev et al. (2017) studied one such event, the flare SOL2012-07-06T01:37 near disk center, of moderately strong class M2.9 on the standard GOES soft X-ray brightness scale (GOES being the Geostationary Operational Environmental Satellite program whose X-ray flux classes calibrate flare magnitude), whose optically thin gyrosynchrotron emission extended to 80 GHz and whose hard X-rays reached 300–800 keV. The polarization sense of the main source changed from extraordinary-like at 17 GHz to ordinary-like at 35 GHz. The authors found no evidence for the positron hypothesis in this flare (no 511 keV electron-positron annihilation-line response in the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) or the Konus gamma-ray spectrometer on the Wind spacecraft, and the positron numbers allowed by the X-ray non-detection are far too small to produce the radio emission); a beamed-electron fit could reproduce the reversal but required a thermal density 3–5 times higher than the X-ray-derived value and an anisotropy whose strong parameter dependence contradicts the observed constancy of the polarization-spectrum shape. They concluded that the reversal is due to propagation through a QT layer: a nonlinear force-free-field extrapolation (a reconstruction of the three-dimensional coronal magnetic field from the photospheric vector magnetogram, assuming the coronal field exerts no net force on the plasma), based on a vector magnetogram from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) satellite places a QT layer below roughly 5 Mm, where the field exceeds 500 G and the transition frequency can exceed 35 GHz, implying that the microwave source lies very low in the corona. This QT conclusion rests, however, on QT-layer parameters inferred from a field extrapolation rather than measured, on single-event forward fits that assume uniform sources with isotropic or simple Gaussian angular distributions, and on a positron non-detection established for one flare only.

Problem Statement

Determine the physical origin of reversals of the circular-polarization sense between approximately 17 GHz and 34/35 GHz in solar microwave bursts — propagation effects or intrinsic source physics — for the population of bursts exhibiting such reversals (about 1% of Nobeyama-recorded events), as left open by Altyntsev et al. (2017) after their case study of SOL2012-07-06. The central question is the population-level attribution of the reversals — which mechanism or mechanisms account for them across the burst population; the aspects below support that determination, with (1) addressing its core and (2) through (4) the supporting analyses: (1) how often quasi-transverse layers with transition frequencies $f_t$ exceeding tens of GHz occur in the low corona and under what active-region conditions, and how many of the observed reversals the QT mechanism can and cannot account for; (2) whether the spatial and temporal structure of the QT layer — its thermal density $n_{\mathrm{th}}$, magnetic-field gradient, and the scale $L$ over which the field direction changes — can be directly measured or tightly constrained for reversal events, rather than inferred from field extrapolations; (3) whether realistic gyrosynchrotron forward models, allowing multi-loop geometries and inhomogeneous source structure instead of uniform sources with isotropic or simple Gaussian angular distributions, discriminate uniquely between the QT, beamed-electron, and positron explanations; and (4) whether positron-dominated emission can be excluded for the reversal population as a whole, rather than for a single event.

References

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