Challenging the paradigms established by S. Dey et al. – Community of European Solar Radio Astronomers

Polarization measurements of solar radio emissions are key diagnostics of coronal plasma, magnetic fields, and propagation effects, and can provide additional constraints on emission mechanisms. At meter wavelengths, circular polarization (CP) has long been exploited in solar radio studies, while linear polarization (LP) was assumed to be absent. This view arose from the expectation that strong coronal Faraday rotation would completely depolarize LP within typical observation bandwidths and instrumental angular resolution (Grognard and McLean 1973; Boischot and Lecacheux 1975).

Consequently, the presence of LP in uncalibrated data sets was routinely attributed entirely to instrumental bias leakage and, in many cases, was used to impose a calibration constraint that LP must be zero. Here we report the first unambiguous detection of intrinsic LP at metric wavelengths. To rule out possible instrumental systematics and analysis artifacts, these observations were carried out simultaneously using two independent telescopes: the Murchison Widefield Array (MWA) in Australia and the Enhanced Giant Meter Wave Radio Telescope (uGMRT) in India, which differ fundamentally in design (MWA: aperture array, linear feed; uGMRT: dish array, circular feed) and are geographically separated by thousands of kilometers.

Observations and results

On June 25, 2022, simultaneous observations were carried out using the MWA and uGMRT covering the frequency range 217 to 247 MHz. Both telescopes detected LPs from two spatially separated sources associated with two type I noise storms and from a short-lived type III explosion. The detections are consistent in the two instruments,

At 218 MHz (Figure 1):

• The brightest source at the western end exhibited LP fractions of approximately 6% (MWA: 6.0 ± 0.3%; uGMRT: 5.9 ± 0.5%).
• The weaker source of the eastern limb showed a stronger LP, about 13% (MWA: 13.5 ± 0.5%; uGMRT: 12.3 ± 2.0%).

The LP fraction also showed rapid variability: at the far eastern source, it ranged from ~2% to ~31% over short time-frequency intervals. During a type III burst at 04:14:14 UTC, the LP fraction dropped sharply from >10% to <5% within one second at 217–220 MHz.

The morphological evolution of the Stokes parameters further reinforces the solar origin of LP. During the type III burst, Stokes Q reversed sign, Stokes U elongated, and Stokes V evolved from a monopolar structure to a bipolar structure, while Stokes I remained comparatively morphologically stable (Figure 2). This independent evolution between Q, U and V cannot be explained by the Stokes I instrumental fugue.

Figure 1. Simultaneous detection of linearly polarized emission at 218 MHz with MWA (top panels) and uGMRT (bottom panels). The left panels show the field of view of the entire disk with two sources of bright limbs marked; Center and right panels provide expanded views. Red contours denote linear polarization intensity at 0.4, 0.6, 0.8, and 0.9 of the peak, and black circles mark the optical solar disk. Both sources exhibit partial linear polarization. The fainter eastern source in Stokes I shows higher LP fractions of 13.5 ± 0.5% (MWA) and 12.3 ± 2.0% (uGMRT), while the brighter western source shows lower LPs: 6.0 ± 0.3% (MWA) and 5.9 ± 0.5% (uGMRT).

Figure 2. Time evolution of the polarization maps at 220 MHz for the eastern source during a type III burst. The four rows show Stokes I (total intensity), Q, U and V, each in units of kJy/beam. The maps show uncorrelated morphologies between Stokes parameters. At 04:14:14.2 UTC, Stokes Q reverses sign while Stokes U lengthens, clear evidence that these polarization features are intrinsic to solar emission and not due to instrumental leakage.

Discussion

These observations provide definitive evidence that LP at meter wavelengths can be observed during active solar emissions. This directly challenges the long-standing assumption of complete depolarization of the LP due to Faraday rotation.

The observed persistence and variability of LP requires reexamining the role of coronal propagation effects. Two plausible origins in this case are:

• Mode coupling in quasitransverse regions provides a natural mechanism to generate partial LP from plasma emission (Zheleznyakov and Zaitsev 1970; Melrose 1974).
• Reflection at the limits of plasma density contrasts can also produce LP (Bastian et al. 2022), although the rapid time-frequency variability of Stokes Q and U observed here disfavors this dominant mechanism.

The implications of this work extend beyond the Sun. The presence of LPs has often been treated as a reference in support of evidence against plasma emission and in favor of electron cyclotron maser processes in coherent stellar explosions (Lynch et al. 2017; Callingham et al. 2021). This is guided by the traditional consensus of the non-existence of linear polarization of solar radio emissions, which are predominantly plasma emissions.

Conclusions

We present the first robust detection of linear polarization in solar metric emissions, with independent confirmation from two widely separated radio interferometers of very different designs. These detections demonstrate that LP is of solar origin, persists despite the expected depolarization of coronal propagation, and displays a rich temporal and spectral structure.

This work leads to the following key implications:

• The presence of LP in active solar emissions cannot be ignored. Calibration schemes that assume zero PL also bias CP estimation.
• LP provides a new probe of the effects of corona and propagation, providing an opportunity to improve our understanding of coronal magnetic fields and propagation.

Based on a recent article by Soham Dey, Devojyoti Kansabanik, Divya Oberoi and Surajit Mondal, “First robust detection of linear polarization from metric solar emissions: challenging established paradigms,” 2025 ApjL, 988, L73

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