First robust detection of linear polarization from solar radio bursts

For decades, the focus of polarimetric solar radio studies has been solely on circular polarization. This stemmed from the fact that the strong differential Faraday rotation in coronal plasma should completely obliterate any linear polarization component even if it were to be present (e.g., Grognard & McLean, 1973). Consequently, after a few reports in the late 50s and early 60s of successful detection of linearly polarized radio emissions from the Sun, the  consensus has essentially been to dismiss any detected linear polarization in the observed dynamic spectra as instrumental artifacts (e.g., Grognard & McLean, 1972; Boischot & Lecacheux, 1975). This assumption has been routinely used in calibrating solar polarimetric observations, even for recent studies (Morosan et al. 2022). The state-of-the-art polarimetric calibration algorithm, P-AIRCARS (Kansabanik et al., 2022a, 2022b, 2023) does not rely on any such assumptions. It provides high-fidelity and high-dynamic-range spectropolarimetric snapshot solar radio images using a new-generation instrument, the Murchison Widefield Array (MWA). This enables us to explore a part of phase space which was hitherto unexplored.

Using P-AIRCARS and the MWA, we present the first robust imaging-based evidence for linearly polarized emission in metre-wavelength solar radio bursts. This finding is corroborated by simultaneous observations with the upgraded Giant Metrewave Radio Telescope at the same spectral band. Moreover, our estimated upper limit on the Rotation Measure (RM) of ~50 rad m-2 are orders of magnitude lower than the previous estimates based on coronal models (e.g., ~103 rad m-2 by Bhonsle & McNarry, 1964). This low RM implies that the linear polarized emission has traversed much lower electron column densities, suggesting that it originates at much higher coronal heights. Interestingly, detections of linear polarization in stellar radio bursts have also been reported recently (Callingham et al., 2021; Bastian et al., 2022). We conclude by exploring some physical scenarios which can potentially give rise to such linearly polarized emissions in the solar corona.