Polarization measurements of photo voltaic radio emissions are key diagnostics of coronal plasma, magnetic fields, and propagation results, and might present further constraints on the emission mechanisms. At meter wavelengths, round polarization (CP) has lengthy been exploited in photo voltaic radio research, whereas linear polarization (LP) has been assumed to be absent. This view arose from the expectations that sturdy coronal Faraday rotation would fully depolarize LP inside typical observational bandwidths and instrumental angular decision (Grognard & McLean 1973; Boischot & Lecacheux 1975).
Consequently, the presence of LP in uncalibrated datasets was routinely attributed totally to instrumental polarization leakage and, in lots of instances, used to impose a calibration constraint that LP have to be zero. Right here we report the primary unambiguous detection of intrinsic LP at metric wavelengths. To rule out any potential instrumental systematics and evaluation artefacts, these observations have been carried out concurrently utilizing two unbiased telescopes – the Murchison Widefield Array (MWA) in Australia and the upgraded Big Metrewave Radio Telescope (uGMRT) in India – which differ basically in design (MWA: aperture array, linear feeds; uGMRT: dish array, round feeds) and are geographically separated by 1000’s of kilometers.
Observations and Outcomes
On 2022 June 25, simultaneous observations have been carried out utilizing the MWA and the uGMRT masking the 217–247 MHz frequency vary. Each telescopes detected LP from two spatially separated sources related to two kind I noise storms, and from a short-lived kind III burst. The detections are constant throughout the 2 devices,
At 218 MHz (Determine 1):
- The brighter west-limb supply exhibited LP fractions of about 6% (MWA: 6.0 ± 0.3%; uGMRT: 5.9 ± 0.5%).
- The fainter east-limb supply confirmed stronger LP, about 13% (MWA: 13.5 ± 0.5%; uGMRT: 12.3 ± 2.0%).
The LP fraction additionally confirmed fast variability: within the east-limb supply, it ranged from ~2% to ~31% over brief time–frequency intervals. Throughout a sort III burst at 04:14:14 UTC, the LP fraction dropped sharply from >10% to <5% inside one second over 217–220 MHz.
The morphological evolution of the Stokes parameters additional reinforces the photo voltaic origin of LP. Through the kind III burst, Stokes Q reversed signal, Stokes U elongated, and Stokes V developed from a monopolar construction right into a bipolar construction, whereas Stokes I remained comparatively steady morphologically (Determine 2). Such unbiased evolution throughout Q, U, and V can’t be defined by instrumental leakage from Stokes I.
Determine 1. Simultaneous detection of linearly polarized emission at 218 MHz with the MWA (high panels) and uGMRT (backside panels). The left panels present the full-disk discipline of view with two shiny limb sources marked; the center and proper panels present zoomed-in views. Crimson contours denote linear polarization depth at 0.4, 0.6, 0.8, and 0.9 of the height, and the black circles mark the optical photo voltaic disk. Each sources exhibit partial linear polarization. The japanese supply, weaker in Stokes I, reveals larger LP fractions of 13.5 ± 0.5% (MWA) and 12.3 ± 2.0% (uGMRT), whereas the brighter western supply reveals decrease LP: 6.0 ± 0.3% (MWA) and 5.9 ± 0.5% (uGMRT).
Dialogue
These observations present definitive proof that LP at meter wavelengths can certainly be noticed throughout energetic photo voltaic emissions. This instantly challenges the long-held assumption of full depolarization of LP as a consequence of Faraday rotation.
The persistence and noticed variability of LP require re-examining the position of coronal propagation results. Two believable origins on this case are:
- Mode coupling in quasi-transverse areas offers a pure mechanism to generate partial LP from plasma emission (Zheleznyakov & Zaitsev 1970; Melrose 1974).
- Reflection on the boundaries of plasma density contrasts can also produce LP (Bastian et al. 2022), although the fast time–frequency variability of Stokes Q and U noticed right here disfavors this because the dominant mechanism.
The implications of this work lengthen past the Solar. The presence of LP has typically been handled as a reference in assist of the proof in opposition to plasma emission and in favour of electron cyclotron maser processes in coherent stellar bursts (Lynch et al. 2017; Callingham et al. 2021). That is guided by the standard consensus of the non-existence of linear polarization from photo voltaic radio emissions, that are dominantly plasma emissions.
Conclusions
We report the primary sturdy detection of linear polarization in photo voltaic metric emissions, with unbiased affirmation from two broadly separated radio interferometers of very totally different designs. These detections display that LP is of photo voltaic origin, persists regardless of depolarization anticipated from coronal propagation, and shows wealthy temporal and spectral construction.
This work results in the next key implications:
- The presence of LP in energetic photo voltaic emissions can’t be ignored. Calibration schemes that assume zero LP additionally bias the estimation of CP.
- LP offers a brand new probe of the corona and propagation results, offering a chance to enhance our understanding of the coronal magnetic fields and propagation.
Primarily based on latest paper by Soham Dey, Devojyoti Kansabanik, Divya Oberoi and Surajit Mondal, “First Sturdy Detection of Linear Polarization from Metric Photo voltaic Emissions: Difficult Established Paradigms”, 2025 ApjL, 988, L73
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