Polarization measurements of photo voltaic radio emissions are key diagnostics of coronal plasma, magnetic fields, and propagation results, and may present extra 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 utterly 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 fully 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 impartial telescopes – the Murchison Widefield Array (MWA) in Australia and the upgraded Large Metrewave Radio Telescope (uGMRT) in India – which differ essentially 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 overlaying the 217–247 MHz frequency vary. Each telescopes detected LP from two spatially separated sources related to two sort I noise storms, and from a short-lived sort 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. Throughout the sort 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 impartial 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 (prime panels) and uGMRT (backside panels). The left panels present the full-disk area of view with two vivid limb sources marked; the center and proper panels present zoomed-in views. Pink 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, exhibits greater LP fractions of 13.5 ± 0.5% (MWA) and 12.3 ± 2.0% (uGMRT), whereas the brighter western supply exhibits decrease LP: 6.0 ± 0.3% (MWA) and 5.9 ± 0.5% (uGMRT).
Determine 2. Temporal evolution of polarization maps at 220 MHz for the japanese supply throughout a sort III burst. The 4 rows present Stokes I (whole depth), Q, U, and V, every in items of kJy/beam. The maps show uncorrelated morphologies throughout the Stokes parameters. At 04:14:14.2 UTC, Stokes Q reverses signal whereas Stokes U turns into elongated—clear proof that these polarization options are intrinsic to the photo voltaic emission and never attributable to instrumental leakage.
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 attributable to Faraday rotation.
The persistence and noticed variability of LP require re-examining the function of coronal propagation results. Two believable origins on this case are:
- Mode coupling in quasi-transverse areas gives a pure mechanism to generate partial LP from plasma emission (Zheleznyakov & Zaitsev 1970; Melrose 1974).
- Reflection on the boundaries of plasma density contrasts might 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 usually been handled as a reference in help of the proof towards 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 impartial affirmation from two broadly separated radio interferometers of very totally different designs. These detections reveal 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 gives a brand new probe of the corona and propagation results, offering a possibility to enhance our understanding of the coronal magnetic fields and propagation.
Based mostly on latest paper by Soham Dey, Devojyoti Kansabanik, Divya Oberoi and Surajit Mondal, “First Strong Detection of Linear Polarization from Metric Photo voltaic Emissions: Difficult Established Paradigms”, 2025 ApjL, 988, L73
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