Gyroresonance emission as a quantitative magnetic-field diagnostic for coronal mass ejections in the middle corona: harmonic identification, mechanism discrimination, and field determination

The first possible detection of thermal gyroresonance emission from a coronal mass ejection (CME), reported from OVRO-LWA commissioning observations, leaves open whether meter-wave continuum emission can serve as a quantitative probe of CME magnetic fields in the middle corona. The detection's field estimates (5.6-7.9 G at 4.9-7.5 solar radii, assuming a third-harmonic break) rest on three unresolved issues explicitly posed by the source work: the harmonic number of the break frequency is unconstrained, inhomogeneous gyrosynchrotron models can reproduce the same total-intensity spectra, and the filling factor of the emitting high-field knots -- hence the volume-averaged CME field -- is unmeasured. The problem is to establish (or refute) gyroresonance emission as a CME magnetic-field diagnostic by identifying the emission harmonic, discriminating it from fine-tuned gyrosynchrotron alternatives, and determining the relation of the derived fields to the CME's volume-averaged field, using full-Stokes calibrated imaging spectroscopy co-registered with white-light observations or equivalent decisive evidence.

Background

A coronal mass ejection (CME) is a large-scale eruption of magnetized plasma from the solar atmosphere into interplanetary space; the strength and orientation of its magnetic field largely determine its geoeffectiveness (its capacity to disturb Earth's magnetosphere), so measuring the CME magnetic field before arrival at Earth is a long-standing goal of solar and space-weather physics. Between the photosphere, where fields are measured routinely, and the heliocentric distances later reached by spacecraft, the field of the CME body is essentially unmeasured. Existing indirect methods -- band splitting of type II radio bursts (emission from the CME-driven shock), standoff geometry between shock and ejecta, and Faraday rotation of polarized background sources -- constrain only shock fields or line-of-sight averages (see the review by Gary 2023).

Radio emission offers the most direct route. Electrons gyrating in a magnetic field $B$ radiate at the electron gyrofrequency $f_B \approx 2.8\,B$ MHz ($B$ in gauss) and at its low harmonics $s f_B$ with $s = 2, 3, 4,\dots$. Thermal gyroresonance emission -- absorption and emission by the thermal electron distribution near harmonics of $f_B$ -- is optically thick below a break frequency and optically thin above it, so the break frequency $f_{\rm br}$ yields the field via $B = f_{\rm br}/(2.8\,s)$; this technique has long furnished the total coronal field above active regions, but the harmonic $s$ of the resonant layer must be identified, otherwise $B$ is uncertain by the factor $1/s$. Gyrosynchrotron emission -- broadband continuum from mildly relativistic nonthermal electrons -- is the other relevant mechanism and the one previously detected from CMEs: Bastian et al. (2001) first imaged a CME at radio wavelengths; Mondal, Oberoi and Vourlidas (2020) traced faint gyrosynchrotron emission to high coronal heights; Kansabanik et al. (2023) obtained the first spectropolarimetric images of CME gyrosynchrotron emission (Stokes $I$ total intensity plus Stokes $V$ circular polarization), and Kansabanik et al. (2024) showed that homogeneous source models cannot reproduce Stokes $I$ and $V$ simultaneously, forcing inhomogeneous models. Chen et al. (2025) combined microwave imaging spectroscopy from the Expanded Owens Valley Solar Array (EOVSA) with meter-wave OVRO-LWA observations to measure the field of an erupting flux rope (a twisted, rope-like bundle of magnetic field lines) from $0.02$ to $1.83$ solar radii ($R_\odot$), about 300 G down to about 0.6 G -- the first such measurement spanning the low to middle corona.

Against this backdrop, Mondal et al. (2025) reported the first possible detection of thermal gyroresonance emission from a CME, in Owens Valley Long Wavelength Array (OVRO-LWA) commissioning observations at tens of MHz. Spectra of the CME core show a break frequency that decreases with time as the CME rises; assuming the break is the third harmonic, the field is $7.9$-$5.6$ G over $4.9$-$7.5\,R_\odot$ -- remarkably strong fields at these heights, with no comparable measurement of the total CME field there. Three gaps, however, keep this a candidate detection rather than a measurement. First, the data (Stokes $I$ only, commissioning-level flux calibration) cannot constrain the harmonic $s$, and the derived field scales as $1/s$. Second, the authors themselves show that inhomogeneous gyrosynchrotron models -- a source occupying well under $1\%$ of the telescope beam (filling factor $\eta \ll 1$) with a contrived line-of-sight magnetic-field profile -- can reproduce the same spectra, so the emission mechanism is not uniquely determined. Third, the observed peak brightness temperature (about $0.1$ MK, i.e. about $10^5$ K, versus $\gtrsim 1$ MK expected for a filled source) implies the emission arises in small knots, plausibly the small magnetic islands now resolved in high-resolution white-light imaging; the volume-averaged CME field therefore remains unconstrained, and extrapolating the knot fields to lower heights relies on empirical scalings fitted to single prior events.

Problem Statement

Can thermal gyroresonance emission be established as a quantitative remote-sensing diagnostic of the magnetic field entrained in coronal mass ejections in the middle corona? Starting from the first possible detection of Mondal et al. (2025) -- OVRO-LWA spectra of a CME core whose break frequency decreases as the CME rises, implying $B = 5.6$-$7.9$ G over $4.9$-$7.5\,R_\odot$ if the break is the third harmonic -- an answer must determine, for meter-wave continuum emission from CMEs at these heights: (1) the harmonic number $s$ of the electron gyrofrequency at which the spectral break is produced, since the derived field scales as $1/s$ and the present data cannot constrain it; (2) whether the emission is thermal gyroresonance at all, rather than gyrosynchrotron emission from an inhomogeneous source with a filling factor well below $1\%$ and a contrived line-of-sight magnetic-field profile, which has been shown to reproduce the same total-intensity spectra; and (3) the filling factor and physical nature of the high-field emitting regions (e.g., small magnetic islands), so that radio-derived field strengths can be interpreted as local or volume-averaged CME fields rather than assumption-dependent estimates. These three determinations jointly serve one objective -- validating gyroresonance emission as a CME field diagnostic -- and an answer that resolves only one of them leaves the diagnostic unvalidated. The route anticipated by the source is absolutely calibrated, full-Stokes imaging spectropolarimetry at multiple frequencies, co-registered with high-resolution white-light imaging, together with joint Stokes $I$ and $V$ modeling; evidence from any suitable instrument (e.g., OVRO-LWA; the Murchison Widefield Array, MWA; or the low-frequency Square Kilometre Array, SKA-Low) or decisive modeling studies is equally admissible. A complete answer either delivers a mechanism-confirmed CME field measurement with quantified harmonic and filling-factor uncertainties, or demonstrates quantitatively that the gyroresonance interpretation is untenable.

References

1. Mondal, S., Chen, B., Chen, X., Yu, S., Gary, D., Zhang, P., et al. 2025, 'Possible First Detection of Gyroresonance Emission from a Coronal Mass Ejection in the Middle Corona', The Astrophysical Journal 992, 143, DOI 10.3847/1538-4357/ae061f, arXiv:2509.16453, https://doi.org/10.3847/1538-4357/ae061f
2. Chen, X., Chen, B., Yu, S., Mondal, S., Stiefel, M. Z., Zhang, P., Gary, D. E., et al. 2025, 'Measuring the Magnetic Field of a Coronal Mass Ejection from the Low to Middle Corona', The Astrophysical Journal Letters 990, L50, DOI 10.3847/2041-8213/adfa71, arXiv:2508.08970, https://doi.org/10.3847/2041-8213/adfa71
3. Kansabanik, D., Mondal, S., & Oberoi, D. 2024, 'Spectropolarimetric Radio Imaging of Faint Gyrosynchrotron Emission from a CME: A Possible Indication of the Insufficiency of Homogeneous Models', The Astrophysical Journal 968, 55, DOI 10.3847/1538-4357/ad43e9, arXiv:2404.14714, https://doi.org/10.3847/1538-4357/ad43e9
4. Kansabanik, D., Mondal, S., & Oberoi, D. 2023, 'Deciphering Faint Gyrosynchrotron Emission from a Coronal Mass Ejection Using Spectropolarimetric Radio Imaging', The Astrophysical Journal 950, 164, DOI 10.3847/1538-4357/acc385, https://doi.org/10.3847/1538-4357/acc385
5. Bastian, T. S., Pick, M., Kerdraon, A., Maia, D., & Vourlidas, A. 2001, 'The Coronal Mass Ejection of 1998 April 20: Direct Imaging at Radio Wavelengths', The Astrophysical Journal 558, L65, DOI 10.1086/323421, https://doi.org/10.1086/323421
6. Mondal, S., Oberoi, D., & Vourlidas, A. 2020, 'Estimation of the Physical Parameters of a CME at High Coronal Heights Using Low-frequency Radio Observations', The Astrophysical Journal 893, 28, DOI 10.3847/1538-4357/ab7fab, https://doi.org/10.3847/1538-4357/ab7fab
7. Gary, D. E. 2023, 'New Insights from Imaging Spectroscopy of Solar Radio Emission', Annual Review of Astronomy and Astrophysics 61, 427, DOI 10.1146/annurev-astro-071221-052744, https://doi.org/10.1146/annurev-astro-071221-052744
8. Patra, D., Majee, P., Dey, S., Kansabanik, D., Oberoi, D., Kumari, A., Mondal, S., Deshpande, K., & Paliwal, D. 2026, 'Coronal Magnetography using Spectropolarimetry with SKA Telescopes', arXiv:2607.03620, https://arxiv.org/abs/2607.03620
9. Mondal, S., Shaik, S. B., Howard, R. A., Zhang, P., Chen, B., Chen, X., Yu, S., Gary, D., et al. 2026, 'Estimating Electron Densities in the Middle Solar Corona Using White-light and Radio Observations', The Astrophysical Journal 999, 237, DOI 10.3847/1538-4357/ae4353, arXiv:2602.09819, https://doi.org/10.3847/1538-4357/ae4353