Coronal mass ejections (CMEs) are the dominant drivers of space weather, and their geo-effectiveness is set by their magnetic fields, which cannot be directly measured by visible-light coronagraphs. Gyrosynchrotron (GS) emission from mildly relativistic electrons trapped in CME plasma offers a remote-sensing route to spatially resolved coronal magnetic field estimates, but every inversion performed to date assumes a homogeneous, isotropic source. Spectropolarimetric imaging with the Murchison Widefield Array has shown, for one CME, that no homogeneous, isotropic GS model within the explored family could fit the observed Stokes I and Stokes V simultaneously — a possible indication of the insufficiency of homogeneous models — and that inhomogeneity in the magnetic field and nonthermal electron parameters systematically biases the fitted parameters. The open problem is to construct and validate physics-based forward models of CME GS emission that incorporate realistic spatial inhomogeneity and anisotropic electron distributions, compute both total intensity and circular polarization, remain constrained enough to permit inversion from finite spectropolarimetric datasets, and to quantify where the classical homogeneity assumptions are acceptable. As of 2026 this remains open: the newest observational inversions still use homogeneous fitting, and the first three-dimensional magnetohydrodynamic-coupled forward simulations compute only total intensity and perform no observational inversion.

Background

A coronal mass ejection (CME) is a large-scale expulsion of magnetized plasma from the Sun into interplanetary space; its geomagnetic impact at Earth is governed primarily by the strength and orientation of its magnetic field. White-light coronagraphs image CME geometry but cannot measure their magnetic fields, so the field in the corona (roughly 2 to 10 solar radii) is essentially unmeasured, which is a central gap for space-weather forecasting. Radio astronomy offers a diagnostic: gyrosynchrotron (GS) emission, broadband continuum radiation produced by mildly relativistic electrons spiraling along CME magnetic field lines. The GS spectrum and its circular polarization depend on the magnetic field strength, the angle between the field and the line of sight (LoS), and the density, energy spectrum, and pitch-angle distribution (the distribution of electron velocities relative to the magnetic field) of the emitting electrons. Spectral modeling of spatially resolved GS emission has therefore been used since Bastian et al. (2001) to estimate CME magnetic fields, most recently out to heliocentric distances of about 8 solar radii with the Murchison Widefield Array (MWA), a low-frequency radio interferometer, using the high-dynamic-range (high contrast between bright and faint emission) calibration pipeline P-AIRCARS. A long-standing limitation is parameter degeneracy: even the simplest GS model has about ten free parameters, several of which cannot be disentangled from the total-intensity (Stokes I) spectrum alone. Adding circular-polarization (Stokes V) measurements breaks some degeneracies, and Kansabanik et al. (2023) demonstrated the first joint Stokes I + Stokes V upper-limit modeling of a CME. However, all modeling studies from Bastian et al. (2001) through Mondal et al. (2020) to the present have assumed that the plasma parameters are homogeneous along the LoS and that the electron pitch-angle distribution is isotropic — assumptions made out of necessity, because available data barely constrain even the simplest model, even though CMEs are known to be strongly inhomogeneous structures. The thesis of Kansabanik (2023) changed this picture: for one CME, a robust Stokes V detection at a single spectral slice could not be reproduced by any homogeneous, isotropic GS model that also fit the Stokes I spectrum and the Stokes V upper limits (published as Kansabanik et al. 2024), and toy simulations showed that inhomogeneity in the magnetic field strength, the electron energy spectral index, and the low-energy cutoff produces spectra that homogeneous fits misrepresent. The thesis therefore proposed — but did not implement — a physics-based forward-modeling program: instead of fitting each telescope-resolution element independently, build a single three-dimensional model of the CME plasma parameters, synthesize the full spectropolarimetric GS image cube using fast GS radiation codes (numerical codes that rapidly compute GS emission and absorption coefficients for arbitrary electron distributions), and iteratively adjust the model parameters to minimize the difference between synthetic and observed image cubes. This forward-modeling approach has been demonstrated for solar flares but not for CMEs. Three developments since then frame the current status: Chen et al. (2025) combined microwave imaging from the Extended Owens Valley Solar Array (EOVSA) and meterwave imaging from the Owens Valley Long Wavelength Array (OVRO-LWA) to measure a CME flux rope (a twisted magnetic rope structure) field from 300 G near the limb to 0.6 G at 1.83 solar radii — still with homogeneous-source fitting; Husidic et al. (2025) coupled a three-dimensional magnetohydrodynamic (MHD) coronal model with a particle-transport code and fast GS codes to synthesize GS spectra from an erupting flux rope with realistic anisotropic electron distributions — but only in total intensity and without attempting to fit observations; and the 2026 SKA science review by Kansabanik et al. still lists accounting for inhomogeneous plasma structure in GS modeling as future work requiring the sensitivity of the Square Kilometre Array.

Problem Statement

Construct and validate a physics-based forward model of gyrosynchrotron emission from coronal mass ejection plasma that can be inverted against real radio spectropolarimetric observations. Specifically: (a) the forward model must incorporate realistic spatial inhomogeneity of the CME plasma parameters — for example the magnetic field strength and orientation, the thermal and nonthermal plasma density, and the nonthermal electrons' energy spectra and pitch-angle distributions — along and across the line of sight, and must synthesize both total-intensity (Stokes I) and circular-polarization (Stokes V) image cubes rather than per-pixel spectra, using a single coherent three-dimensional CME model; (b) it must remain sufficiently constrained — for example through regularization exploiting the spatial continuity of plasma parameters, geometric constraints from multi-vantage-point white-light observations, and parsimonious parametrizations — that its parameters, notably the CME magnetic field strength, can be estimated from finite spectropolarimetric datasets of the kind now obtained (MWA, OVRO-LWA, EOVSA, MeerKAT — the Meer Karoo Array Telescope in South Africa) or expected from the SKA; and (c) the modeling framework must be used to quantify the magnitude and statistical impact of the errors introduced by the classical homogeneity and isotropy assumptions as a function of observational regime (spectral coverage including whether the spectral peak is sampled, signal-to-noise ratio, and spatial resolution), thereby mapping where simple homogeneous models are acceptable and where full forward modeling is required. Validation may be carried out on realistically simulated spectropolarimetric image cubes rather than on real observations: it suffices to build the cubes from a known inhomogeneous, anisotropic ground-truth CME model that is matched to the spectral coverage, signal-to-noise ratio, and spatial resolution of the instruments named above, and to demonstrate that the inversion recovers the ground-truth magnetic field strength and inhomogeneity parameters within their stated uncertainties, with the residual homogeneity bias quantified as required in (c).

References

  1. Kansabanik, D. (2023). Deciphering Radio Emission from Solar Coronal Mass Ejections using High-fidelity Spectropolarimetric Radio Imaging. PhD thesis, Tata Institute of Fundamental Research. DOI: 10.48550/arXiv.2310.16072. https://arxiv.org/abs/2310.16072
  2. 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://arxiv.org/abs/2301.06522
  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. https://arxiv.org/abs/2404.14714
  4. 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–L69. DOI: 10.1086/323421. https://doi.org/10.1086/323421
  5. 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
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