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Published October 10, 2025 | Version Published
Journal Article Open

Probing the Turbulent Corona and Heliosphere Using Radio Spectral Imaging Observation during the Solar Conjunction of the Crab Nebula

Creators

  • 1. ROR icon New Jersey Institute of Technology
  • 2. ROR icon University Corporation for Atmospheric Research
  • 3. ROR icon California Institute of Technology
  • 4. ROR icon Jet Propulsion Lab
  • 5. ROR icon Arizona State University
  • 6. ROR icon George Mason University
  • 7. ROR icon University of New Mexico
  • 8. Real-Time Radio Systems Ltd, Bournemouth, Dorset BH6 3LU, UK
  • 9. ROR icon Rice University

Abstract

Measuring plasma parameters in the upper solar corona and inner heliosphere is challenging because of the region's weakly emissive nature and inaccessibility for most in situ observations. Radio imaging of broadened and distorted background astronomical radio sources during solar conjunction can provide unique constraints for the coronal material along the line of sight. In this study, we present radio spectral imaging observations of the Crab Nebula (Tau A) from 2024 June 9 to June 22 when it was near the Sun with a projected heliocentric distance of 5–27 solar radii, using the Owens Valley Radio Observatory's Long Wavelength Array at multiple frequencies in the 30–80 MHz range. The imaging data reveal frequency-dependent broadening and distortion effects caused by anisotropic wave propagation through the turbulent solar corona at different distances. We analyze the brightness, size, and anisotropy of the broadened images. Our results provide detailed observations showing that the eccentricity of the unresolved source increases as the line of sight approaches the Sun, suggesting a higher anisotropic ratio of the plasma turbulence closer to the Sun. In addition, the major axis of the elongated source is consistently oriented in the direction perpendicular to the radial direction, suggesting that the turbulence-induced scattering effect is more pronounced in the direction transverse to the coronal magnetic field. Lastly, when the source undergoes large-scale refraction as the line of sight passes through a streamer, the apparent source exhibits substructures at lower frequencies. This study demonstrates that observations of celestial radio sources with lines of sight near the Sun provide a promising method for measuring turbulence parameters in the inner heliosphere.

Copyright and License

© 2025. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

P.Z. acknowledges support for this research by the NASA Living with a Star Jack Eddy Postdoctoral Fellowship Program, administered by UCAR’s Cooperative Programs for the Advancement of Earth System Science (CPAESS) under award 80NSSC22M0097. The OVRO-LWA expansion project was supported by NSF under grant AST-1828784. OVRO-LWA operations for solar and space weather sciences are supported by NSF under grant AGS-2436999.

Facilities

OVRO:SA - Owens Valley Radio Observatory's Solar Array.

Software References

astropy.

Files

Zhang_2025_ApJ_992_128.pdf

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2506.01632 (arXiv)

Funding

National Aeronautics and Space Administration
80NSSC22M0097
University Corporation for Atmospheric Research
National Science Foundation
AST-1828784
National Science Foundation
AGS-2436999

Dates

Accepted
2025-08-20

Caltech Custom Metadata

Caltech groups
Astronomy Department, Owens Valley Radio Observatory, Division of Physics, Mathematics and Astronomy (PMA)
Publication Status
Published