OVSA Science Highlight No. 5: Is CME's Magnetic Flux Conserved?
Contributed by Xingyao Chen1 (1Center for Solar-Terrestrial Research, New Jersey Institute of Technology, 323 Martin Luther King Jr Blvd., Newark, NJ 07102-1982, USA); Edited by B. Chen. Posted on September 19, 2025.
Probably yes, if a magnetic flux rope truly comprises the “core” of the CME. However, we have not had direct proof heretofore. One of the main challenges is to measure the magnetic field of a CME itself, let alone track its evolution during the eruption (e.g., a review by P. Chen et al. 2011, and references therein).
Spatially resolved radio spectral analysis of gyrosynchrotron radiation, emitted by nonthermal electrons trapped in the CME body, provides a unique means for constraining the magnetic field of CMEs (e.g., T. Bastian et al. 2001). However, such a technique has only been applied to a few CME cases in the literature. Also, limited by the bandwidth of the instrumentation, all previous measurements were made at a given or narrow range of coronal heights (e.g., S. Mondal et al. 2020, B. Chen et al. 2020).
Here we use ultrabroadband radio imaging spectroscopy, combining microwave data from the Expanded Owens Valley Solar Array (EOVSA) in 1–18 GHz and meter-decameter-wave observations in 32–87 MHz made by the Owens Valley Radio Observatory’s Long Wavelength Array (OVRO-LWA), to measure the evolving magnetic field of an erupting CME from the low to the middle corona. These measurements reveal a magnetic field strength of ~300-400 G at a coronal height of ~0.02 R⊙ in the flux rope legs soon after the CME initiation, decreasing to ~1 G at 1.83 R⊙ near the CME front as it propagates to the middle corona. By combining with constraints of the cross-section of the CME at these different periods, we found strong evidence that the magnetic flux of the CME flux rope is conserved.
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Figure 1: (a) EOVSA microwave spectral imaging of the CME initiation, and (b) a schematic diagram illustrating microwave sources associated with the CME driven by an erupting magnetic flux rope in the low corona. (c) OVRO-LWA metric-decametric-wave spectral imaging of the CME as it propagates to the middle corona, and (d) a schematic depiction of metric-decametric-wave sources in relation to the CME/flux rope system. |
This study takes advantage of joint microwave and meter-wave observations of the gyrosynchrotron sources of the erupting CME bubble, bridging the observational gap between the CME initiation and evolution. These results demonstrate the unique power of broadband radio imaging spectroscopy to probe CMEs’ magnetic fields over a wide range of coronal heights – a key observational input to modeling CMEs for space weather predictions.
Based on the recent paper by Xingyao Chen, Bin Chen, Sijie Yu, Surajit Mondal et al. (2025), "Measuring the Magnetic Field of a Coronal Mass Ejection from the Low to Middle Corona," The Astrophysical Journal Letters, 990, L50.
References
- Chen, P. (2011), "Coronal Mass Ejections: Models and Their Observational Basis," Living Reviews in Solar Physics, 8, 1.
- Bastian, T. et al. (2001), "The Coronal Mass Ejection of 1998 April 20: Direct Imaging at Radio Wavelengths," ApJL, 558, L65
- Mondal, S. et al. (2020), "Estimation of the Physical Parameters of a CME at High Coronal Heights Using Low-frequency Radio Observations," ApJ, 893, 28
- Chen, B. et al. (2020), "Microwave Spectral Imaging of an Erupting Magnetic Flux Rope: Implications for the Standard Solar Flare Model in Three Dimensions," ApJL, 895, L50