OVSA Science Highlight No. 6: Detection of Radio Gyroresonance Emission from a CME

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Contributed by Surajit Mondal1 (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 26, 2025.

The space weather potential of a coronal mass ejection (CME) depends significantly on its magnetic field. Hence, considerable effort has been invested in developing various techniques to measure the CME magnetic field. Radio observations are particularly useful for this purpose. The nonthermal gyrosynchrotron emission, produced by the interaction of nonthermal electrons with the CME's magnetic field, provides an excellent means for diagnosing the CME magnetic field (e.g., Bastian et al. 2001, Mondal et al. 2020). However, such diagnostics are only possible in regions where the CME's magnetic field structure is “illuminated” by accelerated nonthermal electrons. The quest continues in searching for new or alternative means of magnetic field diagnostics.

In this work, we report the first detection of thermal gyroresonance radio emission associated with a CME eruption in the middle corona, using observations made by the newly commissioned Owens Valley Radio Observatory’s Long Wavelength Array (OVRO-LWA) in the 13–87 MHz frequency range. The emission processes of thermal gyroresonance emission are very similar to nonthermal gyrosynchrotron, but the source electrons are thermal and are more ubiquitous throughout the CME. However, the radio surface brightness of this emission is low, making its detection difficult in the past. With the high sensitivity and dynamic range offered by OVRO-LWA’s more than 300 antenna elements, we are able to make the first detection of this emission and, in turn, use it to measure the local magnetic field and track its evolution with time.

The CME was first detected by both LASCO C2 and OVRO-LWA on March 9, 2024, around 22:12 UT. After the initial detection, we used OVRO-LWA data to track the CME till about 00:20 UT on March 10, 2024, when the sun set at the telescope site. In Figure 1, we have overlaid radio contours at 39 MHz (red) and 80 MHz (blue) contours on top of a LASCO C2 white light image. The white light image is shown using an inverted color scale, with darker colors implying brighter emission. We observe that the radio emission mostly tracks the dense prominence material. This is in stark contrast to previous reports of nonthermal gyrosynchrotron emission, where the emission is primarily observed from the CME flanks (Bastian et al. 2001, Mondal et al. 2020).

SMondal2025 fig1.jpg Figure 1: 39 and 80 MHz contours in red and blue, respectively, are overlaid on a LASCO C2 white light image. The white-light image is shown in an inverted color scale (i.e., black is bright). The observation time of the radio image is shown in the title of each panel. The time of the white-light image is indicated in each panel. We have been able to track the CME from the time it was visible from Earth till sunset at the observatory. We also find that the radio emission detected here primarily originates from the dense plasma visible in white-light images.
SMondal2025 fig2.jpg Figure 2: The top four rows show OVRO-LWA radio images at a few example frequencies and times. The time corresponding to each column is indicated at the top of each column. The contour levels are at 0.02, 0.04, 0.08, 0.16, 0.32, 0.64, and 1 MK. In the bottom row, we have shown the nearest white light image from the LASCO C2. The time corresponding to the white light image is written in each panel. The magenta ellipse in each panel shows the region from which the spectrum shown in Figure 3 has been extracted.

We extract the spectrum ahead of the bulk of the CME core and find that it exhibits a typical gyroresonance spectrum. At lower frequencies, the spectrum is optically thick. The brightness temperature falls sharply with an increase in the observation frequency. In Figure 2, we show radio images at a few selected times and frequencies. The regions from which the spectra are extracted are also indicated with the red ellipse. The spectra corresponding to each region are shown in the left panel of Figure 3. In the right panel of Figure 3, we also fit an example spectrum with a thermal gyroresonance model. With the spectral analysis, we estimate that the magnetic field of the radio-emitting regions varies from 7.9–5.6 G between 4.9-7.5 R⊙ . This is consistent with the magnetic field estimates obtained earlier from gyro-synchrotron emitting CMEs, if we extrapolate the obtained magnetic fields to lower heights using typical scaling relations (Koya et al. 2024).

SMondal2025 fig3.jpg
Figure 3: The left panel shows the spectrum from different regions on the CME. The region from which the spectrum is extracted is shown in Figure 2. We find that the spectrum is optically thick at lower frequencies and shows a sharp drop in brightness temperature at higher frequencies. This is consistent with gyroresonance emission from a thermal population of electrons. Right panel: Model spectrum shown in blue, which describes the observed spectrum for 00:16:05 shown in the left panel. The parameters of the model are indicated in the figure as well.

Based on the recent paper by Surajit Mondal, Bin Chen, Xingyao Chen, Sijie Yu, Dale Gary, Peijin Zhang et al. (2025), "Possible First Detection of Gyroresonance Emission from a Coronal Mass Ejection in the Middle Corona," The Astrophysical Journal, in press.

References

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