Possible First Detection of Gyroresonance Emission from a Coronal Mass Ejection in the Middle Corona

TL;DR

This paper presents the first potential detection of gyroresonance emission from a coronal mass ejection in the middle corona as a remote diagnostic of CME magnetic fields. By analyzing OVRO-LWA observations across $13.4$–$86.9$ MHz and associating the observed spectra with gyroresonance, the authors estimate a magnetic field of $B \approx 5.6$–$7.9$ G over $4.9$–$7.5\,R_\\odot$, under the assumption that the break frequency corresponds to the third harmonic ($\nu_g=2.8 s B$ MHz, $s\approx3$). They argue that the high field is localized to small magnetic islands within the CME, consistent with high-resolution white-light images showing fine-scale magnetic structure. This work demonstrates a promising remote-sensing pathway to map spatially varying CME magnetic fields and motivates further imaging spectropolarimetric observations to refine the method and its physical interpretations.

Abstract

Routine measurements of the magnetic field of coronal mass ejections (CMEs) have been a key challenge in solar physics. Making such measurements is important both from a space weather perspective and for understanding the detailed evolution of the CME. In spite of significant efforts and multiple proposed methods, achieving this goal has not been possible to date. Here we report the first possible detection of gyroresonance emission from a CME. Assuming that the emission is happening at the third harmonic, we estimate that the magnetic field strength ranges from 7.9--5.6 G between 4.9-7.5 $R_\odot$. We also demonstrate that this high magnetic field is not the average magnetic field inside the CME, but most probably is related to small magnetic islands, which are also being observed more frequently with the availability of high-resolution and high-quality white-light images.

Figures (6)

• Figure 1: Left panel: Location of Solar Orbiter, Earth, and Sun. Middle and right panel: SolO/FSI 304Å images at two different times showing the early times of the solar eruption. The time indicated on the title of the panels show the observation time, after correcting for the light travel time between SolO and Earth.
• Figure 2: OVRO-LWA 39 and 80 MHz contours overlaid on LASCO C2 images at multiple timestamps beginning at a time when the CME is not yet in the LASCO C2 field of view, till the time of sunset at the observatory. The 39 and 80 MHz contours are drawn at 0.07 and 0.025 MK, respectively. The title of each panel shows the time of the radio image. The time written inside each panel shows the time of the background white-light image, which is shown in inverted grayscale (i.e., dark color is brighter). Due to the limited time cadence of LASCO C2, the nearest available white-light image is chosen as the background.
• Figure 3: Shows the dynamic spectrum from OVRO-LWA in uncalibrated units. The red vertical lines show the times within the interval shown for which we find spectra consistent with gyroresonance origin. There is also a data gap between 22:56 -- 23:24 UT and has been darkened.
• Figure 4: The top four rows show OVRO-LWA radio images at a few example frequencies and times during the WL CME. 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 spectrum, shown in Figure \ref{['fig:spectra']} has been extracted.
• Figure 5: Spectra for different times are shown. The region from which the spectrum has been extracted is shown with an ellipse over the image at the corresponding time in Figure \ref{['fig:cme_multi_freq_images']}. Circles and triangles show the detections and upper limits respectively.