OVSA Science Highlight No. 7: Are Nonthermal Electrons Ubiquitously Present in the Quiet Middle Corona?

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 January 30, 2026.

The answer is probably yes. However, we did not yet have direct evidence. While radio observations are very sensitive to the presence of nonthermal electrons compared to most other wavebands, detecting the emission from relatively weak nonthermal electrons requires very high sensitivity and high-dynamic-range imaging observations at low radio frequencies. This requirement has now been fulfilled with the successful commissioning of the Owens Valley Radio Observatory’s Long Wavelength Array (OVRO-LWA).

In this study, the authors present multiple instances of extended weak radio emissions, even in the absence of any other apparent activity. The left and middle panels of Figure 1 show the radio emission using contours overlaid on top of a white-light image. The presence of a weak extended emission is evident. The detected emission is weaker than the quiet Sun, and only becomes visible with the availability of the high-fidelity images produced by OVRO-LWA. The right panel shows the spectrum extracted from the elliptical region shown in the left panel. Although in each instance, the emission is detected for a few minutes, such emissions are often detected at the same sky location on multiple occasions. Figure 2 shows multi-frequency radio contours at multiple times, overlaid on the nearest available LASCO C2 white light image. In both Figures 1 and 2, no apparent activity is observed at the same sky location as the radio emission, in white-light images. Using context data from white-light observations as well as the spectral shape of the detected radio emission, this study demonstrates that the detected emission is nonthermal in nature. The available data were insufficient to determine if the emissions were gyrosynchrotron in nature or were powered by the plasma emission mechanism. However, the data clearly demonstrate that the emission is powered by nonthermal electrons and thus unambiguously prove that nonthermal electrons are indeed present in the quiescent middle corona. How ubiquitous they are remains an open question for future research.

Figure 1: Left panel shows the 43 MHz contours overlaid on top of a LASCO C2 white light difference image at 17:16 UT. The lowest contour is at 0.04 MK and then increases in multiples of 2. Middle panel: Shows 0.04 MK contours at multiple frequencies on the same white-light difference image. The extended weak emission towards the south-eastern limb is evident. Right panel shows the spectrum extracted from the region using the red ellipse in the left and middle panels, which clearly deviates from the free-free emission expected from the coronal plasma (black line).

Figure 2: Multi-frequency radio contours at 0.05 MK over the nearest available LASCO C2 white light difference image. Weak extended emission towards the south-eastern limb is observed at 16:57 and 17:16 UT, but is not present at the other times shown here.

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Based on the recent paper by Surajit Mondal, Bin Chen, Sijie Yu, Xingyao Chen, Peijin Zhang, Dale Gary, et al. (2025), "Enigmatic Centi-SFU and mSFU Nonthermal Radio Transients Detected in the Middle Corona," The Astrophysical Journal, 994, 254.