OVSA Science Highlight No. 8: Megaelectronvolt electrons in a coronal source of a solar flare

Contributed by Gregory Fleishman¹ (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.

Analysis of γ-rays in solar flares (blue spectral component in Fig. 1a) has suggested a distinct continuum component dominating at MeV energies (blue curve in Fig. 1b), which differs from the well-studied X-ray continuum (red spectral component in Fig. 1a) produced by flare-accelerated electrons with spectra steeply falling with energy (red line in Fig. 1b). The origin, emission mechanism, precise spatial location, and extent of this mysterious MeV component have all been unknown up to now. If it is produced by bremsstrahlung, such a γ-ray component requires an unusual population of electrons peaked at a few MeV (blue curve in Fig. 1b).

The article, recently published in Nature Astronomy, reports a joint study of this MeV-peaked electron population in the celebrated 2017 September 10 solar flare with Fermi/GBM's MeV γ-ray data and spatially-resolved microwave imaging spectroscopy data obtained by the Expanded Owens Valley Solar Array (EOVSA). We demonstrate that the microwave spectrum from the MeV-peaked distribution has a distinctly different shape (blue curves in Fig. 2) from that produced by the electrons with falling energy spectrum (red curves in Fig. 2). We inspected microwave maps of the flare (Fig. 3) and identified an evolving area (region of interest (ROI) 3 inside a blue poly-gon) where the measured microwave spectra (blue symbols in Fig. 2 that measured from a pixel marked by empty blue circle in Fig. 3) matched the theoretically expected ones for the MeV-peaked population, thus pinpointing the site where this MeV component resides.

This study reveals the location and extent of a distinct group of extremely energetic electrons in a large solar flare. Their energies peak at a few million electron volts—several times higher than the rest-mass energy of electrons.

Fig. 1. Fermi/GBM spectrum of one-minute accumulation during the 2017 September 10 flare and its spectral fit (a). Inferred shape of the MeV-peaked electron spectrum implied by this spectral fit. Fig. 2. Example of normal (red) and abnormally steep (blue) microwave spectra from different flare locations. The blue spectrum is produced by MeV-peaked electrons. Fig. 3. The distribution of the brightness temperature of microwave emission observed with EOVSA at (a) 8.42 and (b) 17.92 GHz at 15:57:48UT, along with three different ROIs defined in the legend. The images show different morphologies. The brightness temperature of the high-frequency source (b) has a peak value of 10,000 million Kelvin, indicative that MeV particles are the main contributors to this emission. The red square and blue circle symbols mark the selected pixels corresponding to the spectra presented in Fig. 2. Blue and red arrows show relationships between different ingredients of this study.

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Based on the recent paper by Gregory Fleishman, Ivan Oparin, Gelu Nita, Bin Chen, Sijie Yu, Dale Gary (2025), "Megaelectronvolt-peaked electrons in a coronal source of a solar flare," Nature Astronomy.