Solar Flare Hosts MeV-peaked Electrons in a Coronal Source

Solar flares promptly release large amounts of free magnetic energy in the solar corona to produce substantial populations of high-energy charged particles, both ions and electrons. These particles are detected when they radiate microwaves in solar magnetic fields and X- and γ-rays when they encounter matter. Analysis of γ-rays in solar flares has revealed a distinct continuum component dominating at MeV energies, which differs from the well-studied X-ray continuum produced by flare-accelerated electrons with steeply falling energy spectra. The origin and precise spatial location and extent of this mysterious MeV component have 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. Here we report a joint study of this MeV-peaked electron population in the 2017-Sep-10 solar flare with Fermi MeV γ-ray data and EOVSA spatially resolved microwave imaging spectroscopy data. We demonstrate that the microwave spectrum from the peaked MeV distribution has a distinctly different shape from that produced by the well-known population of electrons with falling energy spectrum. We inspected microwave maps of the flare and identified an evolving area where the measured microwave spectra matched the theoretically expected one for the MeV-peaked population, thus pinpointing the site where this MeV component resides in the flare. The locations are in a coronal volume adjacent to the region where prominent release of magnetic energy and bulk electron acceleration were detected, which implies that transport effects play a key role in forming this population.

💡 Research Summary

The paper presents a comprehensive multi‑wavelength investigation of the 2017‑September‑10 solar flare, combining high‑energy γ‑ray observations from the Fermi Gamma‑ray Burst Monitor (GBM) with spatially resolved microwave imaging spectroscopy from the Expanded Owens Valley Solar Array (EOVSA). The central discovery is the identification of a distinct population of electrons (or positrons) whose energy distribution peaks at a few mega‑electron‑volts (MeV) and resides in a coronal volume (designated ROI 3) adjacent to the primary energy‑release sites.

γ‑ray spectral analysis, following the methodology of previous work (reference 6), reveals a continuum component that rises with photon energy up to ~1–5 MeV, requiring an electron spectrum that increases as E^{δ₁} (δ₁ ≤ 2) and then falls sharply as E^{−δ₂} (δ₂ = 2–8). This “MeV‑peaked” electron distribution is markedly different from the steeply falling power‑law spectra (δ ≈ 3–5) that dominate hard X‑ray bremsstrahlung below a few hundred keV.

EOVSA provides microwave images at 8.42 GHz and 17.92 GHz. In the high‑frequency map, brightness temperatures reach ~10¹⁰ K, implying that the emitting particles have mean energies of order 1 MeV (since for incoherent gyrosynchrotron emission the brightness temperature approximates the mean particle energy in the optically thick regime). More importantly, the microwave spectrum extracted from ROI 3 shows an unusually steep rise, F ∝ f^{β} with β ≈ 6, far steeper than the β ≲ 3 expected for the optically thick gyrosynchrotron emission of a conventional power‑law electron population.

The authors model the microwave emission using gyrosynchrotron theory and Markov Chain Monte Carlo (MCMC) fitting. They find that reproducing the observed steep spectrum requires an electron distribution with a low‑energy cutoff E_min ≈ 2–3.6 MeV, a high‑energy cutoff E_max ≈ 6 MeV, and a power‑law index δ ≈ 2. These parameters match precisely those inferred from the γ‑ray analysis, establishing that the same MeV‑peaked electrons (or positrons) are responsible for both the γ‑ray continuum and the anomalous microwave emission.

Three possible origins for this population are examined: (i) direct creation via β‑decay of radioactive nuclei or π‑decay of flare‑accelerated ions; (ii) a highly selective direct electric‑field acceleration that lifts a minuscule fraction (~10⁻⁷) of the thermal electrons to MeV energies; and (iii) collisional evolution of an initially standard power‑law electron population, where low‑energy electrons are rapidly lost while MeV electrons survive longer. Scenario (i) is disfavored because the decay timescales of β‑emitters (tens of seconds) do not match the observed γ‑ray timing, and there is no accompanying high‑energy π⁰‑decay γ‑ray signature. Scenario (ii) would require an electric field of order 2 × 10⁻⁵ V cm⁻¹ (≈E_D/30) and an acceleration path length ~10¹¹ cm, far exceeding the size of ROI 3, making it implausible.

Scenario (iii) emerges as the most viable. Using the measured coronal density n_th ≈ 2.5 × 10¹¹ cm⁻³ and magnetic field B ≈ 400–500 G, the Coulomb loss time for 1 MeV electrons is ~30 s, while for 3–5 MeV electrons it is on the order of 2 minutes. Thus, after the primary energy release ceases, sub‑MeV electrons are quickly thermalized, leaving a residual distribution that peaks at a few MeV. This naturally explains the observed duration of the MeV‑peaked component (~8 minutes) and its upward motion as the coronal source expands.

The study therefore resolves the long‑standing ambiguity about the origin of the flat MeV γ‑ray continuum: it is bremsstrahlung from a genuine MeV‑peaked electron population. Moreover, by pinpointing the spatial location of this population in the corona, the work demonstrates that transport effects—specifically collisional energy‑dependent loss—play a crucial role in shaping the high‑energy electron spectra in solar flares. The authors suggest that such MeV‑peaked electron populations should be a common, though often hidden, feature of flares, consistent with the detection of a similar γ‑ray component in a summed spectrum of 48 weaker events. This insight bridges microwave and γ‑ray diagnostics, offering a unified picture of electron acceleration, transport, and radiation in solar eruptive phenomena.