Evidence that SOL2012-06-03 Late Phase $γ$ Rays are Produced by $>$300 MeV Protons from CME-Shock Acceleration of Suprathermals from the Flare

A recent paper on SOL2012-06-03 reported the detection for the first time of two distinct phases of $>$100 MeV $γ$-radiation indicating separate acceleration processes. But such two-phase emission has been seen before and was first observed in SOL1982-06-03. The second phase is known as Late Phase Gamma-Ray Emission (LPGRE) and was cataloged for $>$40 solar eruptions, including SOL2012-06-03. Here we provide evidence that the second SOL2012-06-03 $π$-decay peak is the onset of LPGRE that lasted for $>$8 min. Its delay from the impulsive X-ray peak is consistent with the time it would take flare-produced suprathermal protons to overtake the expanding CME and be accelerated by its shock. The high accelerated ion-to-electron ratio in SOL2012-06-03 and other LPGRE events is consistent with the ratio observed in gradual SEP events produced by shocks and is inconsistent with ratios typically found in impulsive flares and solar energetic particle events produced by reconnection.

💡 Research Summary

The paper revisits the 2012 June 3 solar eruption (SOL2012‑06‑03) and challenges the claim that it provided the first observation of two distinct >100 MeV γ‑ray phases indicating separate acceleration mechanisms. By placing the event in the broader context of Late Phase Gamma‑Ray Emission (LPGRE), a phenomenon first identified in the 1982 June 3 flare and subsequently cataloged for more than forty eruptions, the authors argue that the second >100 MeV γ‑ray peak in SOL2012‑06‑03 is simply the onset of LPGRE rather than a novel flare‑related acceleration episode.

Key observational evidence includes: (1) a 17 s delay between the impulsive hard X‑ray peak and the second γ‑ray peak, which matches the time required for suprathermal protons (∼75 keV) produced at the flare peak to overtake a 900 km s⁻¹ CME, encounter its shock at ≈1.2 R☉, and be accelerated to >300 MeV within ≈10 s; (2) a hardening of the γ‑ray spectral index from 5.6 ± 1.4 during the first peak to 4.3 ± 1.0 during the second, consistent with a π‑decay dominated spectrum typical of LPGRE; (3) an increase in the late‑to‑impulsive phase flux ratio from ∼4 to ∼15 when the second peak is interpreted as LPGRE, indicating a distinct acceleration regime.

The authors also estimate particle numbers: ≈3 × 10³¹ protons above 100 MeV and ≈1.5 × 10³⁴ electrons between 300–700 keV, yielding an electron‑to‑proton ratio of ∼500. This ratio matches that observed in gradual solar energetic particle (SEP) events, which are known to be shock‑accelerated, and is orders of magnitude lower than the ratios typical of impulsive SEP events. Consequently, the particle composition strongly supports a CME‑shock origin for the late‑phase γ‑rays.

Comparisons with earlier LPGRE cases (e.g., SOL1982‑06‑03, SOL1984‑04‑25) show that the temporal characteristics—onsets within a minute of the impulsive phase and durations ranging from a few to tens of minutes—are reproduced in SOL2012‑06‑03. The paper further critiques the statistical analysis presented in Pesce‑Rollins et al. (2025), noting mis‑plotted uncertainties in logarithmic spectra and an F‑test that only reaches 90 % confidence, insufficient to claim a definitive two‑Gaussian fit over a single component.

In summary, the second >100 MeV γ‑ray peak of SOL2012‑06‑03 is best interpreted as the beginning of LPGRE, driven by CME‑shock acceleration of flare‑produced suprathermal ions. This interpretation aligns the event with a well‑established class of long‑duration γ‑ray flares, reinforces the role of CME‑driven shocks in producing high‑energy solar particles, and underscores the importance of considering flare‑CME interactions when diagnosing solar energetic particle acceleration mechanisms.