On the Origin of Solar Long-Duration Gamma-Ray Flares
| Nugget | |
|---|---|
| Number: | 509 |
| 1st Author: | Alessandro BRUNO |
| 2nd Author: | et al. |
| Published: | November 3, 2025 |
| Next Nugget: | TBD |
| Previous Nugget: | FAI and GOES eclipses |
Introduction
Do we really need a CME to produce a long-duration solar gamma-ray event (LDGRF)? Long-duration gamma-ray flares (LDGRFs) are solar events in which >100 MeV γ-ray emission persists up to tens of hours after the flare impulsive phase. These photons are mostly produced in the interaction as secondary emissions, following the primary interactions of of >300 MeV protons and >200 MeV/n &\alpha;-particles in the photosphere. These produce copious pions that quickly decay, with γ-ray emission. Notably the mechanism accelerating the parent ions remains unresolved. Two leading scenarios are (a) back-propagation of particles accelerated at large-scale shock waves driven by coronal mass ejections (Ref. [1]); and (b) particle trapping with stochastic acceleration inside large-scale coronal loops (Ref. [2]). Support for the CME/shock paradigm mainly comes from observed statistical associations between LDGRFs, fast CMEs and large solar energetic particle (SEP) events, yet these correlations driven by biggest events are neither universal nor proof of causation (Ref. [3]). A challenging counter-example can be found in the SOL2024-07-16 event, described here and in our paper (Ref. [4]).
Observations
The parent GOES class X1.9 flare peaked at 13:26 UT and was at the western limb (S05W83). Fermi/LAT began detecting >100 MeV γ-rays around 14 UT, with emission continuing for ~7 hours (see Figure 1). The measured photon energies exceeded 1 GeV, and the inferred parent ion spectrum was among the hardest reported. In contrast, the accompanying CME was relatively slow (~600 km/s); the associated type-II radio burst was faint and short-lived, not extending to kHz frequencies; and, as shown in Figure 2, SEP signatures were weak and limited to a few tens of MeV, even at magnetically well-connected locations (see Figure 3a), with a barely-visible increase in the GOES >10 MeV proton channel. Meanwhile, giant coronal loops appeared around 13:30 UT in the 94 EUV images from SDO/AIA and GOES/SUVI, persisting well beyond the end of the LDGRF (see Figure 3b-d).
Discussion
All the shock-related phenomena accompanying this event were weak and exhibited no apparent link with the γ-ray emission. This suggests that the CME-driven shock was not strong enough to accelerate ions to energies well above the pion-production threshold needed to account for the measured γ-ray emission. The alternative coronal-loop origin scenario is instead favored by the observation of large-scale structure of hot plasma over the source region for the entire duration of this LDGRF.
Conclusions
The 2024 July 16 event demonstrates that a very fast CME resulting in a high-energy SEP event in the interplanetary space is not a necessary condition for the occurrence of an LDGRF. These findings challenge the idea that the high-energy γ-ray emission is produced by the back-precipitation of ions accelerated at CME-driven shocks into the solar surface.
Acknowledgements
The co-authors of this Nugget and Ref. [1] are M. Pesce-Rollins, S. Dalla, N. Omodei, I.G. Richardson and J.M. Ryan
References
[1] [https://ui.adsabs.harvard.edu/abs/1993ICRC....3...91C "On the Origin of Gamma-Ray Emission from the Behind-the-Limb Flare on 29 September 1989"}
[2] Ryan, J. M. & Lee, M.A. 1991, "On the Transport and Acceleration of Solar Flare Particles in a Coronal Loop", ApJ, 368, 316, https://ui.adsabs.harvard.edu/abs/1991ApJ...368..316R
[3] Bruno, A., et al. 2023, "Statistical Relationship between Long-duration High-energy Gamma-Ray Emission and Solar Energetic Particles", ApJ, 953, 187, https://iopscience.iop.org/article/10.3847/1538-4357/ace24c