Fermi-LAT observations of the 2017 September 10$^{th}$ solar flare (1803.07654v1)

Abstract: The Fermi-Large Area Telescope (LAT) detection of the X8.2 GOES class solar flare of 2017 September 10 provides for the first time observations of a long duration high-energy gamma-ray flare associated with a Ground Level Enhancement (GLE). The >100 MeV emission from this flare lasted for more than 12 hours covering both the impulsive and extended phase. We present the localization of the gamma-ray emission and find that it is consistent with the active region (AR) from which the flare occurred over a period lasting more than 6 hours contrary to what was found for the 2012 March 7 flares. The temporal variation of the proton index inferred from the gamma-ray data seems to suggest two phases in acceleration of the proton population. Based on timing arguments we interpret the second phase to be tied to the acceleration mechanism powering the GLE, believed to be particle acceleration at a coronal shock driven by the CME.

Analysis of Fermi-LAT Observations of the 2017 September 10 Solar Flare

The paper "Fermi-LAT observations of the 2017 September 10$^{th}$ solar flare" by Omodei et al. presents an in-depth investigation of an X8.2 class solar flare detected by the Fermi Large Area Telescope (Fermi-LAT), emphasizing the distinctive long-duration high-energy gamma-ray emissions observed in conjunction with a Ground Level Enhancement (GLE). The research offers substantial advancements in our understanding of solar flare physics, particularly concerning particle acceleration mechanisms.

Key Observations and Data Analysis

The 2017 September 10 solar flare was characterized by the persistence of gamma-ray emissions exceeding 100 MeV for over 12 hours. Notably, the emission encompassed both the impulsive and extended phases of the flare, marking it as a rare event of prolonged high-energy activity. The flare ensued from an active region on the Sun, with gamma-ray emissions spatially consistent with this region for more than six hours. This behavior contrasts with previous observations, such as those from the 2012 March 7 flares, where the gamma-ray emission source appeared to traverse the solar disk.

Temporal variations in the inferred proton index from gamma-ray data suggest dual acceleration phases for the proton population. Initially, between 15:58-16:08 UT, the proton index showed a softening trend, following the descending phase of hard X-ray emissions. A subsequent hardening phase overlapped a resurgence in gamma-ray flux, with the proton index hardening once more when the Sun re-entered the Fermi-LAT field of view post-17:33 UT. This cyclic behavior implies distinct particle acceleration mechanisms active at varying phases of the flare.

Spectral and Spatial Analysis

Spectral analysis involved fitting the Fermi-LAT data with power-law and pion-decay spectral models, revealing significant fluctuations in the photon index and cutoff energy. The proton spectral index exhibited clear phases of softening and hardening, which align with changes expected if multiple acceleration mechanisms are at play. Spatially, the localization analysis corroborates that the gamma-ray emission stemmed from the active region, further supporting hypotheses linking flare-induced proton acceleration with the originating active region.

Implications and Future Directions

The findings enrich the current understanding of solar flares, particularly in linking gamma-ray emissions and GLEs with coronal mass ejection (CME)-driven shocks. The demonstrated connection between the temporal evolution of gamma-ray emissions and variations in the proton index indicates complex interactions at play during such solar events. Future research should focus on high-resolution temporal and spatial correlation analyses of gamma-ray, CME, and SEP data to refine models of particle acceleration in solar flares.

Conclusion

The observations documented in this paper offer comprehensive insights into the dynamics of the 2017 September 10 solar flare. By employing advanced methods for spectral analysis and spatial localization, the research effectively characterizes the long-duration nature of the flare's gamma-ray emissions. Subsequent investigations should explore the varied particle acceleration mechanisms suggested by the multifaceted proton index evolution, enhancing the broader theoretical framework underpinning solar energetic events.