High-Energy Gamma-Ray Emission From Solar Flares: Summary of Fermi LAT Detections and Analysis of Two M-Class Flares

TL;DR

This paper reports the detection of 19 high-energy gamma-ray solar flares by Fermi LAT, analyzes two specific long-lasting events, and suggests proton acceleration is common even in modest, long-duration flares.

Contribution

It provides the first detailed analysis of long-duration gamma-ray emissions from solar flares and supports the idea that proton acceleration occurs more frequently than previously believed.

Findings

19 flares detected with Fermi LAT in four years

Proton acceleration occurs even in modest, long-lasting flares

Gamma-ray emission duration can extend up to 13 hours

Abstract

We present the detections of 19 solar flares detected in high-energy gamma rays (above 100 MeV) with the Fermi Large Area Telescope (LAT) during its first four years of operation. Interestingly, all flares are associated with fairly fast Coronal Mass Ejections (CMEs) and are not all powerful X-ray flares. We then describe the detailed temporal, spatial and spectral characteristics of the first two long-lasting events: the 2011 March 7 flare, a moderate (M3.7) impulsive flare followed by slowly varying gamma-ray emission over 13 hours, and the 2011 June 7 M2.5 flare, which was followed by gamma-ray emission lasting for 2 hours. We compare the Fermi-LAT data with X-ray and proton data measurements from GOES and RHESSI. We argue that a hadronic origin of the gamma rays is more likely than a leptonic origin and find that the energy spectrum of the proton distribution softens after the 2011 March 7 flare, favoring a scenario with continuous acceleration at the flare site. This work suggests that proton acceleration in solar flares is more common than previously thought, occurring for even modest X-ray flares, and for longer durations.