Fermi Gamma-ray Space Telescope

Evidence for Shock Source of Sustained Gamma-ray Emission from the Sun

Nat Gopalswamy
(P. Makela, S. Yashiro, S. Akiyama, H. Xie, R. J. MacDowall)


Gamma-ray emission from the Sun that lasts far beyond the flare impulsive phase has come into focus again because of the large number of such events observed by Fermi Large Area Telescope (Fermi/LAT). Such events are now called sustained gamma ray emission (SGRE) events. Some SGRE events last for almost a day. SGRE is thought to be pion continuum resulting due to >300 MeV protons impacting the solar chromosphere. Two possibilities have been discussed in the literature: (i) continued acceleration/trapping of protons in large-scale magnetic structures in the flare and (ii) precipitation of shock-accelerated protons. It is well established that long-lasting solar energetic particle (SEP) events are due to shocks driven by coronal mass ejections (CMEs). One of the best observational signatures of shocks is the interplanetary type II radio emission due to nonthermal electrons accelerated in the shock front. Shocks start accelerating particles very close to the Sun and often continue to do so far into the IP medium resulting in kilohertz radio emission. We examined the type II burst association of 14 SGRE events detected by Fermi/LAT that have durations exceeding ~5 hours. Every one of the SGRE events is associated with an IP type II radio burst. Furthermore, the SGRE duration is highly correlated with the duration over which the type II bursts last and anti-correlated with the ending frequency of the type II bursts. This result strongly supports the idea that protons accelerated at the shock front travel back to the Sun to precipitate and produce SGRE. The protons must be traveling along field lines threading the shock front and lying at the periphery of the CME flux rope. Initial estimates show that the shocks at a distance of several tens of solar radii when the SGRE and type II bursts end. However, >300 MeV SEP events are not observed in most of the events. This can be explained by the fact that SEP events need magnetic connectivity to the observer, whereas type II bursts and SGRE are electromagnetic emissions and hence do not have the connectivity requirement.