Solar Physics Laboratory, NASA Goddard Space Flight Center
Source of High-Energy Protons Responsible for Neutral Pion Gamma-Ray Continuum from the Sun
Forrest et al. (1985) identified gamma-ray continuum emission well after the impulsive phase of a solar flare. They attributed the gamma-rays to the decay of neutral pions based on the characteristic peak of this emission around 70 MeV. We refer to this emission as sustained gamma-ray emission (SGRE). The neutral pions are produced due to the collision of protons accelerated in the eruption with those in the chromosphere. The origin of the energetic protons has been an unsolved problem for more than 30 years. The prevalent ideas are: (i) particles accelerated in the associated flare are somehow trapped in magnetic structures and diffuse slowly to the chromosphere, and (ii) particles accelerated at the associated shock diffuse back to the Sun. The issue with the flare mechanism is that SGRE can last for hours after the flare has ended and one has to come up with unusual conditions in post-eruption structures that trap the flare particles and release slowly. While there were only a couple of SGRE events with hours-long duration, the Fermi Large Area Telescopes have revealed that SGRE events are rather common and in some cases the duration can be almost a day. In addition to being time-extended, the SGRE events seem to be spatially extended as well, which is not consistent with the angular extent of flare structures. The only other source of energetic particles shock driven by the associated coronal mass ejection (CME). We know that CME-driven shocks accelerate particles for more than a day as inferred from interplanetary type II bursts and energetic storm particle events. We recently investigated this possibility and obtained a quantitative relation between SGRE and type II radio burst properties: the SGRE duration is very similar to the type II burst duration. Furthermore, the longer the SGRE duration, the smaller is the ending frequency of type II bursts suggesting the shock travels to larger distances. These results strongly support the shock source of >300 MeV protons and settle a long-standing issue that persisted for more than three decades.