| Name: | Nat Gopalswamy |
| Affiliation: | NASA Goddard Space Flight Center |
| Conference ID : | ASI2023_223 |
| Title : | New insights into the origin of long-lived solar gamma-ray emission |
| Authors : | Nat Gopalswamy, Pertti Mäkelä, Sachiko Akiyama, Hong Xie, and Seiji Yashiro
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| Mode of Presentation: | Oral |
| Abstract Category : | Sun and the Solar System |
| Abstract : | Sustained gamma ray emission (SGRE) from the Sun were discovered by the Gamma Ray Spectrometer on board the Solar Maximum Mission by Forrest et al. (1985). SGRE often peaks well after the end of the impulsive phase of the associated flare and can last up to a day. There has been intense debate over how the SGRE-causing >300 MeV particles are accelerated: by flare reconnection or MHD shocks driven by coronal mass ejections (CMEs). The Fermi Large Area Telescope (Fermi/LAT) has detected dozens of SGRE events since 2011 compared to only a handful of events in the pre-Fermi era. Examination of the underlying CMEs reveals that the CMEs have very high speeds (average ~2000 km/s) and very wide (halo CMEs), similar to CMEs causing ground level enhancement (GLE) events. The SGRE duration also matched that of interplanetary type II radio bursts suggesting that the same shock accelerates electrons to produce type II bursts and protons to produce SGRE. The close SGRE - fast halo association supports the shock paradigm for SGRE. The close association prompts the question: are all fast halos associated with SGRE events? To answer this, we investigated the SGRE association of 22 ultrafast halo CMEs selected with the criterion that the CME sky-plane speed ≥1800 km/s as observed by the Solar and Heliospheric Observatory (SOHO). All the halos are associated with large SEP events and IP type II bursts. The average speed of the 22 CMEs is ~2100 km/s, indicating that these CMEs are capable of accelerating >300 MeV particles. We find that all frontside halos are associated with SGRE, strongly supporting the shock paradigm. Even among the 9 backside events, 2 were associated with SGRE, further indicating CME shock as the source of the energetic particles. |
