A Study on the Near-Sun Speed and Acceleration of CMEs Associated with Sustained Gamma-Ray Emission Events Observed by FERMI-LAT

Sustained gamma-ray emission (SGRE) events, also called late-phase gamma-ray emission (LPGRE) events, are solar eruptive events that produce long-lasting >100 MeV gamma-ray flux. Enhanced gamma-ray emission can extend several hours after the impulsive phase of the associated solar flare. SGRE events have been detected since 1980s, but the highly sensitive Large Area Telescope (LAT) on the FERMI spacecraft has observed SGREs more frequently since 2010. The >100 MeV gamma-ray emission is generated by >300 MeV protons via neutral pion production and decay. Flare-related processes or shock waves driven by coronal mass ejections (CMEs) have been suggested as possible acceleration mechanisms of the >300 MeV protons. Previous studies have shown that SGRE events are associated with fast and wide halo CMEs. We describe in detail the near-Sun speed and acceleration of SGRE-event-associated CMEs and compare them with other CME populations without SGRE events. Fast acceleration suggests early shock formation with high speeds that are typical of ground level enhancement events and SGRE events indicating acceleration of high-energy particles. The height-time profiles are obtained by fitting 3D flux rope and shock wave models to the EUV and white-light images of the CME.