Probing the Puzzle of Fermi Long-Duration Gamma-Ray Flares by Data-driven Global MHD Simulations

Abstract:

With the ever growing number of long-duration, >100 MeV gamma-ray solar flares observed by Fermi/LAT, it poses a puzzle on the underlying particle acceleration and transport mechanisms. Further challenges come from (i) recent detection of gamma-rays in behind-the-limb (BTL) flares (e.g., the 2014 September 1 event), in which the gamma-ray emission region is located away from the BTL flare site by tens of degrees in heliographic longitude, and (ii) migration of gamma-ray emission centroids on the solar disk hours past the impulsive phase (e.g., the 2012 March 7 event). Most of the long-duration events are associated with fast CMEs, it is thus necessary to understand the role of CMEs and CME-driven shocks in these events. To probe this puzzle, we perform data-driven, global magnetohydrodynamics simulations of CMEs associated with the long-duration gamma-ray flares. We investigate the magnetic connectivity and evolution of the CME-driven shocks, and their relationship, in both space and time, with the observed gamma-ray emission. Specifically, we derive and track the time-varying shock parameters over the area that is magnetically connected to the gamma-ray emission region. Based on the modeling results, we discuss the causes of Fermi long-duration gamma-ray events. In particular, we address the possibility of CME shock-accelerated particles traveling back to the Sun to produce gamma-rays, a scenario that bears potentially paradigm-shifting implications on particle acceleration and transport in solar eruptive events including flares and CMEs.