The magnetic connectivity of coronal shocks from behind-the-limb flares to the visible solar surface during γ-ray events
1 Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse, France
2 CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
3 Department of Astronomy, University of Maryland, College Park, MD 20742, USA
4 National Observatory of Athens, PO Box 20048, Thissio, 11810 Athens, Greece
Received: 17 March 2017
Accepted: 28 July 2017
Context. The observation of >100 MeV γ-rays in the minutes to hours following solar flares suggests that high-energy particles interacting in the solar atmosphere can be stored and/or accelerated for long time periods. The occasions when γ-rays are detected even when the solar eruptions occurred beyond the solar limb as viewed from Earth provide favorable viewing conditions for studying the role of coronal shocks driven by coronal mass ejections (CMEs) in the acceleration of these particles.
Aims. In this paper, we investigate the spatial and temporal evolution of the coronal shocks inferred from stereoscopic observations of behind-the-limb flares to determine if they could be the source of the particles producing the γ-rays.
Methods. We analyzed the CMEs and early formation of coronal shocks associated with γ-ray events measured by the Fermi-Large Area Telescope (LAT) from three eruptions behind the solar limb as viewed from Earth on 2013 Oct. 11, 2014 Jan. 06 and Sep. 01. We used a 3D triangulation technique, based on remote-sensing observations to model the expansion of the CME shocks from above the solar surface to the upper corona. Coupling the expansion model to various models of the coronal magnetic field allowed us to derive the time-dependent distribution of shock Mach numbers and the magnetic connection of particles produced by the shock to the solar surface visible from Earth.
Results. The reconstructed shock fronts for the three events became magnetically connected to the visible solar surface after the start of the flare and just before the onset of the >100 MeV γ-ray emission. The shock surface at these connections also exhibited supercritical Mach numbers required for significant particle energization. The strongest γ-ray emissions occurred when the flanks of the shocks were connected in a quasi-perpendicular geometry to the field lines reaching the visible surface. Multipoint, in situ, measurements of solar energetic particles (SEPs) were consistent with the production of these SEPs by the same shock processes responsible for the γ-rays. The fluxes of protons in space and at the Sun were highest for the 2014 Sep. 01, which had the fastest moving shock.
Conclusions. This study provides further evidence that high-energy protons producing time-extended high-energy γ-ray emission likely have the same CME-shock origin as solar energetic particles measured in interplanetary space.
Key words: Sun: corona / Sun: flares / Sun: magnetic fields / Sun: X-rays, gamma rays / Sun: particle emission
© ESO, 2017