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The magnetic connectivity of coronal shocks to the visible solar surface during long-duration gamma-ray events View all abstracts by submitter

Illya Plotnikov   Submitted: 2017-03-23 08:26

Context: Solar gamma-ray events measured near Earth can last several hours during so-called Long Duration Gamma Ray Flares (LDGRFs). LDGRFs suggest that a particle-acceleration mechanism operates over many hours to produce energetic protons that stream continually towards the solar surface. Coronal shocks, driven by the expansion of Coronal Mass Ejections (CMEs), could be the source of these energetic particles. For this hypothesis to work, the shock must be magnetically connected to the solar disk visible from Earth in order for particles accelerated at the shock to be channeled towards and impact the visible chromosphere. LDGRFs that occur when solar eruptions erupt on the far side of the Sun and during which the flare loops and footpoints are not visible from Earth, provide favourable case studies to isolate the possible role of shocks driven by CMEs in producing the LDGRFs. Aims: In this paper, we investigate if the spatial and temporal evolution of the coronal shocks, inferred from stereoscopic observations, could be the accelerators of the particles producing the LDGRFs. Methods: We analyse three CMEs that (1) erupted behind the solar limb viewed from Earth, (2) were associated with the early formation of coronal shocks measured by ground-based radio spectrographs, and (3) were associated with gamma-ray events measured by the Fermi-Large Area Telescope (LAT) instrument. A 3D triangulation technique, based on remote-sensing observations is employed to model the expansion of these three CME shocks from above the solar surface to the upper corona. Coupling the expansion model to different models of the coronal magnetic field allows 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: For all events, the reconstructed shock front was magnetically connected to the visible solar surface after the start of the flare and just before the onset of the >100 MeV gamma-ray emission observed by Fermi-LAT gamma-ray emission. The shock surface also exhibits super-critical Mach numbers required for significant particle energisation. The strongest gamma-ray emissions occur when the flanks of the shock exhibiting a quasi-perpendicular geometry are connected to the visible surface. Multi-point in-situ measurements of solar energetic particles (SEPs) during the events studied reveals that the flux of protons with energies between 10 and 100 MeV is highest for the fastest shock and the strongest LDGRF measured on 2014 Sep 1. Conclusion: This study provides further evidence that the high-energy protons producing the long duration high-energy -ray emission has likely the same CME shock origin as the solar energetic particles measured in interplanetary space.

Authors: Illya Plotnikov, Alexis P. Rouillard, Gerald H. Share

Publication Status: Submitted to A&A
Last Modified: 2017-03-23 14:50
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