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The magnetic connectivity of coronal shocks to the visible solar surface during long-duration gamma-ray events  

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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Long-Term Tracking of Corotating Density Structures using Heliospheric Imaging  

Illya Plotnikov   Submitted: 2016-06-10 03:00

The systematic monitoring of the solar wind in high-cadence and high-resolution heliospheric images taken by the Solar-Terrestrial Relation Observatory (STEREO) spacecraft permits the study of the spatial and temporal evolution of variable solar wind flows from the Sun out to 1 AU, and beyond. As part of the EU Framework 7 (FP7) Heliospheric Cataloguing, Analysis and Techniques Service (HELCATS) project, we have generated a catalogue listing the properties of 190 corotating structures well-observed in images taken by the Heliospheric Imager (HI) instruments on-board STEREO-A (ST-A). Based on this catalogue, we present here one of very few long-term analyses of solar wind structures advected by the background solar wind. We concentrate on the subset of plasma density structures clearly identified inside corotating structures. This analysis confirms that most of the corotating density structures detected by the heliospheric imagers comprises a series of density inhomogeneities advected by the slow solar wind that eventually become entrained by stream interaction regions. We have derived the spatial-temporal evolution of each of these corotating density structures by using a well-established fitting technique. The mean radial propagation speed of the corotating structures is found to be 311 ± 31 km s-1. Such a low mean value corresponds to the terminal speed of the slow solar wind rather than the speed of stream interfaces, which is typically intermediate between the slow and fast solar wind speeds (~400 km s-1). Using our fitting technique, we predicted the arrival time of each corotating density structure at different probes in the inner heliosphere. We find that our derived speeds are systematically lower by ~100 km s-1 than those measured in situ at the predicted impact times. Moreover, for cases when a stream interaction region is clearly detected in situ at the estimated impact time, we find that our derived speeds are lower than the speed of the stream interface measured in situ by an average of 55 km s-1 at ST-A and 84 km s-1 at STEREO-B (ST-B). We show that the speeds of the corotating density structures derived using our fitting technique track well the long-term variation of the radial speed of the slow solar wind during solar minimum years (2007-2008). Furthermore, we demonstrate that these features originate near the coronal neutral line that eventually becomes the heliospheric current sheet.

Authors: I. Plotnikov, A.P. Rouilllard, J.A. Davies, V. Bothmer, J.P. Eastwood, P. Gallagher, R.A. Harrison, E. Kilpua, C. Möstl, C.H. Perry, L. Rodriguez, B. Lavraud, V. Génot, R.F. Pinto, E. Sanchez-Diaz
Projects: STEREO

Publication Status: Accepted for publication in Solar Physics
Last Modified: 2016-06-10 15:24
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The magnetic connectivity of coronal shocks to the visible solar surface during long-duration gamma-ray events
Long-Term Tracking of Corotating Density Structures using Heliospheric Imaging

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