Identifying the physical mechanisms that produce the most energetic particles is a long-standing observational and theoretical challenge in astrophysics. Strong shock waves have been proposed as efficient accelerators both in the solar physics and astrophysical contexts via various acceleration mechanisms. The proposed processes rely on shock waves being super-critical or moving several times faster than the characteristic speed of the medium they propagate through. Using recent imaging of the NASA STEREO, SoHO and SDO spacecraft, we determine the time-dependent 3-dimensional distribution of the expansion speed and Mach number of a number of coronal shocks that formed during intense Solar Energetic Particle (SEPs) events. Although supercritical shocks are found to form along field lines threading the close and open corona, the highest Mach numbers are derived along the open field lines adjacent to helmet streamers. In this region the coronal fast mode speed drops to the lowest estimated values due to a significant expansion of magnetic flux tubes.
We compare the 3-D evolution of triangulated shocks with the properties of SEPs measured near 1AU and with solar radio waves, hard X-rays and gamma rays. Overall these observations reveal that the most energetic SEPs (GeVs) are produced near the Sun when the shock becomes super-critical. We will present a new modeling framework that exploits the 3-D shock triangulation technique as input to a numerical model of diffusive-shock acceleration. The results of these simulations for a specific event (17 May 2012) show that particles can be accelerated to GeV energies a few minutes after the shock becomes super-critical.
On a purely observational basis the properties (timing, intensity and spatial spread) of moderate gradual SEPs cannot be easily interpreted in terms of individual shock properties (Mach, geometry, speed). Further modeling that, in addition to the time-dependent non-linear effects of the acceleration process, also accounts for transport processes in the corona and in the interplanetary combined with SEP measurements obtained much closer to the source with Solar Orbiter and Parker Solar Probe will shed new light on these events.
We use our modeling technique to study the evolution of solar hard X rays and gamma-ray events measured by the FERMI LAT and GBM instruments. We focus on three CME events where the flare is occulted behind the solar limb. We show that the onset of these gamma-ray events is consistent with the time when quasi-perpendicular and super-critical parts of the shocks become connected with the visible surface of the Sun. These are favorable conditions for the production of energetic electrons and protons that can stream towards and interact with the visible chromosphere to produce hard X rays and gamma rays. Finally we discuss current challenges and the planned improvements to our modeling framework through the presenter’s recently funded COROSHOCK project to investigate the origin of long-duration gamma ray events and in preparation for upcoming missions.