TESTING THE SHOCK HYPOTHESIS AS THE ACCELERATOR OF SOLAR ENERGETIC PARTICLES – COROSHOCK

The COROSHOCK project will address one of the remaining unsolved fundamental problems in solar physics: how are solar particles accelerated to high energies? The origin(s) of Solar Energetic Particles (SEPs) and of high-energy electromagnetic radiations such as ? rays are still highly debated. Two main particle-energisation mechanisms proposed are diffusive-shock acceleration ahead of expanding Coronal Mass Ejection (CMEs) and reconnection occurring in solar flares situated below CMEs and likely involved in the formation process of magnetic flux ropes. Comparison of in-situ measurements near 1AU and remote-sensing observations of the corona show that particle acceleration to high energies (> 1 GeV) occurs in the first tens of minutes of the initiation of a CME in the solar corona somewhere below 5 solar radii (Rs), a region that is not yet accessible to in-situ measurements. The onset of solar ?-ray events also occurs during these first minutes and are measured near Earth by the Fermi satellite. ?-rays are produced by the highest energy ions and electrons accelerated at the Sun and they provide our only direct (albeit secondary) knowledge about the properties of the accelerator(s). ?-rays measured from Earth’s perspective could be produced by shock-accelerated particles propagating sunwards and impacting the solar chromosphere visible from Earth (Cliver et al. 1996) or else from particles trapped on coronal loops. Another puzzle relates to the origin of Long Duration Gamma Ray Flares that can last several hours to several days well after the impulsive phase of solar flares and at times when the associated CME is already far away from the Sun. Long duration flares suggest that a particle-acceleration mechanism operates over many hours to produce energetic protons that stream continually towards the solar surface. The COROSHOCK project focuses mostly on evaluating the role of coronal shocks as strong accelerators of the energetic particles that form the SEPs measured in situ near 1AU and the high-energy electromagnetic radiations observed near the Sun. The fundamental questions to be addressed are:
Q1: Is the variability and long-duration of gamma ray events controlled by the 3-D evolution of shock waves?
Q2: Can particle acceleration at coronal shocks explain simultaneously the properties of SEPs and LDGRFs?
We will test the shock hypothesis as the prime particle accelerator by adapting existing advanced numerical models and exploiting remote-sensing and in-situ data in an integrated manner. The COROSHOCK projects aims at combining our advanced coronal shock models with models that account for the physical processes known to operate in the corona and interplanetary medium during the transport of particles and the conversion of energised beams of particles into electromagnetic radiations. The project will not develop new models of the microphysics of particle acceleration but instead exploit existing diffusive-shock acceleration models to derive a parameterisation of particle production based on the properties of coronal shocks. The ultimate aim of the project is to determine how well we can reproduce the spectra and time-varying fluxes of particles and electromagnetic particles measured near 1AU. Our ambitions are technically challenging and require to connect widely different datasets and develop new techniques that go well beyond the current state-of-the-art. COROSHOCK will take full advantage of spaceborne and ground-based remote-sensing observations of the solar corona and integrates in a comprehensive way in-situ measurements of the thermal and non-thermal particles transported in the solar wind. COROSHOCK is tailored to exploit currently available missions if the planned NASA Solar Probe Plus, ESA PROBA-3 and ESA Solar Orbiter instrumentation provide as scheduled new data, these will be directly integrated in the analysis during the later phase of the project (2019-2021).