Heliospheric ion energization due to emerging CME shocks

摘要

Formation of inhomogeneous electromagnetic structures with gradients in magnetic field and intrinsic electric fields is commonly observed in magnetized plasmas. Heliospheric plasmas are susceptible to formation of localized, supersonically propagating electromagnetic inhomogeneities. Coronal relaxation may result in an emergence close to solar surface of a shock wave which traverses significant parts of the heliosphere. Shocks of solar origin may accelerate ions to high energies, and some of these ions can be trapped in quasi-stable orbits of planetary magnetospheres. The observationally deduced ion acceleration close to the Sun, at low Mach numbers and low turbulence levels, poses a dilemma regarding the energization mechanism. When the magnetic ramp of an obliquely propagating electromagnetic substructure narrows to a size of a fraction of ion skin depth, as conjectured during merging of successively propagating shocks, the trajectories of some ions exhibit strongly nonadiabatic characteristics. Subset of ions is energized while surfing along the shock due to the combined forces of magnetic fields and cross-shock electric potential gradient, forming a high-energy tail. This tail may be additionally accelerated after traversing the shock multiple times as a result of scattering and reflection due to Alfvenic wave diffusion. We follow the orbits of seed ions in a presence of a stationary, fluid-based, self-consistent model, and investigate their behavior for a variety of plasma parameters and geometries. The results indicate that (1) the energization of ions for low Mach numbers, as observed for emerging shocks close to the Sun, depends crucially on the narrowness of the electromagnetic structures, (2) the sufficiently narrow heliospheric structure can energize thermal protons and a subset of rare ions which were enriched due to impulsive coronal processes, (3) the energization is sensitive to the pitch angle of the seed population, indicating dependence on the geometry of the shock-plasma flow system, and (4) the preacceleration by surfing mechanism is a prerequisite for an additional energization due to diffusive shock acceleration. We conclude that the best configuration for an effective acceleration due to an emerging shock at small heliocentric distances and low Mach number consists of a narrow electromagnetic substructure which energizes heliospheric thermal protons directly from their thermal level, as well as trace elements which enrich the seed population due to previous coronal processes. The narrowness of the shock or its substructure and the surfing mechanism may help in explaining the observed energization when other mechanisms become inefficient due to an insufficient level of the turbulence. The energetic ion populations may have a direct profound impact on human space exploration.