Solar Particle Transport in a Dynamical Quasi‐linear Theory

Abstract

Current theories for parallel diffusion of high-energy particles in the heliosphere are fairly well accepted, and supported by both observations and simulation results, if recently found evidence for the magnetic fluctuations consisting of a partly slab and partly two-dimensional composite geometry is taken into account. However, there are important outstanding questions pertaining at medium to low energies where dynamical effects in the solar wind, such as propagation and thermal damping of waves, and time-dependent decorrelation of magnetic fluctuations, have a strong influence on the scattering mean free path. A model is presented that addresses the above effects and is able to explain the observed rigidity dependence of particle mean free paths ranging from keV electrons to GeV protons. The dynamical processes, leading to a strongly nonresonant pitch-angle scattering through 90° at low rigidities, can be described by a single parameter that is determined from the observed density, temperature, and magnetic field strength in the solar wind. This leaves the slab content of the fluctuations, which is most effective in particle scattering, as the only remaining input parameter. Mean free paths for electrons and protons determined for a solar particle event were compared with predictions from the model based on an estimate of the slab component from magnetic field fluctuations observed during the time of the event. We find that the model reproduces the observations well, suggesting that a relation between the properties of solar particle transport and of solar wind turbulence on an event-by-event basis has finally become possible. Consequences for other energetic particle processes in the heliosphere and possibilities to use solar electrons to probe properties of the solar wind turbulence in regions not accessible by spacecraft are discussed.