A unified model of solar energetic particle transport in structured solar wind

Solar energetic particle (SEP) fluxes, after their propagation from the particles' source to the Earth's orbit, depend on the state of solar wind, which is known to be highly variable in both time and space. Commonly used SEP transport models are based on the assumption of the standard interplanetary magnetic field, which would be the case for a uniform, steady state expansion of solar wind. Modeling of SEP transport in the standard solar wind can be facilitated by the use of a corotating reference frame, wherein the solar wind speed is parallel to interplanetary magnetic field at each point and the magnetic field is static. However, this approach is not possible in the realistic solar wind. This necessitates development of a more general SEP model applicable to particle transport in arbitrarily structured solar wind and in interplanetary coronal mass ejections, magnetic clouds, and shocks. In the framework of focused transport theory, we formulate a practical model of SEP transport in an evolving, structured solar wind. This unified model accommodates the results of three-dimensional MHD modeling of solar wind based on observations of the sun, solar wind, and SEPs in a particular event. A relation between the generalized focused transport model and the diffusion-convection equation of cosmic ray transport is discussed.