Random Walk and Trapping of Interplanetary Magnetic Field Lines: Global Simulation and Implications for Solar Energetic Particles and Magnetic Connectivity

Abstract:

The random walk of magnetic field lines is an important ingredient in understanding the connectivity of the magnetic field and the spatial transport of charged particles. As solar energetic particles (SEPs) propagate away from near-solar sources, their interaction with both the smooth and the unresolved (or random) components of the magnetic field can influence where (and when) they are observed. We develop a formalism in which the differential equation describing the field line random walk contains a contribution from the large-scale expansion, i.e., a non-stochastic contribution, and an additional contribution due to localized magnetic displacements, based on standard assumptions made in nonlinear diffusion theory. We also estimate a "filamentation distance", i.e., the heliocentric distance up to which field lines originating in magnetic islands can remain strongly trapped in filamentary structures, which can lead to so-called "dropouts" in SEP observations. Furthermore, we investigate the increase in field-line length due to random walk, and derive estimates based on different possible symmetries of the fluctuations. The formalism developed above is then applied within a global magnetohydrodynamic simulation of the inner-heliospheric solar wind, coupled with a turbulence transport model, to estimate the diffusive spreading of magnetic field lines that originate at different regions on the solar surface. Conversely, we estimate the size of the source region from which a given field line (observed at a spacecraft location) could have originated. We thus quantify the potential uncertainty in calculations that aim to establish connectivity of spacecraft observations to solar sources, focusing on the Parker Solar Probe as an example. Partially supported by grant RTA6280002 from Thailand Science Research and Innovation.