Search

Modeling of time profiles of solar energetic particle (SEP) observations often considers transport along a large-scale magnetic field with a fixed path length from the source to the observer. Here we point out that variability in the turbulent field line path length can affect the fits to SEP data and the inferred mean free path and injection profile. To explore such variability, we perform Monte Carlo simulations in representations of homogeneous 2D MHD + slab turbulence adapted to spherical geometry and trace trajectories of field lines and full particle orbits, considering proton injection from a narrow or wide angular region near the Sun, corresponding to an impulsive or gradual solar event, respectively. We analyze our simulation results in terms of field line and particle path length statistics for $1^\circ\times 1^\circ$ pixels in heliolatitude and heliolongitude at 0.35 and 1 AU from the Sun, for different values of the turbulence amplitude $b/B_0$ and turbulence geometry as expressed by the slab fraction $f_s$. Maps of the most probable path lengths of field lines and particles at each pixel exhibit systematic patterns that reflect the fluctuation amplitudes experienced by the field lines, which in turn relate to the local topology of 2D turbulence. We describe the effects of such path length variations on SEP time profiles, both in terms of path length variability at specific locations and motion of the observer with respect to turbulence topology during the course of the observations., Comment: 18 pages, 6 figures

The statistics of the magnetic field line separation provide insight into how a bundle of field lines spreads out and the dispersion of non-thermal particles in a turbulent environment, which underlies various astrophysical phenomena. Its diffusive character depends on the distance along the field line, the initial separation, and the characteristics of the magnetic turbulence. This work considers the separation of two magnetic field lines in general transverse turbulence in terms of the magnetic power spectrum in three-dimensional wavenumber space. We apply non-perturbative methods using Corrsin's hypothesis and assume random ballistic decorrelation to calculate the ensemble average field line separation for general transverse magnetic turbulence. For 2D+slab power spectra, our analytic formulae and computer simulations give similar results, especially at low slab fraction. Our analytical expression also demonstrates several features of field line separation that are verified by computer simulations.

The investigation of the diffusive transport of charged particles in a turbulent magnetic field remains a subject of considerable interest. Research has most frequently concentrated on determining the diffusion coefficient in the presence of a mean magnetic field. Here we consider diffusion of charged particles in fully three-dimensional isotropic turbulent magnetic fields with no mean field, which may be pertinent to many astrophysical situations. We identify different ranges of particle energy depending upon the ratio of the Larmor radius of the charged particle to the characteristic outer length scale of the turbulence. Two different theoretical models are proposed to calculate the diffusion coefficient, each applicable to a distinct range of particle energies. The theoretical results are compared with those from computer simulations, showing good agreement.