Early propagation of energetic particles across the mean field in turbulent plasmas

Laitinen, Timo Lauri mikael orcid iconORCID: 0000-0002-7719-7783, Dalla, Silvia orcid iconORCID: 0000-0002-7837-5780 and Marriott, D. (2017) Early propagation of energetic particles across the mean field in turbulent plasmas. Monthly Notices of the Royal Astronomical Society, 470 (3). pp. 3149-3158. ISSN 0035-8711

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Official URL: https://doi.org/doi/10.1093/mnras/stx1509


Propagation of energetic particles across the mean field direction in turbulent magnetic fields is often described as spatial diffusion. Recently, it has been suggested that initially the particles prop- agate systematically along meandering field lines, and only later reach the time-asymptotic diffusive cross-field propagation. In this paper, we analyse cross-field propagation of 1–100 MeV protons in composite 2D-slab turbulence superposed on a constant background magnetic field, using full-orbit particle simulations, to study the non-diffusive phase of particle propagation with a wide range of turbulence parameters. We show that the early-time non-diffusive propagation of the particles is consistent with particle propagation along turbulently meandering field lines. This results in a wide cross-field extent of the particles already at the initial arrival of particles to a given distance along the mean field direction, unlike when using spatial diffusion particle transport models. The cross-field extent of the particle distribution remains constant for up to tens of hours in turbulence environ- ment consistent with the inner heliosphere during solar energetic particle events. Subsequently, the particles escape from their initial meandering field lines, and the particle propagation across the mean field reaches time-asymptotic diffusion. Our analysis shows that in order to understand so- lar energetic particle event origins, particle transport modelling must include non-diffusive particle propagation along meandering field lines.
Key words: Sun: particle emission – diffusion – magnetic fields – turbulence

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