Pitch-Angle Dependent Perpendicular Diffusion of Energetic Particles Interacting With Magnetic Turbulence

Qin, Gang; Shalchi, A.

doi:10.5539/apr.v6n1p1

Cited by 16 publications

(13 citation statements)

References 44 publications

(65 reference statements)

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“…The assumption that D ⊥ ∼ |μ| is supported by the test-particle simulations performed by Qin and Shalchi (2014). Furthermore, this pitch-angle dependence is also provided by the FLRW limit (see, e.g., Eq.…”

Section: Derivation From the Fokker-planck Equationmentioning

confidence: 77%

Perpendicular Transport of Energetic Particles in Magnetic Turbulence

Shalchi

2020

Space Sci Rev

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Scientists have explored how energetic particles such as solar energetic particles and cosmic rays move through a magnetized plasma such as the interplanetary and interstellar medium since more than five decades. From a theoretical point of view, this topic is difficult because the particles experience complicated interactions with turbulent magnetic fields. Besides turbulent fields, there are also large scale or mean magnetic fields breaking the symmetry in such systems and one has to distinguish between transport of particles parallel and perpendicular with respect to such mean fields. In standard descriptions of transport phenomena, one often assumes that the transport in both directions is normal diffusive but non-diffusive transport was found in more recent work. This is in particular true for early and intermediate times where the diffusive regime is not yet reached. In recent years researchers employed advanced numerical tools in order to simulate the motion of those particles through the aforementioned systems. Nevertheless, the analytical description of the problem discussed here is of utmost importance since analytical forms of particle transport parameters need to be known in several applications such as solar modulation studies or investigations of shock acceleration. The latter process is directly linked to the question of what the sources of high energy cosmic rays are, a problem which is considered to be one of the most important problems of the sciences of the 21st century. The present review article discusses analytical theories developed for describing particle transport across a large scale magnetic field as well as field line random walk. A heuristic approach explaining the basic physics of perpendicular transport is also presented. Simple analytical forms for the perpendicular diffusion coefficient are proposed which can easily be incorporated in numerical codes for solar modulation or shock acceleration studies. Test-particle simulations are also discussed together with a comparison with analytical results. Several applications such as cosmic ray propagation and diffusive shock acceleration are also part of this review.

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Section: Derivation From the Fokker-planck Equationmentioning

confidence: 77%

Perpendicular Transport of Energetic Particles in Magnetic Turbulence

Shalchi

2020

Space Sci Rev

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“…On the other hand, the spreading of particles across the mean magnetic field direction due to propagation along meandering field lines requires large particle velocities along the field lines, i.e., | | m~1. Thus, pitch angle dependence of the cross-field particle diffusion particle transport may be more complicated than the recently discussed proportionality to | | m or ( ) m -1 2 (see, e.g., Dröge et al 2010;Qin & Shalchi 2014;Strauss & Fichtner 2015, and discussion therein). Overall, our simulations show the importance of understanding the microphysics of the particle decoupling from their original field lines for understanding the propagation of particles across the mean magnetic field in turbulent plasmas.…”

Section: Discussionmentioning

confidence: 99%

Energetic Particle Transport Across the Mean Magnetic Field: Before Diffusion

Laitinen¹,

Dalla

2017

ApJ

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Current particle transport models describe the propagation of charged particles across the mean field direction in turbulent plasmas as diffusion. However, recent studies suggest that at short timescales, such as soon after solar energetic particle (SEP) injection, particles remain on turbulently meandering field lines, which results in nondiffusive initial propagation across the mean magnetic field. In this work, we use a new technique to investigate how the particles are displaced from their original field lines, and we quantify the parameters of the transition from field-aligned particle propagation along meandering field lines to particle diffusion across the mean magnetic field. We show that the initial decoupling of the particles from the field lines is slow, and particles remain within a Larmor radius from their initial meandering field lines for tens to hundreds of Larmor periods, for 0.1-10 MeV protons in turbulence conditions typical of the solar wind at 1 au. Subsequently, particles decouple from their initial field lines and after hundreds to thousands of Larmor periods reach time-asymptotic diffusive behavior consistent with particle diffusion across the mean field caused by the meandering of the field lines. We show that the typical duration of the prediffusive phase, hours to tens of hours for 10 MeV protons in 1 au solar wind turbulence conditions, is significant for SEP propagation to 1 au and must be taken into account when modeling SEP propagation in the interplanetary space.

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“…The pitch angle dependence of the cross-field diffusion coefficient has also been discussed recently, with suggestions for using κ ⊥ ∝ |µ| (e.g. Qin & Shalchi, 2014) or κ ⊥ ∝ r L , the particle Larmor radius (Dröge et al, 2010). While the suggested different forms of pitch angle dependence of κ ⊥ influence particle cross-field propagation (e.g.…”

Section: Fp Modelmentioning

confidence: 96%

Solar energetic particle access to distant longitudes through turbulent field-line meandering

Laitinen

Kopp

Effenberger

et al. 2016

A&A

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Context. Current solar energetic particle (SEP) propagation models describe the effects of interplanetary plasma turbulence on SEPs as diffusion, using a Fokker-Planck (FP) equation. However, FP models cannot explain the observed fast access of SEPs across the average magnetic field to regions that are widely separated in longitude within the heliosphere without using unrealistically strong cross-field diffusion. Aims. We study whether the recently suggested early non-diffusive phase of SEP propagation can explain the wide SEP events with realistic particle transport parameters. Methods. We used a novel model that accounts for the SEP propagation along field lines that meander as a result of plasma turbulence. Such a non-diffusive propagation mode has been shown to dominate the SEP cross-field propagation early in the SEP event history. We compare the new model to the traditional approach, and to SEP observations. Results. Using the new model, we reproduce the observed longitudinal extent of SEP peak fluxes that are characterised by a Gaussian profile with σ = 30−50• , while current diffusion theory can only explain extents of 11• with realistic diffusion coefficients. Our model also reproduces the timing of SEP arrival at distant longitudes, which cannot be explained using the diffusion model. Conclusions. The early onset of SEPs over a wide range of longitudes can be understood as a result of the effects of magnetic fieldline random walk in the interplanetary medium and requires an SEP transport model that properly describes the non-diffusive early phase of SEP cross-field propagation.

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Pitch-Angle Dependent Perpendicular Diffusion of Energetic Particles Interacting With Magnetic Turbulence

Cited by 16 publications

References 44 publications

Perpendicular Transport of Energetic Particles in Magnetic Turbulence

Perpendicular Transport of Energetic Particles in Magnetic Turbulence

Energetic Particle Transport Across the Mean Magnetic Field: Before Diffusion

Solar energetic particle access to distant longitudes through turbulent field-line meandering

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