Asymmetric diffusion of cosmic rays

Medvedev, Mikhail V.; Medvedev, Viktor V.

doi:10.1063/1.4928942

Cited by 8 publications

(4 citation statements)

References 10 publications

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“…In deriving a diffusive transport formalism for charged particles accelerated in a "sea of magnetic islands," Zank et al (2014) and le Roux et al (2015) make the important implicit assumption that the timescale over which the particle distribution is averaged is much longer than the trapping time for particles trapped in individual plasmoids. More energetic particles are less effectively trapped than lower energy particles (Medvedev & Medvedev 2015). We therefore expect that there exists a threshold energy above which charged particles may be regarded as propagating diffusively and below which particle trapping in islands directly affects the distribution function.…”

Section: Particle Acceleration By Reconnection and Magnetic Islandsmentioning

confidence: 99%

“…As discussed above, we anticipate that below some threshold energy range, the transport formalism (1) and (2) no longer holds and that particle trapping needs to be included specifically (Medvedev & Medvedev 2015). In Figure 14, we plot the Voyager 2 LECP data from ∼28 keV to ∼3 MeV (Decker et al 2008) using the format of Figure 13.…”

Section: Appendix Energetic Particle Observations At Lower Energiesmentioning

confidence: 99%

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Diffusive Shock Acceleration and Reconnection Acceleration Processes

Zank

Hunana

Mostafavi

et al. 2015

ApJ

179

147

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Shock waves, as shown by simulations and observations, can generate high levels of downstream vortical turbulence, including magnetic islands. We consider a combination of diffusive shock acceleration (DSA) and downstream magnetic-island-reconnection-related processes as an energization mechanism for charged particles. Observations of electron and ion distributions downstream of interplanetary shocks and the heliospheric termination shock (HTS) are frequently inconsistent with the predictions of classical DSA. We utilize a recently developed transport theory for charged particles propagating diffusively in a turbulent region filled with contracting and reconnecting plasmoids and small-scale current sheets. Particle energization associated with the antireconnection electric field, a consequence of magnetic island merging, and magnetic island contraction, are considered. For the former only, we find that (i) the spectrum is a hard power law in particle speed, and (ii) the downstream solution is constant. For downstream plasmoid contraction only, (i) the accelerated spectrum is a hard power law in particle speed; (ii) the particle intensity for a given energy peaks downstream of the shock, and the distance to the peak location increases with increasing particle energy, and (iii) the particle intensity amplification for a particular particle energy, f x c c f c c , 0 , , 0 0 () () is not 1, as predicted by DSA, but increases with increasing particle energy. The general solution combines both the reconnection-induced electric field and plasmoid contraction. The observed energetic particle intensity profile observed by Voyager 2 downstream of the HTS appears to support a particle acceleration mechanism that combines both DSA and magnetic-island-reconnectionrelated processes.

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Section: Particle Acceleration By Reconnection and Magnetic Islandsmentioning

confidence: 99%

Section: Appendix Energetic Particle Observations At Lower Energiesmentioning

confidence: 99%

Diffusive Shock Acceleration and Reconnection Acceleration Processes

Zank

Hunana

Mostafavi

et al. 2015

ApJ

179

147

View full text Add to dashboard Cite

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“…This trapping effect can also remove the singularity in parallel diffusion coefficient at 90 • (Cesarsky & Kulsrud 1973), which is a fundamental difficulty of the QLT (Jokipii 1966). Trapping of CRs by large-scale magnetic irregularities was earlier studied by, e.g., Fermi (1949); Noerdlinger (1968); Cesarsky & Kulsrud (1973); Klepach & Ptuskin (1995); Zirakashvili (2001); Medvedev & Medvedev (2015), but it has not been investigated in the framework of modern theories of MHD turbulence.…”

Section: Introductionmentioning

confidence: 99%

Trapping of cosmic rays in MHD turbulence

Xu,

Lazarian

2020

Preprint

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Astrophysical plasmas are turbulent and magnetized. The interaction between cosmic rays (CRs) and magnetohydrodynamic (MHD) turbulence is a fundamental astrophysical process. Based on the current understanding of MHD turbulence, we revisit the trapping of CRs by magnetic mirrors in the context of MHD turbulence. In compressible MHD turbulence, isotropic fast modes dominate both trapping and gyroresonant scattering of CRs. The presence of trapping significantly suppresses the pitch-angle scattering and the spatial diffusion of CRs along the magnetic field. The resulting parallel diffusion coefficient has a weaker dependence on CR energy at higher energies. In incompressible MHD turbulence, the trapping by pseudo-Alfvén modes dominates over the gyroresonant scattering by anisotropic Alfvén and pseudo-Alfvén modes at all pitch angles and prevents CRs from diffusion.

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“…Nonresonant interactions of CRs with MHD turbulence have also been investigated for studying CR diffusion and stochastic acceleration (Noerdlinger 1968;Cesarsky & Kulsrud 1973;Ptuskin 1988;Klepach & Ptuskin 1995;Cho & Lazarian 2006;Brunetti & Lazarian 2007;Medvedev & Medvedev 2015;Brunetti & Lazarian 2016;Xu & Zhang 2017;Lazarian & Xu 2021;Bresci et al 2022). The acceleration of CRs by magnetic compressions on scales larger than their Larmor radii was studied by Cho & Lazarian (2006), where, as the diffusion due to pitch-angle scattering was invoked, significant compression of gas is required for a net energy gain in average (see also Drury 2012).…”

Section: Introductionmentioning

confidence: 99%

Mirror Acceleration of Cosmic Rays in a High-β Medium

Lazarian,

2023

ApJ

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In a weakly compressible high-β medium, pitch-angle scattering and the associated scattering acceleration of cosmic rays (CRs) by anisotropic Alfvén and slow modes of magnetohydrodynamic (MHD) turbulence is inefficient. To tap the energy from magnetic compressions for efficient particle acceleration, a diffusion mechanism that can effectively confine particles in space without causing their trapping or pitch-angle isotropization is needed. We find that the mirror diffusion in MHD turbulence recently identified in Lazarian & Xu satisfies all the above conditions and serves as a promising diffusion mechanism for efficient acceleration of CRs via their stochastic nonresonant interactions with magnetic compressions/expansions. The resulting mirror acceleration is dominated by the slow-mode eddies with their lifetime comparable to the mirror diffusion time of CRs. Consequently, we find that the acceleration time of mirror acceleration is independent of the spatial diffusion coefficient of CRs. The mirror acceleration brings new life for the particle acceleration in a weakly compressible/incompressible medium and has important implications for studying CR reacceleration in the high-β intracluster medium.

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Asymmetric diffusion of cosmic rays

Cited by 8 publications

References 10 publications

Diffusive Shock Acceleration and Reconnection Acceleration Processes

Diffusive Shock Acceleration and Reconnection Acceleration Processes

Trapping of cosmic rays in MHD turbulence

Mirror Acceleration of Cosmic Rays in a High-β Medium

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