Abstract
Nonthermal acceleration of particles in magnetohydrodynamic (MHD) turbulence plays a central role in a wide variety of astrophysical sites. This physics is addressed here in the context of a strong turbulence, composed of coherent structures rather than waves, beyond the realm of quasilinear theory. The present description tracks the momentum of the particle through a sequence of frames in which the electric field vanishes, in the spirit of the original Fermi scenario. It connects the sources of energy gain (or loss) to the gradients of the velocity of the magnetic field lines, in particular the acceleration and the shear of their velocity flow projected along the field line direction, as well as their compression in the transverse plane. Those velocity gradients are subject to strong intermittency: they are spatially localized, and their strengths obey power law distributions, as demonstrated through direct measurements in the incompressible MHD simulation of the Johns Hopkins University database. This intermittency impacts the acceleration process in a significant way, which opens up prospects for a rich phenomenology. In particular, the momentum distribution, which is here captured through an analytical random walk model, displays extended power law tails with soft-to-hard evolution in time, in general agreement with recent kinetic numerical simulations. Extensions to this description and possible avenues of exploration are discussed.
Highlights
The remarkable wealth of physical phenomena that inhabit the cascades of magnetohydrodynamic (MHD) turbulence [1,2,3], from the large scales of stirring motions down to the microscopic dissipative layers, the ubiquity of magnetized turbulence in space plasmas, and the decisive roles that it plays in various astrophysical settings [4,5,6,7], have turned its study into a field of research of its own right, with broad interdisciplinary connections
In the context of quasilinear theory, particle acceleration scales with the power spectrum of magnetic fluctuations, with the turbulent magnetic energy density, while in the present nonresonant description, acceleration scales with the shear of the field line velocity flow
This paper has discussed the physics of particle acceleration in strong MHD turbulence characterized by an ensemble of structures, rather than by the linear superposition of random waves of quasilinear theory
Summary
The remarkable wealth of physical phenomena that inhabit the cascades of magnetohydrodynamic (MHD) turbulence [1,2,3], from the large scales of stirring motions down to the microscopic dissipative layers, the ubiquity of magnetized turbulence in space plasmas, and the decisive roles that it plays in various astrophysical settings [4,5,6,7], have turned its study into a field of research of its own right, with broad interdisciplinary connections. Accounting for nonlinear effects, such as resonance broadening related to the finite lifetime of the eigenmodes, can at best lead to scattering timescales of the order of the coherence scale of the turbulence, at least for for those modes that are subject to anisotropy This renders hazardous the extrapolation of predictions based on quasilinear calculations to the regime of strong turbulence; this point will be detailed in the following Sec. II.
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