Velocity-Space Cascade in Magnetized Plasmas: Numerical Simulations

TL;DR

This paper uses numerical simulations to explore velocity-space cascades in magnetized plasmas, confirming theoretical predictions of spectral behavior and revealing anisotropy and intermittency linked to turbulence structures.

Contribution

It provides the first numerical evidence of velocity-space cascade features in a strongly magnetized plasma, supporting recent theoretical and observational models.

Findings

Velocity-space spectral anisotropy due to magnetic field

Energy distribution follows P(m) ~ m^{-2}

Velocity-space activity is intermittent near coherent structures

Abstract

Plasma turbulence is studied via direct numerical simulations in a two-dimensional spatial geometry. Using a hybrid Vlasov-Maxwell model, we investigate the possibility of a velocity-space cascade. A novel theory of space plasma turbulence has been recently proposed by Servidio {\it et al.} [PRL, {\bf 119}, 205101 (2017)], supported by a three-dimensional Hermite decomposition applied to spacecraft measurements, showing that velocity space fluctuations of the ion velocity distribution follow a broad-band, power-law Hermite spectrum $P(m)$, where $m$ is the Hermite index. We numerically explore these mechanisms in a more magnetized regime. We find that (1) the plasma reveals spectral anisotropy in velocity space, due to the presence of an external magnetic field (analogous to spatial anisotropy of fluid and plasma turbulence); (2) the distribution of energy follows the prediction $P(m)\sim m^{-2}$, proposed in the above theoretical-observational work; and (3) the velocity-space activity is intermittent in space, being enhanced close to coherent structures such as the reconnecting current sheets produced by turbulence. These results may be relevant to the nonlinear dynamics weakly-collisional plasma in a wide variety of circumstances.