Finite Larmor radius effects on non-diffusive tracer transport in a zonal flow

TL;DR

This study investigates how finite Larmor radius effects influence non-diffusive, asymmetric particle transport in a simplified model of zonal flows with drift waves, revealing significant impacts on transport scaling and PDF shapes.

Contribution

It introduces a gyroaveraged model incorporating FLR effects into chaotic transport, showing how these effects alter transport scaling and particle displacement distributions.

Findings

FLR effects eliminate the transition in second moment scaling

PDFs exhibit algebraic decay with Larmor radius-dependent exponents

Effective fractional diffusion models accurately reproduce observed PDFs

Abstract

Finite Larmor radius (FLR) effects on non-diffusive transport in a prototypical zonal flow with drift waves are studied in the context of a simplified chaotic transport model. The model consists of a superposition of drift waves of the linearized Hasegawa-Mima equation and a zonal shear flow perpendicular to the density gradient. High frequency FLR effects are incorporated by gyroaveraging the ExB velocity. Transport in the direction of the density gradient is negligible and we therefore focus on transport parallel to the zonal flows. A prescribed asymmetry produces strongly asymmetric non- Gaussian PDFs of particle displacements, with L\'evy flights in one direction but not the other. For zero Larmor radius, a transition is observed in the scaling of the second moment of particle displacements. However, FLR effects seem to eliminate this transition. The PDFs of trapping and flight events show clear evidence of algebraic scaling with decay exponents depending on the value of the Larmor radii. The shape and spatio-temporal self-similar anomalous scaling of the PDFs of particle displacements are reproduced accurately with a neutral, asymmetric effective fractional diffusion model.