Séminaire Thierry Passot

Abstract:   Space plasmas are essentially collisionless and permeated by a large-scale magnetic field. Alfvén waves are ubiquitous in these plasmas and, despite the significance of small scale kinetic effects, the large-scale dynamics is often described by fluid models. We will illustrate the use of gyro-fluid models, which incorporating corrections related to finite Larmor radius (FLR) and electron inertia, to describe the dynamics at sub-ion scales in two specific situations: the case of imbalanced turbulence (when there are more outgoing than ingoing waves, as is the case within the the solar wind), and the case of electron-scale turbulence, where several competing instabilities affect the dynamics up to large ion scales. After a brief introduction to heliospheric plasmas, we describe the main properties of gyrofluids and present a background on reduced-MHD turbulence and on “reconnection mediated turbulence”. Simulations of a two-field gyrofluid model for imbalanced turbulence are then presented, which highlight the  helicity barrier mechanism and the formation of a steep spectral transition zone at ion scales, which we show results from the nonlinear interaction of co-propagating kinetic Alfvén waves. Small-scale dynamics exhibit a k^(-8/3) magnetic power spectrum, which results from the role played by reconnection in energy transfer. The final part is devoted to 2D simulations of a four-field model that incorporates all the electron and ion FLRs, for three different regimes. It will focus on the role of reconnection and lower-hybrid drift instabilities in shaping the small-scale dynamics and the ion-scale magnetic spectra.