Thierry Passot - Laboratoire Lagrange, Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, Nice, France - Keywords: Space plasmas, turbulence

Thierry Passot - Laboratoire Lagrange, Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, Nice, France - Keywords: Space plasmas, turbulence

Contribution title: Fluid approaches for sub-ionic turbulence in space plasmas

Turbulence in solar-terrestrial environments, an essentially collisionless magnetized plasma, involves particularly complex physical phenomena with several characteristic spatial and temporal scales which break scale invariance. Earth orbiting satellites provide high-quality in-situ data which clearly display several frequency domains, easily converted to wavenumber ranges by Taylor hypothesis, where the magnetic energy spectral density follows a power law. In spite of many recent theoretical as well as numerical advances in the understanding of the mechanisms at play in this turbulent medium, several important questions remain open. A first one concerns the relative importance of wave versus strong turbulence. Another question is related to the type of waves that dominates, depending on parameters such as the ratio of thermal to magnetic pressures. The main issues, currently the object of intense debates, nevertheless remain the origin and mechanisms of energy dissipation, the heating of the plasma and the acceleration of particles. Intermittent structures such as current sheets are commonly observed and are believed to play a crucial role in these questions, as they can in particular be destabilized by magnetic reconnection.

This talk will concentrate on the solar wind plasma at scales ranging from the ionic to the electronic scale. In spite of the lack of collisions, but thanks to the presence of a strong enough ambient magnetic field, fluid modeling is made possible, providing easier theoretical and numerical tools than fully kinetic approaches. A proper description of the plasma at or below the ion gyro-scale nevertheless requires taking into account the lowest order wave-particle resonance (Landau damping) as well as finite Larmor radius corrections. Using a recently developed fluid model that includes these effects, it will be shown that ion Landau dissipation can break the universality of turbulence, making the energy spectrum steeper while still allowing for a power law [1,2]. At scales close to the electron inertial length, where ion velocity and temperature fluctuations become negligible, a description in terms of reduced fluid and gyrofluid models including electron inertia will be discussed, that are suited to study the interplay between turbulence and magnetic reconnection [3]. Phenomenological predictions on energy spectra and cascade directions will also be presented.

1. T. Passot and P.L. Sulem, A model for the non-universal power law of the solar wind sub-ion scale magnetic spectrum, Astrophys. J. Lett. 812:L37 (2015)
2. P.L. Sulem, T. Passot, D. Laveder and D. Borgogno, Influence of the nonlinearity parameter on the solar wind sub-ion magnetic energy spectrum: FLR-Landau fluid simulations, Astrophys. J. 818:66 (2016)
3. T. Passot, P.L. Sulem and E. Tassi, Electron-scale reduced fluid models with gyroviscous effects, J. Plasma Phys. 83, 715830402 (2017).