Turbulence in space plasmas: analysis of observations and theoretical mode

Abstract

Turbulence represents an universal phenomenon characterizing the dynamics of different kinds of fluids, like gases, liquids, plasmas, etc., both in nature and in laboratory devices. It is responsible for the efficient transfer of energy across scales, making the connection between the macroscopic flow and the microscopic dissipation of its energy. Moreover, turbulence plays a key role in determining various phenomena. For instance, the anomalous diffusion of tracers in a flow may be controlled by the properties of turbulence, and the transport of charged particles in astrophysical or laboratory plasmas is determined by the properties of the turbulent magnetic field. Synthetic turbulence models are a useful tool that provide realistic representations of turbulence, necessary to test theoretical results, to serve as background fields in some numerical simulations, and to test analysis tools. Models of 1D and 3D synthetic turbulence previously developed still required large computational resources. A new “wavelet-based” model of synthetic turbulence, able to produce a field with tunable spectral law, intermittency and anisotropy, is presented here. The rapid algorithm introduced, based on the classic p-model of intermittent turbulence, allows to reach a broad spectral range using a modest computational effort. The model has been tested against the standard diagnostics for intermittent turbulence, all showing an excellent response. The same analysis tools have been used to study a more specific subject, of interest in space physics, i.e., the turbulence at the interface between the solar wind and the Earth’s magnetosphere, mediated by the magnetopause. The dynamics occurring at this boundary depends on various aspects as, e.g., the solar wind dynamic pressure or the direction of the Interplanetary Magnetic Field (IMF). If the IMF is directed northward the formation of a wide boundary layer at the low latitude is observed. This boundary layer is thought to be the result of the observed plasma transfer, driven by the development of the Kelvin-Helmholtz instability, originating from the velocity shear between the solar wind and the almost static near-Earth plasma. Our interest is to described these phenomena and build a collection of event related to rolled-up vortices, spatially located on the tail-flank magnetopause, previously studied by Hasegawa et al. (2006) and Lin et al. (2014). The scope is to study the properties of plasma turbulence and intermittency inside the magnetosheath, with the aim to understand the evolution of turbulence, as a result of the development of KH instability. The analysis we present, represents a complete and quantitative characterization of turbulence and associated intermittency in this region. It appears that a fluctuating behaviour during the progressive departure along the Geocentric Solar Magnetosphere (GSM) coordinate system may exist, and it is visible as a quasi-periodic modulation of the exponent. The periodicity associated with such oscillation can be estimated to be approximately 6 − 7 RE, which is consistent with the typical periodicity of the magnetosheath KH. This suggest that a kind of signature related to the development of the KH unstable modes could be present in the statistical properties of the magnetic turbulence.