The Complex Structure of Magnetic Field Discontinuities in the Turbulent Solar Wind

Abstract

Using high resolution Cluster satellite observations, we show that the turbulent solar wind is populated by magnetic discontinuities at different scales, going from proton down to electron scales. The structure of these layers resembles the Harris equilibrium profile in plasmas. Using a multi-dimensional intermittency technique, we show that these structures are connected through the scales. Supported by numerical simulations of magnetic reconnection, we show that observations are consistent with a scenario where many current layers develop in turbulence, and where the outflow of these reconnection events are characterized by complex sub-proton networks of secondary islands, in a self-similar way. The present work establishes that the picture of "reconnection in turbulence" and "turbulent reconnection", separately invoked as ubiquitous, coexist in space plasmas.In the past decades, spacecraft observations suggested that plasma turbulence shares many similarities with classical hydrodynamics. The power spectrum of magnetic field fluctuations as a function of frequencies f manifests an inertial range, with a Kolmogorov-like scaling f −5/3 (see for example Ref . [1]). More recently, high resolution measurements revealed the presence of a secondary inertial sub-range, where the spectrum breaks down and exhibits a power index steeper than −5/3 [2, 3]. The characteristic scales at which this break-down occurs are given by the proton gyro-radius ρ p = v th,p /Ω p (being v th,p the proton thermal speed and Ω p the proton gyrofrequency) and/or the proton skin depth d p = c/ω cp (being c the speed of light and ω cp the proton plasma frequency) [2,[4][5][6]. At these scales the dynamics could be mediated by kinetic-Alfvén fluctuations, whistler-like perturbations and coherent structures such as vortexes and current sheets [7].The most narrow current sheets and filaments are present at electron scales, where turbulent energy eventually dissipates [8,9], even if the energy-dissipation mechanisms in a weakly-collisional plasma such as the turbulent solar wind are far from being understood. Recent kinetic simulations

On the Kinetic Nature of Solar Wind Discontinuities

“…Figures 5c, 5d, and 4d show that j m peaks around the B l reversal and reaches 3 nA/m 2 (comparable with the most intense currents observed in the solar wind; see Greco et al, 2016;Podesta, 2017). Figures 5c, 5d, and 4d show that j m peaks around the B l reversal and reaches 3 nA/m 2 (comparable with the most intense currents observed in the solar wind; see Greco et al, 2016;Podesta, 2017).…”

Reconnection and small-scale fields in 2D-3V hybrid-kinetic driven turbulence simulations

“…An important role can be played by coherent structures, as they represent a different path (with respect to wave modes) to contribute to the turbulent cascade around and below the ion kinetic scales in order to allow and/or speed up the nonlinear transfer of the electromagnetic fluctuations beyond the inertial spectrum. Despite these processes, that come into play well before collisional effects, are not yet well understood, their importance in partially 'replacing' the role of collisional dissipation in plasma turbulence is now widely accepted [18,19]. At the same time the dynamics of these coherent structures may provide some additional self-consistent energy injection [20][21][22] and thus supporting the continuation of the cascade across and below ion kinetic scales.…”

Review article: Wave analysis methods for space plasma experiment

“…Coherent structures (particularly arising out of the turbulent field) are considered ubiquitous in the free solar wind as well as in magnetospheric plasma. Coherent structures populate signals in the solar wind at a very high cadence, on scales on the order of the electron inertial length, playing a role in the low-frequency fluctuations, ω ∼ 0 (Greco et al, 2009(Greco et al, , 2016Bruno and Carbone, 2016).…”