Magnetic Reconnection as a Driver for a Sub-ion-scale Cascade in Plasma Turbulence

Abstract

A new path for the generation of a sub-ion scale cascade in collisionless space and astrophysical plasma turbulence, triggered by magnetic reconnection, is uncovered by means of high-resolution two-dimensional hybrid-kinetic simulations employing two complementary approaches, Lagrangian and Eulerian, and different driving mechanisms. The simulation results provide clear numerical evidences that the development of powerlaw energy spectra below the so-called ion break occurs as soon as the first magnetic reconnection events take place, regardless of the actual state of the turbulent cascade at MHD scales. In both simulations, the reconnection-mediated small-scale energy spectrum of parallel magnetic fluctuations exhibits a very stable spectral slope of ∼ −2.8, whether or not a large-scale turbulent cascade has already fully developed. Once a quasi-stationary turbulent state is achieved, the spectrum of the total magnetic fluctuations settles towards a spectral index of −5/3 in the MHD range and of ∼ −3 at sub-ion scales.

Solar Wind Turbulent Cascade from MHD to Sub-ion Scales: Large-size 3D Hybrid Particle-in-cell Simulations

“…Moreover, the fact that the width of the 2D current sheets in the 3D simulation seems to be of the same order of the 1D current sheets in the 2D simulation (see the bottom panel of Figure 6) might support, although only qualitatively at this level, the idea that the disruption of current structures via magnetic reconnection may be the main factor responsible for the break and the onset of the sub-ion-scale cascade even in more realistic 3D turbulence. Although the complex shape of magnetic and current structures makes the identification of reconnection sites in 3D much more difficult than in 2D, a preliminary analysis (not presented here) shows a correspondence between the first peak in the maximum of the current and the development of the kinetic power law, in agreement with the findings of Franci et al (2017b) in 2D. A more quantitative analysis of the current structures is necessary in order to confirm this scenario.…”

“…Observational results suggest that the transition likely occurs in correspondence with the larger of the two when they are well separated (Chen et al 2014) or a combination of the two in the intermediate-beta regime (Bruno & Trenchi 2014). Although the nature of the power-law behavior of the solar wind fluctuations at kinetic scales is still under debate, an increasing consensus has recently emerged about the fundamental role of coherent structures and magnetic reconnection in shaping the magnetic field spectrum near and below the ion scales (e.g., Franci et al 2016Franci et al , 2017bLoureiro & Boldyrev 2017;Mallet et al 2017).…”

Collisionless Reconnection in Magnetohydrodynamic and Kinetic Turbulence

“…Indeed, for realistic values of the resistivity, one finds that the mechanism breaking the frozen flux condition, and thus enabling reconnection, is likely to be the electron inertia, rather than the resistivity. Second, turbulence in very weakly collisionless plasmas extends to weakly collisional turbulent astrophysical environments; it is possible, as we will show, that reconnection may only become important at those scales Franci et al 2017). This paper presents the first attempt at extending these recent ideas on the role of reconnection in turbulence to accommodate kinetic physics.…”

“…Indeed, the turbulent fluctuations at such scales resemble current sheets (e.g., Boldyrev & Perez 2012;Wan et al 2012Wan et al , 2016TenBarge & Howes 2013;Chen et al 2015;Franci et al 2017), although those turbulent structures (and generally turbulence at kinetic scales) are relatively less understood than their Alfvénic counterpart. If we may assume that, similarly to the Alfvénic case, the dynamics at subproton scales are governed by the fastest growing tearing modes, the reconnection-dominated turbulence should have the same scaling E(k)∝k −8/3 to E(k)∝k −3 that we have derived for the Alfvénic case.…”

“…In particular, we are not familiar with an analytical theory that offers the kinetic equivalent of Equations (11)-(13), implying, therefore, that we cannot know whether the tendency to develop current sheets that is present in the MHD range remains true in the kinetic range. However, numerical simulations and observations (e.g., Boldyrev & Perez 2012;Wan et al 2012Wan et al , 2016TenBarge & Howes 2013;Chen et al 2015;Franci et al 2017) do show evidence for current sheet formation at such scales, and so perhaps it is legitimate to assume that current sheets remain the fundamental units of sub-ion scales turbulence. Note that the argument that follows is completely independent of the nature of the waves that are found in the kinetic regime; indeed, no specific property of the turbulence in the kinetic range is invoked, other than its tendency to form current sheets.…”