Evidence of a "current-mediated" turbulent regime in space and astrophysical plasmas

Abstract

How the turbulent energy cascade develops below the magnetohydrodynamic scales in space and astrophysical plasmas is a major open question. Here, we measure the power spectrum of magnetic fluctuations in Parker Solar Probe's observations close to the Sun and in state-of-the-art numerical simulations of plasma turbulence. Both reveal a power-law behavior with a slope compatible with −11/3 at scales smaller than the ion characteristic scales, steeper than what is typically observed in the solar wind and in the Earth's magnetosheath. We explain such behavior by developing a simple two-fluid model which does not require any kinetic processes nor electron-inertia effects. This is characterized by a significant contribution of the ion kinetic energy to the total turbulent energy cascade at sub-ion scales, although the dynamics is driven by the magnetic field through the current density. We expect that this regime may be relevant for a broad class of low-beta plasmas, e.g. the solar corona, non-relativistic magnetized jets and disks, and laboratory plasmas.

Anisotropic Electron Heating in Turbulence-driven Magnetic Reconnection in the Near-Sun Solar Wind

“…P B hyb is more extended at large scales, since the hybrid simulation box is larger. This allows for the development of a Kolmogorov-like power law with a spectral index of −5/3 for k ⊥ d i  3.5, which then steepens to a power law with a slope compatible with −11/3, as already observed in Franci et al (2020a). P B kin , instead, completely lacks the −5/3 range and is higher at the largest scales.…”

“…We have modeled plasma conditions similar to those measured by PSP during its first solar encounter. This allowed us to extend toward larger wavenumbers the analysis of the ion-scale turbulent cascade performed with hybrid simulations in Franci et al (2020a). Our simulation employs a large box in terms of the electron characteristic scales, i.e., 320 d e × 320 d e (d i = 10 d e , since the proton-to-electron mass ratio has been set to 100) and a spatial resolution Δx = d i /64 ; 0.16 d e , and implements a very large number of particles (1024 ppc ions and 8192 ppc electrons, for a total of more than 40 billion particles in the whole simulation domain).…”

“…We set the ion plasma beta β i = 0.2 and the electron plasma beta β e = 0.5 to mimic average values from PSP measurements at its first perihelion (see Section IIIA of Franci et al 2020a). Initially, we assume a uniform number density n = 1 and no ion or electron temperature anisotropy, i.e., A i,e = T (i,e)⊥ /T (i,e)∥ = 1.…”

“…In order to investigate this and validate our results, we have run a hybrid simulation with the same physical conditions but a much larger box, able to model the turbulent cascade over two decades in wavenumber, one above and one below the ion scales. This simulation has the same parameters and initial conditions as the one described in Franci et al (2020a), so that further information can be found in Appendix A therein. The only (minor) differences are that the grid points are 4000 × 4000 instead of 4096 × 4096 (and correspondingly the box size is 250 d i instead of 256) and 2048 ppc instead of 1024.…”

A Novel Method for Estimating the Intrinsic Magnetic Field Spectrum of Kinetic-Range Turbulence

“…The dependence of the rectified local slopes on the amplitude/power of the magnetic field fluctuations is qualitatively similar to that of [42]; however, for solar wind intervals exhibiting low levels of fluctuations, we observe significantly steeper slopes (∼−8/3). Interestingly, for high P in , the local slope seems to saturate around −11/3, which is a prediction of the current-mediated turbulent regime [43].…”