Can Hall Magnetohydrodynamics explain plasma turbulence at sub-ion scales?

TL;DR

This study compares Hall-MHD and hybrid particle-in-cell simulations to determine if Hall-MHD can accurately describe plasma turbulence from large to sub-ion scales, finding strong agreement in spectral properties and reconnection dynamics.

Contribution

It demonstrates that Hall-MHD simulations effectively replicate key features of plasma turbulence at sub-ion scales observed in more complex HPIC models and solar wind data.

Findings

Spectral indices match between HMHD and HPIC simulations.

Magnetic reconnection is fast and similar in both models.

Reconnection mediates energy transfer at kinetic scales.

Abstract

We investigate the properties of plasma turbulence by means of two-dimensional Hall-magnetohydrodynamic (HMHD) and hybrid particle-in-cell (HPIC) numerical simulations. We find that HMHD simulations exhibit spectral properties that are in most cases in agreement with the results of the HPIC simulations and with solar wind observations. The energy spectra of magnetic fluctuations exhibit a double power-law with spectral index $-5/3$ at MHD scales and $-3$ at kinetic scales, while for velocity fluctuations the spectral index is $-3/2$ at MHD scales. The break between the MHD and the kinetic scales occurs at the same scale in both simulations. In the MHD range the slopes of the total energy and residual energy spectra satisfy a fast Alfv\'en-dynamo balance. The development of a turbulent cascade is concurrently characterized by magnetic reconnection events taking place in thin current sheets that form between large eddies. A statistical analysis reveals that reconnection is qualitatively the same and fast in both the HMHD and HPIC models, characterized by inverse reconnection rates much smaller than the characteristic large-eddy nonlinear time. The agreement extends to other statistical properties, such us the kurtosis of the magnetic field. Moreover, the observation of a direct energy transfer from the large vortices to the small sub-ion scales triggered by magnetic reconnection, further supports the existence of a reconnection-mediated turbulent regime at kinetic scales. We conclude that the Hall-MHD fluid description captures to a large extent the transition of the turbulent cascade between the large MHD scales and the sub-ion scales.