Turbulent spectra and spectral kinks in the transition range from MHD to kinetic Alfv\'en turbulence

TL;DR

This paper investigates the transition from MHD to kinetic Alfvén turbulence, identifying a weakly dispersive sub-range with characteristic spectra and spectral kinks, and proposes a mechanism for selective turbulence dissipation affecting observed spectra.

Contribution

First identification and analysis of the weakly dispersive sub-range in MHD to kinetic Alfvén turbulence transition, revealing spectral features and dissipation mechanisms.

Findings

Steep spectra (~k^{-3} and ~k^{-4}) in the WDR connect MHD and KAW sub-ranges.

Two spectral kinks mark the transition points between different turbulence regimes.

Partial dissipation near the first kink influences intermittency and explains recent observational results.

Abstract

A weakly dispersive sub-range (WDR) of kinetic Alfv\'en turbulence is distinguished and investigated for the first time in the context of MHD/kinetic turbulence transition. We found perpendicular wavenumber spectra ~ k^{-3} and ~ k^{-4} formed in WDR by strong and weak turbulence of kinetic Alfv\'en waves (KAWs), respectively. These steep WDR spectra connect shallower spectra in the MHD and strongly dispersive KAW sub-ranges, which results in a specific double-kink (2-k) pattern often seen in observed turbulent spectra. The first kink occurs where MHD turbulence transforms into weakly dispersive KAW turbulence; the second one is between weakly and strongly dispersive KAW sub-ranges. Our analysis suggests that the partial turbulence dissipation due to amplitude-dependent super-adiabatic ion heating may occur in the vicinity of the first spectral kink. A threshold-like nature of this process results in a conditional selective dissipation affecting only largest over-threshold amplitudes and decreasing intermittency in the range below the first spectral kink. Several recent counter-intuitive observational findings can be explained by the selective dissipation coupled to the nonlinear interaction among weakly dispersive KAWs.