MMS Observations of Kinetic Alfv\'en Wave Turbulence and Steep Kinetic-Range Spectra in the Outer Plasma Sheet Boundary Layer

TL;DR

This study presents MMS observations of Kinetic Alfvén Wave turbulence in the outer plasma sheet boundary layer, revealing steep kinetic-range spectra, electric field structures, and evidence of collisionless damping processes.

Contribution

First observational characterization of KAW turbulence spectral properties and dissipation signatures in the magnetotail boundary layer using high-resolution MMS data.

Findings

Steep kinetic-range spectral slope of -3.48

Broadband KAW turbulence with high electric-to-magnetic field ratio

Impulsive parallel electric fields and density fluctuations during wave activity

Abstract

Energy dissipation and particle acceleration in the collisionless magnetotail plasma remain incompletely understood. While Kinetic Alfv\'en Waves (KAWs) are widely hypothesized to mediate these processes, observational characterization of their spectral properties and dissipation signatures in magnetotail boundary layers remains limited. We report observations of KAW turbulence and parallel electric fields ($E_{\parallel}$) in the outer Plasma Sheet Boundary Layer (PSBL) using high-resolution burst-mode data from the Magnetospheric Multiscale (MMS) mission. For a crossing event on May 31, 2017, we identify broadband KAW turbulence characterized by a normalized electric-to-magnetic field ratio $\mathcal{R} = |\delta E_{\perp}|/(v_A|\delta B_{\perp}|) = 2.5 \pm 1.2$ exceeding the MHD limit, a spectral break near ion scales, a steep kinetic-range spectral slope ($\alpha = -3.48 \pm 0.13$), and low magnetic compressibility ($C_{\parallel} \approx 0.03$). We observe impulsive parallel electric field structures (up to 15~mV/m) and large-amplitude density fluctuations (up to 68\%) during intervals of enhanced wave activity. The steep spectral slope, steeper than theoretical predictions for undamped KAW cascades ($-7/3$ to $-8/3$), is consistent with substantial energy removal from the cascade at kinetic scales. The near-zero correlation between the $E_{\parallel}$ waveform and density fluctuations ($r \approx -0.03$) suggests that the observed $E_{\parallel}$ structures are not straightforwardly organized by compressive density variations, consistent with dissipation through direct wave--particle interaction. Attribution to a specific damping channel (e.g., Landau damping) is not uniquely constrained by the present diagnostics. These observations support collisionless damping of KAW turbulence at kinetic scales in the intermediate-beta, outer PSBL of the terrestrial magnetotail.