Kinetic plasma turbulence in the fast solar wind measured by Cluster

TL;DR

This study uses Cluster spacecraft data to analyze plasma turbulence at ion scales in the fast solar wind, revealing wave propagation nearly perpendicular to the magnetic field and suggesting the presence of kinetic Alfvén waves and vortices.

Contribution

It applies advanced wave analysis techniques to identify wave modes and polarization properties in solar wind turbulence at ion scales, providing new insights into turbulence composition.

Findings

Waves propagate nearly perpendicular to the magnetic field.

Magnetic fluctuations are dominated by perpendicular components.

Evidence suggests the presence of kinetic Alfvén waves and vortices.

Abstract

The k-filtering technique and wave polarization analysis are applied to Cluster magnetic field data to study plasma turbulence at the scale of the ion gyroradius in the fast solar wind. Waves are found propagating in directions nearly perpendicular to the background magnetic field at such scales. The frequencies of these waves in the solar wind frame are much smaller than the proton gyro-frequency. After the wave vector ${\bf k}$ is determined at each spacecraft frequency $f_{sc}$, wave polarization property is analyzed in the plane perpendicular to ${\bf k}$. Magnetic fluctuations have $\delta B_\perp>\delta B_\parallel$ (here the $\parallel$ and $\perp$ refer to the background magnetic field ${\bf B}_0$). The wave magnetic field has right-handed polarization at propagation angles $\theta_{\bf kB}<90^\circ$ and $>90^\circ$. The magnetic field in the plane perpendicular to ${\bf B}_0$ however has no clear sense of a dominant polarization but local rotations. We discuss the merits and limitations of linear kinetic Aflv\'en waves (KAWs) and coherent Alfv\'en vortices in the interpretation of the data. We suggest that the fast solar wind turbulence may be populated with KAWs, small scale current sheets and Alfv\'en vortices at ion kinetic scales.