Coherent structures at ion scales in fast solar wind: {\it Cluster} observations

TL;DR

This study analyzes magnetic turbulence at ion scales in fast solar wind using Cluster data, revealing Alfvénic vortex dominance and structural differences from slow wind, with implications for plasma energy transfer.

Contribution

It provides new insights into the nature of coherent structures in fast solar wind, highlighting the dominance of Alfvén vortices and their alignment, contrasting with slow wind characteristics.

Findings

Fast solar wind is dominated by Alfvén vortices with small compressive parts.

Coherent structures are aligned with the magnetic field and convected with the flow.

No strongly compressive structures are observed in fast wind.

Abstract

We investigate the nature of magnetic turbulent fluctuations, around ion characteristic scales, in a fast solar wind stream, by using {\it Cluster} data. Contrarily to slow solar wind, where both Alfv\'enic ($\delta b_{\perp} \gg \delta b_{\parallel}$) and compressive ($\delta b_{\parallel} \gg \delta b_{\perp}$) coherent structures are observed \cite[]{per16}, the turbulent cascade of fast solar wind is dominated by Alfv\'enic structures, namely Alfv\'en vortices, with small and/or finite compressive part, with the presence also of several current sheets aligned with the local magnetic field. Several examples of vortex chains are also recognized. Although an increase of magnetic compressibility around ion scales is observed also for fast solar wind, no strongly compressive structures are found, meaning that the nature of the slow and fast winds is intrinsically different. Multi-spacecraft analysis applied to this interval of fast wind indicate that the coherent structures are almost convected by the flow and aligned with the local magnetic field, i.e. their normal is perpendicular to {\bf B}, that is consistent with a two dimensional turbulence picture. Understanding intermittency and the related generation of coherent structures could provide a key insight into the nonlinear energy transfer and dissipation processes in magnetized and collisionless plasmas.