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

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 Alfvénic ($\delta b_{\perp }\gg \delta b_{\parallel }$) 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 Alfvénic structures, namely Alfvén 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.

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• Magnetospheric Multiscale Observation of Kinetic Signatures in the Alfvén Vortex
• Fluid simulations of plasma turbulence at ion scales: comparison with Vlasov-Maxwell simulations
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• Direction of cascades in a magnetofluid model with electron skin depth and ion sound Larmor radius scales
• Influence of ion-to-electron temperature ratio on tearing instability and resulting subion-scale turbulence in a low-${\beta }_e$ collisionless plasma
• Statistical analysis of stochastic magnetic fluctuations in space plasma based on the MMS mission
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• Nature of electrostatic fluctuations in the terrestrial magnetosheath
• Ion-Scale Solitary Structures in the Solar Wind Observed by Solar Orbiter and Parker Solar Probe