Coherent events and spectral shape at ion kinetic scales in the fast solar wind turbulence

TL;DR

This study analyzes magnetic fluctuation coherence in the fast solar wind, revealing that a combination of waves and structures shapes the spectral transition from MHD to sub-ion scales, explaining observed spectral variability.

Contribution

It demonstrates the coexistence of Alfvénic waves and vortex-like structures near ion scales and their combined influence on spectral shape in solar wind turbulence.

Findings

Alfvén Ion Cyclotron waves contribute to the spectrum in a narrow frequency range.

Coherent structures influence the spectrum over a broad frequency band.

The spectral shape results from a combination of waves and structures.

Abstract

In this paper we investigate spectral and phase coherence properties of magnetic fluctuations in the vicinity of the spectral transition from large, magnetohydrodynamic (MHD) to sub-ion scales using in-situ measurements of the Wind spacecraft in a fast stream. For the time interval investigated by Leamon et al. (1998) the phase-coherence analysis shows the presence of sporadic quasi-parallel Alfv\'en Ion Cyclotron (AIC) waves as well as coherent structures in the form of large-amplitude, quasi-perpendicular Alfv\'en vortex-like structures and current sheets. These waves and structures importantly contribute to the observed power spectrum of magnetic fluctuations around ion scales; AIC waves contribute to the spectrum in a narrow frequency range whereas the coherent structures contribute to the spectrum over a wide frequency band from the inertial range to the sub-ion frequency range. We conclude that a particular combination of waves and coherent structures determines the spectral shape of the magnetic field spectrum around ion scales. This phenomenon provides a possible explanation for a high variability of the magnetic power spectra around ion scales observed in the solar wind.