Spectral Features of Magnetic Fluctuations at Proton Scales from Fast to Slow Solar Wind

TL;DR

This study analyzes magnetic fluctuation spectra at proton scales in different solar wind conditions, revealing wind-dependent polarization signatures and the depletion of fluctuation power during rarefaction.

Contribution

It provides new insights into how magnetic fluctuation characteristics vary with solar wind speed and structure, especially regarding wave polarization and power anisotropy.

Findings

Fast wind shows both right and left polarized fluctuations, linked to KAW and Ion-Cyclotron waves.

Rarefaction regions exhibit rapid disappearance of ion-cyclotron signatures.

Perpendicular fluctuation power decreases more slowly, maintaining anisotropy.

Abstract

This Letter investigates the spectral characteristics of the interplanetary magnetic field fluctuations at proton scales during several time intervals chosen along the speed profile of a fast stream. The character of the fluctuations within the first frequency decade, beyond the high frequency break located between the fluid and the kinetic regime, strongly depends on the type of wind. While the fast wind shows a clear signature of both right handed and left handed polarized fluctuations, possibly associated with KAW and Ion-Cyclotron waves, respectively, the rarefaction region, where the wind speed and the Alfv\'{e}nicity of low frequency fluctuations decrease, shows a rapid disappearance of the ion-cyclotron signature followed by a more gradual disappearance of the KAWs. Moreover, also the power associated to perpendicular and parallel fluctuations experiences a rapid depletion, keeping, however, the power anisotropy in favour of the perpendicular spectrum.