BRUNO, RobertoRobertoBRUNOTELLONI, DanieleDanieleTELLONI2020-03-252020-03-2520152041-8205http://hdl.handle.net/20.500.12386/23573This research was partially supported by the Italian Space Agency (ASI) under contracts I/013/12/0 and I/022/10/2, and by the European Commission’s Seventh Framework Program under the grant agreement STORM (project No. 313038). Data from WIND were obtained from NASA-CDAWeb website.This Letter investigates the spectral characteristics of 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 kinetic regimes, 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 Kinetic {Alfv}\acute{{{e}}}{{n}} Wave (KAW) and ion-cyclotron waves, respectively, the rarefaction region, where the wind speed and the Alfvénicity of low-frequency fluctuations decrease, shows a rapid disappearance of the ion-cyclotron signature followed by a more gradual disappearance of KAWs. Moreover, the power associated with perpendicular and parallel fluctuations also experiences rapid depletion, however, retaining the power anisotropy in favor of the perpendicular spectrum.STAMPAenArticle10.1088/2041-8205/811/2/L172-s2.0-84945183043000364478600003https://iopscience.iop.org/article/10.1088/2041-8205/811/2/L172015ApJ...811L..17BFIS/05 - ASTRONOMIA E ASTROFISICA