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.Comment: 10 pages, 5 figures, to be published in ApJ
