  A canonical description of a corotating solar wind high speed stream, in
terms of velocity profile, would indicate three main regions:a stream interface
or corotating interaction region characterized by a rapid flow speed increase
and by compressive phenomena due to dynamical interaction between the fast wind
flow and the slower ambient plasma;a fast wind plateau characterized by weak
compressive phenomena and large amplitude fluctuations with a dominant
Alfv\'enic character;a rarefaction region characterized by a decreasing trend
of the flow speed and wind fluctuations dramatically reduced in amplitude and
Alfv\'enic character, followed by the slow ambient wind. Interesting enough, in
some cases the region where the severe reduction of these fluctuations takes
place is remarkably short in time, of the order of minutes, and located at the
flow velocity knee separating the fast wind plateau from the rarefaction
region. The aim of this work is to investigate which are the physical
mechanisms that might be at the origin of this phenomenon. We firstly looked
for the presence of any tangential discontinuity which might inhibit the
propagation of Alfv\'enic fluctuations from fast wind region to rarefaction
region. The absence of a clear evidence for the presence of this discontinuity
between these two regions led us to proceed with ion composition analysis for
the corresponding solar wind, looking for any abrupt variation in minor ions
parameters (as tracers of the source region) which might be linked to the
phenomenon observed in the wind fluctuations. In the lack of a positive
feedback from this analysis, we finally propose a mechanism based on
interchange reconnection experienced by the field lines at the base of the
corona, within the region separating the open field lines of the coronal hole,
source of the fast wind, from the surrounding regions mainly characterized by
closed field lines.
