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A canonical description of a corotating solar wind high speed stream, interms of velocity profile, would indicate three main regions:a stream interfaceor corotating interaction region characterized by a rapid flow speed increaseand by compressive phenomena due to dynamical interaction between the fast windflow and the slower ambient plasma;a fast wind plateau characterized by weakcompressive phenomena and large amplitude fluctuations with a dominantAlfv\'enic character;a rarefaction region characterized by a decreasing trendof the flow speed and wind fluctuations dramatically reduced in amplitude andAlfv\'enic character, followed by the slow ambient wind. Interesting enough, insome cases the region where the severe reduction of these fluctuations takesplace is remarkably short in time, of the order of minutes, and located at theflow velocity knee separating the fast wind plateau from the rarefactionregion. The aim of this work is to investigate which are the physicalmechanisms that might be at the origin of this phenomenon. We firstly lookedfor the presence of any tangential discontinuity which might inhibit thepropagation of Alfv\'enic fluctuations from fast wind region to rarefactionregion. The absence of a clear evidence for the presence of this discontinuitybetween these two regions led us to proceed with ion composition analysis forthe corresponding solar wind, looking for any abrupt variation in minor ionsparameters (as tracers of the source region) which might be linked to thephenomenon observed in the wind fluctuations. In the lack of a positivefeedback from this analysis, we finally propose a mechanism based oninterchange reconnection experienced by the field lines at the base of thecorona, within the region separating the open field lines of the coronal hole,source of the fast wind, from the surrounding regions mainly characterized byclosed field lines.

Sudden depletion of Alfv\'enic turbulence in the rarefaction region of corotating solar wind high speed streams at 1 AU: possible solar origin?