Sudden Depletion of Alfvénic Turbulence in the Rarefaction Region of Corotating Solar Wind High-Speed Streams at 1 AU: Possible Solar Origin?

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 increase in flow speed and by compressive phenomena that are 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 Alfvénic character; and a rarefaction region characterized by a decreasing trend of the flow speed and wind fluctuations that are gradually reduced in amplitude and Alfvénic character, followed by the slow ambient wind. Interesting enough, in some cases, fluctuations are dramatically reduced, and the time window in which the severe reduction of these fluctuations takes place is remarkably short, about some minutes. The region in which the fluctuations are rapidly reduced is located at the flow velocity knee that separates the fast wind plateau from the rarefaction region. The aim of this work is to investigate the physical mechanisms that might be at the origin of this phenomenon. To do this, we searched for any tangential discontinuity that might have inhibited the diffusion of these large-amplitude fluctuations in the rarefaction region as well. We also searched for differences in the composition analysis because minor ions are good tracers of physical conditions in the source regions of the wind under the hypothesis that large differences in the source regions might be linked to the phenomenon observed in situ. We found no positive feedback from these analyses, and finally invoked a mechanism based on interchange reconnection experienced by the field lines at the base of the corona, within the region that separates the open field lines of the coronal hole, which is the source of the fast wind, from the surrounding regions that are mainly characterized by closed field lines. Another possibility clearly is that the observed phenomenon might be due to the turbulent evolution of the fluctuations during the expansion of the wind. However, it is hard to believe that this mechanism would generate a short transition region such as is observed in the phenomenon we discuss. This type of study will greatly benefit from Solar Orbiter observations during the future nominal phase of the mission, when it will be possible to link remote and in-situ data, and from radial alignments between Parker Solar Probe and Solar Orbiter.