Sudden depletion of Alfvénic turbulence in the rarefaction region of solar wind high speed streams: solar origin or dynamic evolution?

Abstract:

A canonical description of a high speed stream in the solar wind 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 Alfvénic character; a rarefaction region characterized by a decreasing trend of the flow speed and wind fluctuations dramatically reduced in amplitude and Alfvénic character. Interesting enough, the region where the quasi disappearance of these fluctuations takes place is remarkably short in time and located at the flow velocity knee separating the fast wind plateau from the rarefaction region. The aim of this work has been to investigate those physical mechanisms that might be at the origin of such phenomenon. We analyzed a variety of high speed streams observed by Wind at 1 AU. We looked at different aspects which might be related to differences in solar wind sources on the Sun or might unravel particular phenomena in the turbulence character of the fluctuations. Having discarded differences in the source regions which might have been unraveled by the analysis based on minor ions parameters, we propose here a mechanism related to the magnetic field topology at the source region of the stream. The effects of this phenomenon is more or less evident, to an observer in the ecliptic, depending on the larger or smaller inclination of the heliomagnetic equator with respect to the ecliptic plane. This kind of study will greatly benefit from the Solar Orbiter observations during the future nominal phase of the mission when it will be possible to link remote and in-situ data.