Dependence of solar wind speed on the local magnetic field orientation: Role of Alfvenic fluctuations

We report an analysis of correlations between magnetic field and velocity fluctuations in the fast solar wind beyond 1 AU at high latitudes. We have found that on scales shorter than the microstream structures, there exists a well-defined dependence of the flow speed on the angle between the magnetic field vector and the radial direction. Solar wind is found to be slightly faster when the measured magnetic field vector is transverse to the velocity, while it is always slower when the magnetic field is parallel, or antiparallel, to the radial direction. We show that this correlation is a direct consequence of the high Alfvenicity of fast wind fluctuations and that it can be reasonably described by a simple model taking into account the main properties of the low-frequency antisunward Alfven fluctuations as observed in the solar wind plasma. We also discuss how switchbacks, short periods of magnetic field reversals, naturally fit in this new observed correlation. Key Points A new correlation between magnetic field direction and solar wind speed is found Low frequency Alfvenic fluctuations are responsible for the observed dynamics Magnetic switchbacks are explained as natural evolution of Alfvenic turbulence