Investigation of Alpha-Proton Drift Speeds in the Solar Wind: WIND and HELIOS Observations

In this paper, we present an analysis of how alpha–proton drift speeds (the difference between the magnitudes of alpha and bulk proton speeds) are constrained in the inner heliosphere using observations from the WIND and twin HELIOS spacecraft. The solar wind is separated based on its bulk proton speed into the fast wind (>600 km/s) and slow wind (<400 km/s). The slow wind is again separated based on its normalized cross-helicity; slow wind intervals with average absolute normalized cross-helicity greater than 0.6 are considered Alfvénic, and those less than 0.6 are considered non-Alfvénic. Analysis of different types of wind intervals between 0.3 to 1 au have shown that the alpha-proton drift speeds are very much constrained by the angle between the B and V vectors for fast and slow Alfvénic wind intervals. Depending on the polarity of the magnetic field, there is a clear correlation or anti-correlation between the drift speeds and the angle between the B and V vectors. Interestingly, we did not observe any such relation in the non-Alfvénic slow wind intervals. Large-amplitude Alfvénic fluctuations present in the fast and slow Alfvénic winds control the drift between the alpha and proton core in the Alfvénic solar wind. The drift speeds can be modeled using the equation +/−VArAcosθBV, where VA is the Alfvén speed and rA is the Alfvén ratio. Because the observations of drift speed constrained by the angle between the B and V vector for the fast and slow Alfvénic wind intervals are observed throughout the inner heliosphere, it is possible to consider this observed behavior to be a universal phenomenon of Alfvénic wind above the Alfvénic surface.