We examine the Alfvenicity of a set of 188 solar wind directional discontinuities (DDs) identified in the Cluster data from 2003 by Knetter (2005), with the objective of separating rotational discontinuities (RDs) from tangential ones (TDs). The DDs occurred over the full range of solar wind velocities and magnetic shear angles. By performing the Walen test in the de Hoffmann–Teller (HT) frame, we show that 77 of the 127 crossings for which a good HT frame was found had plasma flow speeds exceeding 80 % of the Alfven speed at an average angular deviation of 7.7◦; 33 cases had speeds exceeding 90 % of the Alfven speed at an average angle of 6.4◦. We show that the angular deviation between flow velocity (in the HT frame) and the Alfven velocity can be obtained from a reduced form of the Walen correlation coefficient.
Paschmann, G.; Haaland, S.; Sonnerup, B.; and Knetter, T., "Discontinuities and Alfvenic Fluctuations in the Solar Wind" (2013). Open Dartmouth: Faculty Open Access Articles. 430.