Discontinuities and Alfvenic fluctuations in the solar wind

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 degrees; 33 cases had speeds exceeding 90% of the Alfven speed at an average angle of 6.4 degrees. 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. The corresponding results from the Walen test expressed in terms of jumps in flow speed and corresponding jumps in Alfven speed are similar: 66 of the same 127 cases had velocity jumps exceeding 80% with average angular deviation of 5.8 degrees, and 22 exceeding 90% of the jump in Alfven speed, with average angular deviation 6.2 degrees. We conclude that a substantial fraction of the 127 events can be identified as RDs. We present further evidence for coupling across the DDs by showing that, for most of the 127 crossings, the HT frame velocities, evaluated separately on the two sides of the DD, are nearly the same - a result required for RDs but not for TDs. We also show that the degree of Alfvenicity is nearly the same for the DDs and fluctuations in which the DDs are embedded. Whatever process causes deviations from ideal Alfvenicity appears to operate equally for the DDs as for the surrounding fluctuations. Finally, our study has established a unique relation between the strahl electron pitch angle and the sign of the Walen slope, implying antisunward propagation in the plasma frame for all 127 cases.