Abstract
Abstract. We examine the Alfvénicity 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 Walén 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 Alfvén speed at an average angular deviation of 7.7°; 33 cases had speeds exceeding 90% of the Alfvén speed at an average angle of 6.4°. We show that the angular deviation between flow velocity (in the HT frame) and the Alfvén velocity can be obtained from a reduced form of the Walén correlation coefficient. The corresponding results from the Walén test expressed in terms of jumps in flow speed and corresponding jumps in Alfvén speed are similar: 66 of the same 127 cases had velocity jumps exceeding 80% with average angular deviation of 5.8°, and 22 exceeding 90% of the jump in Alfvén speed, with average angular deviation 6.2°. 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 Alfvénicity is nearly the same for the DDs and fluctuations in which the DDs are embedded. Whatever process causes deviations from ideal Alfvénicity 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 Walén slope, implying antisunward propagation in the plasma frame for all 127 cases.
Highlights
We show how to express the cc in terms of a weighted average of the cosine of the angles between pairs of ordinary three-dimensional physical vectors
We do not think those results can be used to negate the Borovsky model, which is based on a much larger data set and has many intuitively attractive features
Summary
We used the set of DDs identified by Knetter (2005) in the high-resolution magnetic field measurements (Balogh et al, 1997) on the Cluster 1 (C1) spacecraft obtained in 2003. As described in Knetter et al (2004), events were selected when either the criteria introduced by Burlaga (1969) or those introduced by Tsurutani and Smith (1979) were met, which both essentially require magnetic field rotations > 30◦ within a 1 or 3 min interval. Given that the Cluster apogee is not much beyond the Earth’s bow shock, the avoidance of bow shock connection biases the data set toward cases where the DD normals have large GSE x components. The 4 s time resolution of HIA means that the DDs could usually not be resolved in the plasma data
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