Discontinuity observations with cluster

Observations of solar wind magnetic field discontinuities using four spacecraft allow their orientation to be estimated by relative timings referred to as triangulation. From February 1st until May 13th 2001 the Cluster orbit traversed the solar wind. During these periods we identified 489 directional discontinuities (DDs) using the Tsurutani-Smith and the Burlaga method. However, out of them only 146 were unambiguously free of foreshock activity and another 17 events failed a visual inspection. Besides triangulation we also applied Minimum Variance Analysis (MVA) to the remaining 129 DDs in order to estimate their normals. Using all events that satisfy λ2/λ3 > 2 MVA results are often far from the triangulation normals. In addition, there are often great discrepancies between the four MVA normals among each other. For a normalized change in magnitude |[B]|/Bmax < 0.2 35% of the DDs have a magnetic field normal component |Bn|/Bmax > 0.4 when we estimate the normals using MVA. This is in good agreement with previous single spacecraft studies where this class of events was suggested to be rotational. Triangulation normal estimates on the other hand yield 0%. However, if we confine ourselves to consider only those DDs with large values of λ2/λ3 we get a consistent picture. We believe that the MVA normal estimates are generally impaired by the presence of non-isotropic magnetic fluctuation due to 3D wave fields.