Three spacecraft observations of solar wind discontinuities

Observations of solar wind magnetic field discontinuities using 3 spacecraft allow their orientations to be estimated. During 5 days when Geotail, Wind and IMP 8 were between 6 x 10(4) and 4 x 10(5) km apart, 35 events identified using the Tsurutani-Smith method were detected in all 3 magnetic field data sets. Normals estimated from interspacecraft timings showed that very few were unambiguous rotational discontinuities, with 77% likely to be tangential, with < 20% of the magnetic field at the discontinuity threading the normal plane. However, previous single spacecraft studies using minimum variance suggest that most discontinuities are rotational. Minimum variance analysis resulted in many normal estimates lying far from the timing-derived normals. While some of this discrepancy is likely to be due to random errors in minimum variance vectors, there appears to be a class of events with small field magnitude changes where the minimum variance directions and discontinuity normals are approximately perpendicular, probably due to surface waves on the discontinuities.