Three spacecraft observations of solar wind discontinuities

Observations of solar wind magnetic field discontinuities using 3 spacecraf t allow their orientations to be estimated. During 5 days when Geotail, Win d and IMP 8 were between 6 x 10(4) and 4 x 10(5) km apart, 35 events identi fied 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 tan gential, with < 20% of the magnetic field at the discontinuity threading th e normal plane. However, previous single spacecraft studies using minimum v ariance suggest that most discontinuities are rotational. Minimum variance analysis resulted in many normal estimates lying far from the timing-derive d normals. While some of this discrepancy is likely to be due to random err ors in minimum variance vectors, there appears to be a class of events with small field magnitude changes where the minimum variance directions and di scontinuity normals are approximately perpendicular, probably due to surfac e waves on the discontinuities.