Three spacecraft observations of solar wind discontinuities

Abstract

Abstract.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 104 and 4 x 105 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.

Interchange instability of a curved current layerconvecting in the magnetosheath from the bow shock towards themagnetopause

“…This is consistent with observations of Horbury et al (2001). The reversal length scale is dependent on the position of the convecting current sheet with respect to the magnetopause.…”

“…Tangential discontinuities carried by the solar wind are typical structures with such interplanetary magnetic field (IMF) variations. Horbury et al (2001) analyzed solar wind discontinuities using three spacecraft observations, and they identified about 14% of the observations as tangential discontinuities which are characterized by large variations of the total magnetic field with respect to its maximum value. The time scale of a tangential discontinuity estimated from the example of Horbury et al (2001) is less than 1 min.…”

“…Horbury et al (2001) analyzed solar wind discontinuities using three spacecraft observations, and they identified about 14% of the observations as tangential discontinuities which are characterized by large variations of the total magnetic field with respect to its maximum value. The time scale of a tangential discontinuity estimated from the example of Horbury et al (2001) is less than 1 min. It is rather difficult to determine the discontinuity scale precisely because of fluctuations of the magnetic field and plasma parameters on either side of a discontinuity.…”

What is the best method to calculate the solar wind propagation delay?

“…More sophisticated methods require in-situ measurements and then trying to estimate the normal from multi spacecraft timing or gradient methods (e.g. Horbury et al, 2001b;Knetter et al, 2004), or use the local field or plasma measurements from a single spacecraft (see e.g. overviews in Sonnerup et al, 2006;Volwerk, 2006).…”

Discontinuities and Alfvénic fluctuations in the solar wind

“…This was the situation until the advent of multi-spacecraft observations. After a study with three widely spaced spacecraft (Horbury et al, 2001) had already thrown doubt on the B n results from MVA, it was the four-point measurements by Cluster that changed the story dramatically. In a paper on 129 DDs observed by Cluster in 2001, Knetter et al (2004) demonstrated that none of the DDs met the requirement for B n /B > 0.2 when the normal directions were "triangulated" from the crossing times recorded by the four spacecraft, which were separated by the…”