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We present a statistical analysis of directional discontinuities in the solar wind at 1 astronomical unit. It is the first study of this kind based on simultaneous observations at four nearby spacecraft. We use recent magnetic field and plasma data from the coordinated Cluster spacecraft flying in formation a few hundred to a few thousand kilometres apart. With four measuring platforms the ability to observe is considerably improved. For example, the simultaneous four-point measurements allow for determining the discontinuity normals by analysing the time differences between the occurrence of the discontinuity at each of the four spacecraft. The events used in our analysis are grouped in three sets according to the period of their observation, i.e., January - May of 2001, 2002 and 2003, respectively. The main distinction between these three data sets is the different spacecraft separation, from 100 km in 2002 to 5000 km in 2003, which enables us to investigate interplanetary discontinuities at different scales. Another difference between the three data sets are the different prevailing solar wind conditions. These include both slow solar winds and fast streams that map back to coronal holes on the Sun. Our analysis demonstrates that the Minimum Variance Analysis (MVA), an established and widely used single-spacecraft technique to compute normals of discontinuities, is much less reliable than previously assumed. We determine new criteria to decide whether or not MVA yields reliable normal estimates. An important parameter is the ratio of intermediate to minimum eigenvalue of the magnetic covariant matrix. Instead of using 2 as the lower limit for this ratio for applicability of MVA, as is common practice, we strongly suggest applying MVA only if the ratio is greater than 10. Furthermore, the accuracy of MVA increases with increasing spreading angle, the angle between the magnetic field vectors on the two sides of the discontinuity. We recommend MVA should not be used for angles less than 60 degree. Inaccurate MVA normal estimates have resulted in a dramatic overestimation of the number of rotational discontinuities in earlier studies. Using the relative timing technique, we do not find a single clear rotational discontinuity. A detailed error analysis of the multi-spacecraft tool puts this new result on a firm footing. Analysis of plasma data shows that many discontinuities do not satisfy the polarisation relation, a necessary condition for rotational discontinuities. Our results allow for the interpretation that the solar wind is dominated by tangential rather than rotational discontinuities which is in apparent contradiction to earlier single-spacecraft studies. This finding is particularly true for fast coronal hole streams where earlier observations claim a high abundance of rotational discontinuities. Our new results entail significant implications for the dynamics of the solar wind micro-structure and generation mechanisms for discontinuities. We also discuss possible consequences for the understanding of the source region on the Sun and for the propagation of cosmic rays through the heliosphere. We study further properties of directional discontinuities, such as their thickness, their orientation in space and their large-scale curvature. We conclude that interplanetary discontinuities are 1-D structures superposed by non-isotropic fluctuations

Topics:
ddc:550

Year: 2005

OAI identifier:
oai:USBKOELN.ub.uni-koeln.de:1508

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Kölner UniversitätsPublikationsServer

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