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Directional discontinuities of the interplanetary magnetic field are ubiquitous features in the solar wind. A key issue has been the identification of the nature of these discontinuities: Are they tangential discontinuities, i.e., surfaces separating plasmas of different densities and temperatures (and, possibly, composition), or are they rotational discontinuities, where the plasmas on the two sides are magnetically connected, and densities and temperatures are, to first order, the same. The answer has important implications for the topology and origin of the interplanetary magnetic field, but is also of practical significance, because it could affect the propagation delay between the observation at an upstream monitor and the arrival at the Earth's magnetopause.
The primary quantity used to distinguish rotational from tangential discontinuities is the magnitude of the normal magnetic field components. Based on discontinuity orientations inferred from minimum-variance analysis of single-spacecraft magnetometer data, past studies had concluded that a large fraction of the directional discontinuities had normal components much in excess of zero, and thus should be identified as rotational discontinuities. With the advent of ESA's Cluster mission, it has been possible to determine the discontinuity orientation, from the crossing times recorded by the four spacecraft. In a seminal paper, Knetter et al. (2004) demonstrated that, based on the timing normals, the normal components were zero within error estimates for all discontinuities they investigated. Their results were thus consistent with an interpretation that the solar wind is dominated by tangential discontinuities, in striking contradiction to the earlier studies. Using the large set of directional discontinuities identified by Thorsten Knetter in the Cluster data, we propose to focus our investigation on four interrelated topics that are of prime importance for the understanding of the nature of solar wind directional discontinuities, but require the advanced analysis techniques members of the team have developed.
The first goal is to put Knetter's result concerning the absence of rotational discontinuities on a firmer footing, by establishing tighter limits on the normal components, using a new method to quantify the four-point timing uncertainties, as described in ISSI SR-008. We will thus be able to confirm or refute the basic conclusion about the nature of the solar wind directional discontinuities.
The second goal concerns the occurrence of Alfvenic fluctuations. In a preliminary analysis we have established that a large fraction of the directional discontinuities are embedded in a sea of magnetic field and plasma flow fluctuations that closely match the requirements for Alfven waves, as evidenced by successful tests of the so-called Walen relation. We will look in detail at the propagation direction of these Alfvenic disturbances relative to the ambient magnetic field, using the wave surveyor technique developed by a member of the team.
The third question addresses the Alfvenic nature of the directional discontinuities themselves. We have already identified a subset of directional discontinuities that are sufficiently resolved in the plasma measurements so that the Walen test can be applied to the samples within the directional discontinuities themselves. The results support the conclusion that they were RDs. Related to this issue is the suggestion that the well-defined Alfvenic fluctuations surrounding the directional discontinuities could have played a role in the formation of the directional discontinuities via nonlinear wave steepening. This process appears to lead to structures in which a very gradual field rotation either precedes or follows a directional discontinuity, which rapidly rotates the field in the opposite sense back to its original direction. We have already identified a number of such cases, and intend to establish their properties by use of the array of analysis tools at our disposal.
The striking failure of the minimum variance analysis technique to infer the normal directions of the directional discontinuities and the differences in the profiles often recorded by the four spacecraft, suggest an internal structure and/or temporal evolution of the directional discontinuities. As our fourth goal, we will therefore search for such local structures, employing two novel techniques. One is the magneto-hydrodynamic (MHD) reconstruction technique, developed by members of the team, that allows reconstruction of two-dimensional coherent MHD structures from time-series of magnetic field and plasma data. A second approach will be based on the lag-covariance matrix of the multivariate spacecraft measurements, which allows determination of the percentage of the magnetic field profiles that cannot be explained by the hypothesis of a one-dimensional time-stationary structure convecting past the four spacecraft.
To facilitate the work at the team meetings, we have started to compile, in the format of a large spreadsheet, all the results that we have obtained for all the directional discontinuities. This will provide a convenient means to filter the data for certain combinations of properties that we want to focus on.