Local structure of directional discontinuities in the solar wind

Abstract

We examine local structures of three directional discontinuities (DDs) observed by Cluster in the solar wind, using reconstruction based on the ideal 2-D MHD equations in a steady state. In this novel application of the technique, our goals are the following: (1) to explain why the minimum variance analysis of the magnetic field (MVAB) often fails to meaningfully predict the vector normal to a DD and (2) to use the reconstructed field maps as an aid in interpreting the differences in the magnetic field profiles recorded by the four Cluster spacecraft. From the maps, we learn that the failure of MVAB as a predictor of the normal direction is due to internal structure such as magnetic islands (flux ropes) within the DDs and also that we can partly understand the differences in the fields observed by the four spacecraft. We find fairly good agreement between the normal directions determined from the four-point timing approach and from MVAB, provided the constraint $\langle$B_n$\rangle$ = 0 is imposed on MVAB. Because of the island structures, the DDs cannot be readily identified as either tangential or rotational discontinuities, although the approximately Alfvénic flows on both sides favor the latter interpretation.