FIRST APPLICATION OF MULTI-POINT MAGNETIC FIELD ANALYSIS TECHNIQUES: THE CURLOMETER AND THE DISCONTINUITY ANALYSER

摘要

The Cluster mission has collected 3-D information on magnetic field structures at small to medium scales in the near-Earth environment for the first time. The interpretation of this information can be assisted using a variety of multi-spacecraft analysis techniques. We focus here on the Curlometer and Discontinuity Analyser (DA) techniques. The First of these uses the measured spatial gradients of the magnetic field to estimate the electric current density from curl(B). The effect of time dependence as well as non-constant spatial gradients over the spacecraft volume can degrade the quality of the Curlometer estimate. For many magnetic field structures, however, the estimated divergence of the magnetic field can be used to monitor (indirectly) the effect of non-constant gradients. The DA determines the orientation and motion of magnetic discontinuities, using inter-spacecraft timing and variance information. Accurate boundary timing depends on the field variations being stationary in the frame of the boundary. Planar boundaries allow the motion to be computed directly, although boundary curvature can be safely computed only when the boundary motion is known to be constant. The applicability of these techniques is tested for selected events within the data set. The application of the DA for planar boundaries (at the bow shock and magnetopause) has shown that significant acceleration (～10 kms~(-2)) of both shock and magnetopause boundaries is nearly always present on scales of ～600-1000 km. A consequence is that previous estimates of boundary scales, based on constant motion between two spacecraft, can be in error by factors of up to two or three.