Abstract
Abstract. A recently developed technique for reconstructing approximately two-dimensional (∂/∂z≈0), time-stationary magnetic field structures in space is applied to two magnetopause traversals on the dawnside flank by the four Cluster spacecraft, when the spacecraft separation was about 2000km. The method consists of solving the Grad-Shafranov equation for magnetohydrostatic structures, using plasma and magnetic field data measured along a single spacecraft trajectory as spatial initial values. We assess the usefulness of this single-spacecraft-based technique by comparing the magnetic field maps produced from one spacecraft with the field vectors that other spacecraft actually observed. For an optimally selected invariant (z)-axis, the correlation between the field components predicted from the reconstructed map and the corresponding measured components reaches more than 0.97. This result indicates that the reconstruction technique predicts conditions at the other spacecraft locations quite well. The optimal invariant axis is relatively close to the intermediate variance direction, computed from minimum variance analysis of the measured magnetic field, and is generally well determined with respect to rotations about the maximum variance direction but less well with respect to rotations about the minimum variance direction. In one of the events, field maps recovered individually for two of the spacecraft, which crossed the magnetopause with an interval of a few tens of seconds, show substantial differences in configuration. By comparing these field maps, time evolution of the magnetopause structures, such as the formation of magnetic islands, motion of the structures, and thickening of the magnetopause current layer, is discussed. Key words. Magnetospheric physics (Magnetopause, cusp, and boundary layers) – Space plasma physics (Experimental and mathematical techniques, Magnetic reconnection)
Highlights
The magnetopause current layer has long been a focus of investigation, because physical processes operating in this region control energy and mass transfer from the solar wind into the magnetosphere
Intercomparison of the two reconstructed maps (Figs. 3 and 5) demonstrates that the magnetopause encountered in this event was a quasi-static, tangential discontinuity (TD)-type current layer, for which the model assumptions appear to be well justified
Similarities of the orientation of the invariant axis, current sheet thickness, and the overall magnetopause structures among the results from C1 and C3 data indicate that mainly two-dimensional structures were present, with superimposed weak threedimensionality and temporal variations
Summary
The magnetopause current layer has long been a focus of investigation, because physical processes operating in this region control energy and mass transfer from the solar wind into the magnetosphere. Field lines encountered on the magnetospheric and magnetosheath sides of the boundary have pairwise the same A value but usually have different Pt values, indicating that the function Pt (A) has two branches (Hu and Sonnerup, 2003) This kind of behavior makes reliable determination of the invariant (z)-axis difficult: one can use only very few data points within the central current sheet for optimization of the choice of invariant axis and the resulting data fit to the functions Pt (A), p(A), and Bz(A). Because of this difficulty, the intermediate variance direction, computed from minimum variance analysis of the measured magnetic field Our procedure to select an optimal invariant axis is described in Appendix A
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