Two-dimensional magnetopause structures reconstructed from Cluster data

Hiroshi Hasegawa
Thayer School of Engineering, Dartmouth College

The Grad-Shafranov reconstruction technique, a data analysis method for 
recovering magnetic field/plasma structures in space, is applied to 
magnetopause traversals and flux transfer events (FTEs) recorded by the 
Cluster spacecraft.  Under the assumptions that the structures are 
two-dimensional and time-independent, the method produces a field map in 
the plane perpendicular to the axis of invariance, using combined 
magnetic field and plasma data from all four spacecraft.  The resulting 
maps show that, even on a scale of a few thousand km, the magnetopause 
surface is not planar, but has significant curvature, often with 
intriguing meso-scale structures embedded in the current layer.  In one 
of the FTEs encountered near the northern cusp, a flux rope is embedded 
in the magnetopause and is more or less rounded, with diameter of ~1 Re, 
and no standard signatures of active reconnection are found.  The 
orientation of the flux rope axis can be fairly well determined by a 
process of optimizing the map, which verifies the presence of a strong 
core field in the FTE.  These features along with the motion of the FTE 
agree with the view that it was created by component merging at low 
latitudes and had reached an approximate equilibrium by the time when 
Cluster encountered it.  The amount of the magnetic flux in the flux 
rope can be calculated from the map and, using the time separation 
between FTEs, can be used to estimate a lower limit of the reconnection 
electric field during the creation of the FTE.