Open geometry of the magnetotail cross section

We present a picture showing the magnetic topology and geometry of Earth's magnetotail and surrounding magnetosheath. The picture is constructed from a 4-year composite of magnetometer data from IMP 8. It represent a cross-sectional plane around 30 R_e tailward from Earth. It depicts the situation that a quasi-equatorial interplanetary magnetic field produces, which is the common situation. It shows that the separatrix between open and closed magnetic field lines lies in the boundary, as expected. It shows that the tail is open at high latitudes, also as expected. However, it shows further that the tail is open along the equatorial flanks, contrary to a standard assumption. That is, the picture in which permanent, continuous boundary segments run along both flanks separating closed field lines from fully disconnected field lines is inconsistent with the data.