OPEN GEOMETRY OF THE MAGNETOTAIL CROSS-SECTION

We present a picture showing the magnetic topology and geometry of Ear th's magnetotail and surrounding magnetosheath. The picture is constru cted from a 4-year composite of magnetometer data from IMP 8. It repre sent a cross-sectional plane around 30 R-e, tailward from Earth. It de picts the situation that a quasi-equatorial interplanetary magnetic fi eld produces, which is the common situation. It shows that the separat rix between open and closed magnetic field lines lies in the boundary, as expected. It shows that the tail is open tit 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 pi cture in which permanent, continuous boundary segments run along both flanks separating closed field lines from fully disconnected field lin es is inconsistent with the data.