Department of Geophysics, Faculty of Science, Kyoto University, Kyoto

Using the Dynamics Explorer (DE) 2 magnetic and electric field and plasma data, B_y-controlled convection and field-aligned currents in the midnight sector for northward interplanetary magnetic field (IMF) are examined. The results of an analysis of the electric field data show that when IMF is stable and when its magnitude is large, a coherent B_y-controlled convection exists near the midnight auroral oval in the ionosphere having adequate conductivities. When B_y is negative, the convection consists of a westward (eastward) plasma flow at the lower latitudes and an eastward (westward) plasma flow at the higher latitudes in the midnight sector in the northern (southern) ionosphere. When B_y is positive, the flow directions are reversed. The distribution of the field-aligned currents associated with the B_y-controlled convection, in most cases, shows a three-sheet structure. In accordance with the convection the directions of the three sheets are dependent on the sign of B_y. The location of disappearance of the precipitating intense electrons having energies of a few keV is close to the convection reversal surface. However, the more detailed relationship between the electron precipitation boundary and the convection reversal surface depends on the case. In some cases the precipitating electrons extend beyond the convection reversal surface, and in others the poleward boundary terminates at a latitude lower than the reversal surface. Previous studies suggest that the poleward boundary of the electrons having energies of a few keV is not necessarily coincident with an open/closed boundary. Thus the open/closed boundary may be at a latitude higher than the poleward boundary of the electron precipitation, or it may be at a latitude lower than the poleward boundary of the electron precipitation. We discuss relationships between the open/closed boundary and the convection reversal surface. When as a possible choice we adopt a view that the open/closed boundary agrees with the convection reversal surface, we can explain qualitatively the configuration of the B_y-controlled convection on the open and closed field line regions by proposing a mapping modified in accordance with IMF B_y.

J. Geophys. Res., 99, No. A4, 6027-6044, Apr. 1994