Identifications of the polar cap boundary and the auroral belt in the high‐altitude magnetosphere: A model for field‐aligned currents

By means of the Ogo 5 Goddard Space Flight Center fluxgate magnetometer data the polar cap boundary is identified in the high‐altitude magnetosphere by a sudden transition from a dipolar field to a more taillike configuration. It is inferred that there exists a field‐aligned‐current layer at the polar cap boundary. In the night side magnetosphere the polar cap boundary is identified as the high‐latitude boundary of the plasma sheet. The field‐aligned current flows downward to the ionosphere on the morning side of the magnetosphere and upward from the ionosphere on the afternoon side. Thus in the high‐latitude boundary layer of the plasma sheet in the tail the current is toward the earth in the postmidnight region and away from the earth in the premidnight region. The basic pattern of the magnetic field variations observed during the satellite's traversal of the auroral belt is presented. This pattern shows the existence of a field‐aligned‐current layer on the low‐latitude part of the auroral belt. Currents flow in opposite directions in the two field‐aligned‐current layers. The current directions in these layers as observed by Ogo 5 in the high‐altitude magnetosphere are the same as those observed at low altitudes by the polar‐orbiting Triad satellite (Armstrong and Zmuda, 1973). The magnetic field in the region where the lower‐latitude field‐aligned‐current layer is situated is essentially meridional. Thus the equatorial current closure of this current system must be via the equatorial current sheet. A model is presented in which two field‐aligned‐current systems, one at the polar cap boundary and the other on the low‐latitude part of the auroral belt, are mainly connected by ionospheric currents flowing across the auroral belt. The existence of field‐aligned currents deduced from the Ogo 5 observations is a permanent feature of the magnetosphere. Even under disturbed conditions the basic pattern of the field variations in the critical regions of the auroral belt and the polar cap boundary is the same as it is at quiet times. Intensifications of the field‐aligned currents and occurrences of multiple pairs of field‐aligned‐current layers characterize the disturbed conditions of these regions.