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

Abstract

Using the OGO-5 GSFC-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 tail-like configuration. It is inferred that there exists a field-aligned current layer at the polar cap boundary. In the nightside magnetosphere the polar cap boundary is identified as the high-latitude boundary of the plasma sheet. The fieldaligned 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 post-midnight region and away from the earth before midnight. 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 equator side of the polar cap boundary. Currents flow in the opposite directions in the two field-aligned current layers. The current directions in these layers as observed by OGO-5 in the highaltitude 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. The two field-aligned current systems, one at the polar cap boundary and the other on the low-latitude side of the auroral belt, are coupled through the Pedersen current in the ionosphere. The coupling is weak during magnetically quiet periods and becomes strong at disturbed times.Even during intense disturbances the basic pattern of the field variations in the critical regions of the auroral belt and the polar cap boundary is the same as at quiet times. Intensifications of the field-aligned currents and the appearances of multiple pairs of field-aligned current layers characterize the disturbed conditions of these regions.

Electric and magnetic fields in the ionosphere and magnetosphere.

“…These results are summarized in a study by SUGIURA (1976a). SUGIURA (1975) andYA5UHARA et al (1975) both utilized polar orbiter magnetometer data to show that there was an excess of current into the ionosphere in the pre-noon quadrant and an excess of current out of the ionosphere in the dusk sector. Since that time there have been well documented definitive studies carried out by PoTEMRA (1976), andPOTEMRA (1976a, b) using magnetometer data from the Triad satellite which have been very useful in defining the patterns of fieldaligned current flow in more detail.…”

Net field-aligned currents observed by Triad

“…current layer presented in Figure 3 above is the same as that in the current layer ?ctected by Ogo 5 in the high altitude magnetosphere; see Figure 8 in Sugiura (1975) for a schematic current flow pattern. Thus we now believe that the current layer, observed by Triad at 800 km altitude is the same current layer as that detected by Ogo 5• As was discussed by Sugiura l (1975) the presence of this field-aligned current layer is more or less a permanent feature of the magnetosphere.…”

“…Based on the Ogo 5 magnetometer results Sugiura (1975) has shown that the transition from the dipolar magnetospheric field to a more taillike field configuration often takes place rather suddenly and that this transition is accomplished by the existence of a field-aligned current layer at the polar cap boundary. He further showed that the directions -11 -of the field-aligned currents, i.e, toward the ionosphere on the morning side and away from it on the afternoon aide, match those of the fieldaligned currents observed at the higi-latitude boundary of the plasma sheet in the tail.…”

“…We speculate that the Sqp current is mainly a Hall current driven by the field-aligned current system presented in this paper. If this is so, the Sq p is driven by the current generator in the tail that drives the field-aligned current (Sugiura, 1975). …”

Seasonal and interplanetary magnetic field dependence of the field-aligned currents for both Northern and Southern Hemispheres

“…Average spatial distributions of the transverse magnetic perturbations associated with FACs were presented by Armstrong and Zmuda (1975) and Zmuda and Armstrong (1974a,b) who used vector magnetic field data from the TRIAD spacecraft and by Sugiura (1975) using OGO 5 magnetometer data. The first spatial distribution of FACs was obtained from TRIAD magnetometer data and used an average over 439 orbits (Iijima and Potemra, 1976a,b).…”