Altitudinal comparison of dayside field‐aligned current signatures by Viking and DMSP‐F7: Intermediate‐scale field‐aligned current systems

Abstract

Dayside large‐scale and intermediate‐scale field‐aligned current (FAC) signatures are examined with multi‐instrument measurements from Viking and DMSP‐F7 at magnetic conjunctions. The present paper reports four such conjunction events, with an emphasis on an event that occurred on October 13, 1986. In these four events both Viking and DMSP‐F7 crossed prenoon FAC systems approximately along meridians. The altitude of DMSP‐F7 was 835 km, whereas that of Viking ranged from 8500 to 12,000 km. The electric to magnetic field ratio measured by Viking indicates that intermediate‐scale FAC systems, as well as large‐scale FAC systems, are often quasi‐stationary. This is also supported by the comparison between the Viking and DMSP‐F7 magnetic measurements. The only obvious exception is the equatorward part of the October 13 event, in which the Viking and DMSP‐F7 measurements are better explained in terms of Alfvén waves. In two other events the Viking signature projected to the DMSP‐F7 altitude is significantly more structured than the DMSP‐F7 signature, although the electric to magnetic field ratio observed by Viking suggests that the associated FACs were quasistationary. This apparent discrepancy is possibly ascribed to the fact that Viking stays longer in FAC systems and therefore has more chance to observe temporal changes in FACs. However, such temporal effects must operate longer than the Alfvén transit time so that FAC systems become quasi‐stationary. Although the generation mechanism(s) of intermediate‐scale FAC systems remains an open question, possibilities include a localized shear of plasma convection and a localized merging between the solar wind and magnetospheric field lines.