Periodic longitudinal structure of field-aligned currents in the dawn sector: Large-scale meandering of an auroral electrojet

The azimuthal structure of field‐aligned current (FAC) systems is one of the target subjects of the Freja satellite's magnetometer observation. In the event of May 28, 1993, the satellite observed large‐amplitude fluctuations of the latitudinal, as well as the longitudinal, magnetic field components along a postmidnight‐to‐morning orbit. By combining radar and ground magnetometer data, we inferred that the associated FAC system has a periodic structure consisting of multiple FAC sheets inclined significantly from the azimuthal (east‐west) direction and moving westward and possibly poleward. Such a structure may be interpreted in terms of the meandering of an auroral electrojet, and perhaps as the ionospheric projection of boundary waves.