A pseudo‐breakup observation: Localized current wedge across the postmidnight auroral oval

Abstract

This study presents a detailed description and analysis of a pseudo‐breakup, which took place in the field of view (FOV) of the Magnetometers–Ionospheric Radars–All‐sky Cameras Large Experiment (MIRACLE) network (IMAGE magnetometers, Scandinavian Twin Auroral Radar Experiment [STARE] radars, and all‐sky cameras) on 3 November 1997 at 2212 UT. The activation lasted ∼10 min, occurred during geomagnetically quiet conditions (Dst ≥ −10 nT), and was followed by a global‐scale substorm about half an hour later (at 2246 UT). Both the pseudo‐breakup and the substorm had onsets at times corresponding to the Wind satellite observations of interplanetary magnetic field (IMF) BZ northward turnings as delayed to the subsolar magnetopause. The auroral display of the pseudo‐breakup was a mesoscale spiral with a counterclockwise winding direction. Simultaneously, STARE recorded clockwise plasma flow in the spiral surroundings. Applying the method of characteristics to the MIRACLE observations revealed that the spiral was associated with a localized current wedge (less than a few hours in MLT and about 4° in latitude), tilted in the northeast‐southwest direction. The maximum upward current density at the western edge of the wedge was about 6 A/km2, which clearly exceeds the threshold of spiral buildup (2.5 A/km2) suggested by earlier theoretical studies. About an hour before the pseudo‐breakup, the energy input from the solar wind to the magnetosphere (quantified by the ϵ parameter) exceeded the substorm threshold and the ionospheric conductivity was high enough to support substorm activity. Two likely reasons for the activation to remain localized were its unfavorable location in the morning side of the Harang discontinuity region and the abrupt decrease of the solar wind energy input at its onset time.