Oscillations of the equatorward boundary of the ion auroral oval – radar observations

Three SuperDARN radars in the afternoon-midnight sector of the auroral oval detected a boundary oscillation, originating near ∼1800 MLT sector. Analysis of the phase of the oscillations measured in three meridians indicates that the disturbance has a longitudinally (azimuthally) isolated source and away from which it propagates. The eastward and westward phase speeds are 2.6 and 3.6 km/s respectively and the period is roughly 28 minutes. An examination of the geo-synchronous magnetic field inclination also revealed oscillations similar to the oscillations of the boundary. Solar wind and IMF conditions were steady during the period except for variations of the IMF By component. The IMF By component showed variations similar to the oscillations in the boundary and the geo-synchronous magnetic field inclination. During reduced and negative IMF By, the boundary was moving equatorward, while during increased or positive IMF By it was moving poleward. The variations in the magnetic field inclination measured at geosynchronous orbit by the GOES satellites were consistent with these boundary motions: decreases (more stretched) and increases (more dipolar) in the inclination corresponded to equatorward and poleward moving boundaries, respectively. Polar cap convection also showed changes in the direction of the convection in response to the change in the IMF By component. Observed oscillation of the boundary can be explained by stretching of the tail field lines due to asymmetric merging associated with changes in the By component of the interplanetary magnetic field.