A dayside ionospheric absorption perturbation in response to a large deformation of the magnetopause

A large deformation of the dayside magnetopause was observed by Interball‐1 from 1132 to 1139 UT on July 24, 1996. The spacecraft, near 0800 LT at the time, transited the magnetosheath twice within 7 minutes. In response to these boundary perturbations, the antarctic stations South Pole and AGO‐P3, located at about the same local time as Interball‐1, observed impulsive magnetic variations. These variations were similar to previously reported magnetic variations observed in the northern hemisphere conjugate region during this event and may be related to discrete field‐aligned currents linking the ionosphere to the perturbed outer magnetosphere. Westward propagation of the magnetic signatures in both ionospheric regions is consistent with a travelling convection vortex event; however, the speed is significantly higher in the north (∼10 km/s) than in the south (∼3 km/s). A localized intensification of energetic electron precipitation (427.8 nm auroral emission and riometer absorption) was observed at South Pole station, but not at AGO‐P3 or at the nominally conjugate locations in Greenland and Canada. The complex, Z‐component variation of the magnetic pulse accompanying the particle precipitation at South Pole may be evidence of a localized ionospheric conductivity enhancement, as proposed in a recent model study of travelling convection vortices.