Reconfiguration of polar-cap plasma in the magnetic midnight sector

Radio tomography and the EISCAT and SuperDARN radars have been used to identify long-lived, high-altitude, cold plasma in the antisunward convective flow across the polar cap. The projection of the feature to later times suggests that it was reconfigured in the Harang discontinuity to form an enhancement that was elongated in longitude in the sunward return flow of the high-latitude convection pattern. Comparison with a tomographic image at a later time supports the interpretation of a polar patch being reconfigured into a boundary blob. There is also evidence for a second plasma enhancement equatorward of the reconfigured blob, likely to have been produced by in situ precipitation. The observations indicate that the two mechanisms proposed in the literature for the production of boundary blobs are operating simultaneously to form two distinct density features separated slightly in latitude.