Observations of two complete substorm cycles during the Cassini Earth swing‐by: Cassini magnetometer data in a global context

Abstract

During the Earth swing‐by of the Cassini spacecraft, a worldwide program of data‐gathering was undertaken to define the prevailing interplanetary and geophysical conditions. This included observations of the interplanetary medium, outer magnetosphere, geostationary orbit, UV aurora, geomagnetic disturbance, and ionospheric flow. These data show that during the Cassini outbound passage through the geomagnetic tail the magnetosphere underwent two complete “classic” substorm cycles. The global data are used to set the Cassini data into a context which allows a much fuller interpretation. The pass took place through the dawn tail, where previous Geotail observations indicate that the plasma sheet usually remains ‘stationary’ at expansion phase onset. Reconnection and plasmoid formation are typically dusk and midnight phenomena. The Cassini observations nevertheless show a marked response of the plasma sheet both to the growth phase (thinning), and expansion phase onset (expansion without heating). Subsequent impulsive expansion of the substorm into the Cassini sector, however, resulted in the prompt appearance of reconnection‐related phenomena (earthward flowing plasma and strongly disturbed fields), though the spacecraft footprint remained poleward of the intense UV auroras. Due to continuing strong southward‐directed IMF during the expansion phases, a quasi‐equilibrium appears to have formed between dayside and near‐Earth nightside reconnection for ∼30 min after onset. Auroral zone recovery began about ∼10 min after a northward turn of the interplanetary magnetic field (IMF) to nearer‐zero values in each case. A net closure of open flux then ensued, leading to deflation of the tail lobe field, ‘dipolarization’ of the near‐Earth tail plasma sheet field, simultaneous reduction in the earthward plasma sheet flow and the flow in the nightside ionosphere, and displacement of the ground‐based disturbance to high latitudes.