Conjugate ionospheric equivalent currents during bursty bulk flows

Ionospheric equivalent currents related to bursty bulk flows (BBF) during Cluster and the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network conjunctions between 2001 and 2006 are studied. A geomagnetically southeast northwest aligned, relatively narrow channel of northwestward equivalent current density with downward field-aligned current at its northeastward flank and upward field-aligned current at its southwestward flank is confirmed as the ionospheric signature of BBFs in the majority of cases, i.e., whenever an ionospheric signature is present. Unlike in previous event studies, all conjugate BBFs between 2001 and 2006 are analyzed and during 19 out of 22 BBFs, the channel is observed in the ionosphere. The mean duration of the BBFs observed when the footprint of Cluster is located close to the poleward boundary of the auroral oval is clearly longer (17 min) than the that of the BBFs observed close to the equatorward boundary of the oval (3 min). A superposed epoch analysis of the local IMAGE AE (IE) index reveals a stronger activity level preceding a substorm-related than a nonsubstorm BBF. After the BBF observation, however, the activity in both cases is on the same level as that preceding the substorm BBF observation. The observation of the substorm BBF coincides with a substorm-like disturbance of the IE, while the observation of the nonsubstorm BBF coincides with a step-like increase of the IE.