Flow Vortex‐Associated Downward Field‐Aligned Current Retreating in the Near‐Earth Plasma Sheet

The formation of field‐aligned currents (FACs) of a substorm current wedge is a key element of substorms. Conjunctive observations of a downward FAC during a substorm on 20 December 2007 were made in space by THEMIS satellites and on the ground by geomagnetic field and aurora stations, which provides a unique opportunity to investigate the generation of downward FAC. On the ground, AL index decreased significantly at 04:17:00 UT, denoting the onset of the substorm. The geomagnetic perturbations observed at the postmidnight station KUUJ show that the FAC of the substorm current wedge flowed into the ionosphere. Meanwhile, in space, two THEMIS satellites, THC and THB, the magnetic footprints of which were close to the postmidnight station KUUJ, were located in the near‐Earth magnetotail and recorded the outside‐in signatures of the plasma sheet thinning first and then the large‐scale inside‐out propagating plasma flow vortex inferred from the rotation of the flow vector. The magnitude of the FAC was estimated by using the bipolar BY perturbations associated with the flow vortex. It is found that this FAC was so strong that it cannot be produced by the time derivative of the vorticity and that the pressure gradient may act in this event. The upward flowing low‐energy electrons are the possible carriers for this FAC. Key Points Conjunctive observations of the downward FAC of a SCW in space and on the ground are reported The observing satellites recorded the outside‐in and the inside‐out signatures sequentially during the substorm in the near‐Earth tail The downward FAC is observed to be associated with a flow vortex inferred from the rotation of the flow vector