Ionospheric signatures of a plasma sheet rebound flow during a substorm onset

Magnetic reconnection in Earth's magnetotail produces fast earthward flows in the plasma sheet. Tailward flows are often observed associated with the earthward flows. Both return flow vortices at the flanks of an earthward flow channel and rebound of the earthward flow from the intense dipolar magnetic field of the inner magnetosphere have been shown to explain tailward flows observed near Earth. We combine plasma sheet measurements from Cluster with conjugate ground‐based magnetic and auroral data to examine the development of earthward and tailward flow signatures during a substorm onset. We show for the first time observations of ionospheric signatures that appear to be associated with rebound flows. Because of the highly dynamic magnetotail configuration, special care is taken with the satellite footprint mapping. The ionospheric footprints produced by the event oriented AM02 model drift equatorward and poleward in response to tail magnetic field stretching and dipolarization, respectively. The footprint motion matches that of the ambient ionospheric structures, and the plasma flow measured by Cluster agrees with that inferred from the conjugate ionospheric observations, confirming the validity of the AM02 mapping. The ionospheric signatures of fast earthward flows during a substorm onset are shown to resemble the known signatures of quiet‐time flows, including equatorward propagating auroral streamers inside a channel of enhanced poleward equivalent current. However, the large‐scale dipolarization results in additional poleward expansion of the signatures, as has been predicted by simulations.