The coupling of tail fast flows to ionospheric flow signatures and their relationship to substorm onset

Earthward convection of the tail plasma sheet is often organized in bursts of fast ion flows restricted in azimuthally narrow channels. It has been shown that Auroral Poleward Boundary Intensifications (PBIs) are often the ionospheric signature of such fast flow channels in the midtail. While PBIs can occur for all IMF orientations and solar wind conditions, they have a clear preference for southward IMF and their occurrence peaks within 1 hour after a substorm onset, with a secondary occurrence peak at 3 hrs after onset. Equatorward flow bursts have been observed in the ionosphere, that are presumably the ionospheric mapping of the tail fast flow channels. We focus on identifying such ionospheric signatures and understanding the physics of this magnetosphere-ionosphere interaction via conjunctions of the THEMIS probes with the Sondrestrom radar. From a number of such conjunctions we find that the onset of a substorm that is soon followed by a PBI has a very distinct signature in the radar data. At onset and expansion the ionospheric flow turns strictly westward. During the PBI, tail fast flows originate in the mid-tail and ionospheric flows turn equatorward. The generality and physical implications of this pattern are explored.