Poleward-moving HF radar flow bursts in the cusp: Transient changes in flow speed or direction?

Poleward-moving line-of-sight velocity "flow bursts" have been observed in the cusp by two southern hemisphere SuperDARN HF radars with overlapping fi elds-of-view. This has allowed the estimation of unambiguous awe-dimensiona l velocity vectors in the vicinity of the "flow bursts". Rather than showin g enhancements in the flow magnitude, the velocity vectors suggest that the line-of-sight velocity enhancements are a result of a change in the direct ion of the flow associated with latitudinal motion of the convection revers al boundary. These observations may have important implications for underst anding the ionospheric footprint of flux transfer events, and also illustra te that caution is needed when interpreting line-of-sight velocity data fro m single radars.