SABRE observations of structured ionospheric flows during substorm expansion phase onset

Abstract. The irregularity velocity patterns observed by the SABRE coherent radar at substorm expansion phase onset, which is identified by magnetometer observations of Pi2 pulsations, are occasionally highly structured. In all the examples of structured velocity patterns examined, the SABRE viewing area is located at longitudes within the inferred substorm current wedge. Three types of structured velocity regime are apparent depending on the level of magnetic activity and the position of the radar viewing area relative to the substorm enhanced currents and the Pi2 pulsation generation region. Firstly, vortex-like velocity patterns are observed and these may be caused by the field-aligned currents associated with the substorm current wedge. Secondly, regions of equatorward velocity are also observed at times of substorm expansion phase onset moving longitudinally across the SABRE viewing area. The longitudinal movement is usually westward although an example of eastward motion has been observed. The phase velocity of these regions of equatorward flow is typically 1-3 km s^-1. The observed equatorward velocities occur at the poleward edge or poleward of the background convection velocities observed by SABRE. These equatorward velocities may be related to the westward travelling surge and to the expansion (eastwards as well as westwards) of the brightening arc region at substorm onset. Thirdly, the flow rotates equatorward within the field of view but does not then appear to move longitudinally. These equatorward velocities may relate to the earthward surge of plasma from the magnetotail at substorm onset.