Motion of auroral ion outflow structures observed with CLUSTER and IMAGE FUV

During February 2001 the CLUSTER satellites recorded a number of perigee passes through the midnight auroral zone. We concentrate on one pass, on 23 February 2001, when structured outflow was observed. Simultaneous observations of the aurora were available from the FUV instrument on IMAGE. The features in the ion outflow observed by the Cluster Ion Spectrometry (CIS) experiment are compared with the auroral activity. Observations from the multiple CLUSTER spacecraft are used to determine the velocity of the outflow structures. We find a good correspondence between the observed ion outflow and the auroral arcs, with the highest energy outflow corresponding to the brightest arcs. The features at the equatorward edge, which are trapped precipitating ions, are stationary. In addition, the increased velocity structure at the poleward edge is also stationary. However, the bulk of the ion outflow structures, which are observed between these boundaries, are moving equatorward with a velocity of roughly 7 km/s, which corresponds to a velocity of 0.7 km/s at 100 km. One feature is observed moving poleward, at the same time that the auroral arc is expanding poleward. Comparisons with the motion of the auroral arcs and with the convection velocity measured by the EDI instrument on CLUSTER show that the motion of the structures in general agrees with the convective motion ofthe field lines.