Nilsson, H.1, Waara, M.1 and Rème, H.2
1 Swedish Institute of Space Physics, Kiruna, Sweden
2 CESR, Toulouse, France
We present Cluster observations of oxygen ion outflow at high altitudes above the polar cap. At altitudes of 5 - 12 Earth radii the oxygen ions are transversely heated by waves as well as accelerated in the parallel direction, mainly by the centrifugal acceleration mechanism. Much of the transverse heating is transient and occur in bursts. The number flux of the oxygen ions also shows clear transients. We have developed a method to automatically detect such transients. We will show statistical results on the distribution of such transients in three year of Cluster data. Initial results show that sudden increases of number flux are typically about 5 minutes apart when observed by Cluster at high altitude. Superposed epoch and multi-spacecraft measurements will be used to relate the transients to surrounding conditions, in particular convection changes in space and time. The robustness of the transient detection technique and the effect of using different thresholds in the detection algorithm will be discussed. Finally we will discuss what this tells us about the ionospheric source. Can we tell whether the main number flux modulation occur in the ionosphere (large upflow) or in a second stage in the upper ionosphere / lower magnetosphere where transverse heating or other mechanisms provide enough energy for the ions to overcome gravity, by comparing with statistical EISCAT results on ion upflow?