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The results of a statistical study of oxygen ionoutflow using Cluster data obtained at high altitudeabove the polar cap is reported. Moment data for bothhydrogen ions (H+) and oxygen ions (O+)from3 years (2001-2003) of spring orbits (January to May)have been used. The altitudes covered were mainlyin the range 5–12 RE geocentric distance.It was found that O+ issignificantly transversely energized at highaltitudes, indicated both by high perpendiculartemperatures for low magnetic field values as well asby a tendency towards higher perpendicular thanparallel temperaturedistributions for the highest observed temperatures.The O+ parallel bulk velocity increases withaltitude in particular for the lowest observedaltitude intervals.O+ parallel bulk velocities in excess of 60 km s-1were found mainly at higher altitudescorresponding to magnetic field strengths of lessthan 100 nT. Forthe highest observed parallel bulk velocitiesof O+ the thermal velocity exceeds the bulkvelocity, indicating that the beam-like character ofthe distribution is lost.The parallel bulk velocity of the H+ and O+was found to typically be close to the samethroughout the observation interval when the H+bulk velocity was calculated for all pitch-angles. Whenthe H+ bulk velocity was calculated for upwardmoving particles only the H+ parallel bulkvelocity wastypically higher than that of O+. The parallel bulk velocity is closeto the same for a wide range of relative abundance ofthe two ion species, including when the O+ ionsdominates.The thermal velocity of O+ was always well belowthat of H+.Thus perpendicularenergization that is moreeffective for O+ takes place, but this is notenough to explain the close to similar parallelvelocities. Further parallel acceleration mustoccur. The results presented constrain the modelsof perpendicular heating and parallel acceleration.In particular centrifugal acceleration of the outflowingions, which may provide the same parallel velocityincrease to the two ion species and atwo-stream interaction are discussed in the contextof the measurements

Characteristics of high altitude oxygen ion energization and outflow as observed by Cluster: a statistical study