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This paper presents a study of nearly field-aligned outflowing ion beamsobserved on the Cluster satellites over the polar cap. Data are taken atgeocentric radial distances of the order of 5–9 RE. The distinction ismade between ion beams originating from the polar cusp/cleft and beamsaccelerated almost along the magnetic field line passing by the spacecraft.Polar cusp beams are characterized by nearly field-aligned proton and oxygenions with an energy ratio EO+ / EH+, of the order of 3 to 4, due tothe ion energy repartition inside the source and to the latitudinalextension of the source. Rapid variations in the outflowing ion energy arelinked with pulses/modifications of the convection electric field. Clusterdata allow one to show that these perturbations of the convection velocity andthe associated ion structures propagate at the convection velocity.In contrast, polar cap local ion beams are characterized by field-alignedproton and oxygen ions with similar energies. These beams show the typicalinverted V structures usually observed in the auroral zone and areassociated with a quasi-static converging electric field indicative of afield-aligned electric field. The field-aligned potential drop fits well theion energy profile. The simultaneous observation of precipitating electronsand upflowing ions of similar energies at the Cluster orbit indicates thatthe spacecraft are crossing the mid-altitude part of the accelerationregion. In the polar cap, the parallel electric field can thus extend toaltitudes higher than 5 Earth radii. A detailed analysis of the distributionfunctions shows that the ions are heated during their parallel accelerationand that energy is exchanged between H+ and O+. Furthermore,intense electrostatic waves are observed simultaneously. These observationscould be due to an ion-ion two-stream instability

A multi-satellite study of accelerated ionospheric ion beams above the polar cap