Abstract. The Fast Auroral Snapshot (FAST) satellite has made observations in the Auroral Kilometric Radiation (AKR) source region with unprecedented frequency and time resolution. We confirm the AKR source is in a density depleted cavity and present examples in which cold electrons appeared to have been nearly evacuated (ttho t > ticold ). Electron distributions were depleted at lowenergies and up-going ion beams were always present. Source region amplitudes were far greater than previously reported, reaching 2x10 '4 (V/m)2/Hz (300 mV/m) in short bursts with bandwidths generally < 1 kHz. Intense emissions were often at the edge of the density cavity. Emissions were near or below the cold plasma electron cyclotron frequency in the source region, and were almost entirely electromagnetic. The IEI/IBI ratio was constant as a function of frequency and rarely displayed any features that would identify a cold plasma cutoff or resonance.

We have studied in detail multi‐spacecraft observations of the exterior cusp on 04 February 2001, during a steady northward Interplanetary Magnetic Field (IMF) interval. At a radial distance of 11 Re, Cluster encountered a well‐bounded region where the magnetic field exhibited very low diamagnetic values and the ions displayed high levels of isotropisation. We refer to this region as the Stagnant Exterior Cusp (SEC). Its equatorward edge is magnetopause like, whereas on the poleward side of the SEC, high‐speed plasma jets were observed consistent with a reconnection site poleward of the cusp. The SEC/magnetosheath boundary is characterized by abrupt changes in the magnetic field and plasma parameters that satisfy the Walén test, and by an S‐shaped magnetic hodogram. The latter may suggest the presence of an intermediate/slow transition.

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Abstract.The results of a statistical study of oxygen ion outflow using Cluster data obtained at high altitude above the polar cap is reported. Moment data for both hydrogen ions (H + ) and oxygen ions (O + ) from 3 years (2001)(2002)(2003) of spring orbits (January to May) have been used. The altitudes covered were mainly in the range 5-12 R E geocentric distance. It was found that O + is significantly transversely energized at high altitudes, indicated both by high perpendicular temperatures for low magnetic field values as well as by a tendency towards higher perpendicular than parallel temperature distributions for the highest observed temperatures. The O + parallel bulk velocity increases with altitude in particular for the lowest observed altitude intervals. O + parallel bulk velocities in excess of 60 km s −1 were found mainly at higher altitudes corresponding to magnetic field strengths of less than 100 nT. For the highest observed parallel bulk velocities of O + the thermal velocity exceeds the bulk velocity, indicating that the beam-like character of the distribution is lost. The parallel bulk velocity of the H + and O + was found to typically be close to the same throughout the observation interval when the H + bulk velocity was calculated for all pitch-angles. When the H + bulk velocity was calculated for upward moving particles only the H + parallel bulk velocity was typically higher than that of O + . The parallel bulk velocity is close to the same for a wide range of Correspondence to: H. Nilsson (hans.nilsson@irf.se) relative abundance of the two ion species, including when the O + ions dominates. The thermal velocity of O + was always well below that of H + . Thus perpendicular energization that is more effective for O + takes place, but this is not enough to explain the close to similar parallel velocities. Further parallel acceleration must occur. The results presented constrain the models of perpendicular heating and parallel acceleration. In particular centrifugal acceleration of the outflowing ions, which may provide the same parallel velocity increase to the two ion species and a two-stream interaction are discussed in the context of the measurements.

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