Abstract. Hyperboloid is a multi-directional mass spectrometer measuring ion distribution functions in the auroral and polar magnetosphere of the Earth in the thermal and suprathermal energy range. The instrument encompasses two analyzers containing a total of 26 entrance windows, and viewing in two almost mutually perpendicular half-planes. The nominal angular resolution is de®ned by the ®eld of view of individual windows %13°´12.5°. Energy analysis is performed using spherical electrostatic analyzers providing dierential measurements between 1 and 80 eV. An ion beam emitter (RON experiment) and/or a potential bias applied to Hyperboloid entrance surface are used to counteract adverse eects of spacecraft potential and thus enable ion measurements down to very low energies. A magnetic analyzer focuses ions on one of four micro-channel plate (MCP) detectors, depending on their mass/charge ratio. Normal modes of operation enable to measure H + , He + , O ++ , and O + simultaneously. An automatic MCP gain control software is used to adapt the instrument to the great¯ux dynamics encountered between spacecraft perigee (700 km) and apogee (20 000 km). Distribution functions in the main analyzer half-plane are obtained after a complete scan of windows and energies with temporal resolution between one and a few seconds. Three-dimensional (3D) distributions are measured in one spacecraft spin period (120 s). The secondary analyzer has a much smaller geometrical factor, but oers partial access to the 3D dependence of the distributions with a few seconds temporal resolution. Preliminary results are presented. Simultaneous, local heating of both H + and O + ions resulting in conical distributions below 80 eV is observed up to 3 Earth's radii altitudes. The thermal ion signatures associated with large-scale nightside magnetospheric boundaries are investigated and a new ion out¯ow feature is identi®ed associated to the polar edge of the auroral oval. Detailed distribution functions of injected magnetosheath ions and ou¯owing cleft fountain ions are measured down to a few eVs in the dayside.
In the subauroral ionosphere, there are often observed narrow streams of fast subauroral ion drift to the west near the projection of the plasmopause at the height of the F-layer of the ionosphere. They are most noticeable during storms/substorms on the background of the large-scale plasma convection (Anderson et al., 1991;. For the first time, such streams were recorded with the Soviet satellite Kosmos-184, and they were called "polarization jet" (PJ; Gal'perin et al., 1973Gal'perin et al., , 1974. In the scientific literature, this phenomenon was also called "SubAuroral Ion Drifts" (SAIDs) after the paper of Spiro et al. (1979) where narrow streams of ion drifts were studied using data from the American satellite Atmosphere Explorer C (AE-C). The bandwidth of PJ/SAID at ionospheric heights is 1°-2° in latitude, and the drift velocity is ∼1 km/s and more in the westerly direction and is recorded in the evening and night MLT sectors. At the same time, broad flows are encountered in the evening sector (<20 MLT;Figueiredo et al., 2004;Karlsson et al., 1998;Yeh et al., 1991). Foster and Burke (2002) combined these two types of observations of subauroral electric fields: narrow jets of ion drift (PJ) and wide regions of ionospheric convection to the west with high velocities, called SubAuroral Polarization Stream (SAPS). These phenomena (SAPS and PJ) were studied with the satellite data, measurements by a chain of ground-based ionospheric stations, SuperDARN HF radars, and with the coherent scattering of radio signals (e.g.,
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