Abstract
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 defined by the field of view of individual windows ≈13° × 12.5°. Energy analysis is performed using spherical electrostatic analyzers providing differential 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 effects 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 flux 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 offers 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 outflow feature is identified associated to the polar edge of the auroral oval. Detailed distribution functions of injected magnetosheath ions and ouflowing cleft fountain ions are measured down to a few eVs in the dayside.Key words. Ionosphere (auroral ionosphere; particle acceleration; ionosphere-magnetosphere interactions)
Highlights
Dubouloz et al.: Thermal ion measurements on board Interball Auroral Probe by the Hyperboloid experiment characteristics of these outows were obtained during the Dynamics Explorer (DE) and Akebono missions
Classical polar wind theory is not able to explain other ion outows characterized by temperatures much larger than typical ionospheric temperatures, or by the presence of heavy ions escaping along the ®eld lines (Gurgiolo and Burch, 1982; Green and Waite, 1985)
Beside technical modes intended to check the health of the electronics and dump the memory, and diagnostic modes used to monitor any evolution in the physical response of the instrument optics and detectors, scienti®c modes consist in repetitive measurements where both energy and view direction are swept in sequence during several spacecraft spin periods
Summary
In order to ful®ll the assigned scienti®c objectives, the Hyperboloid ion mass spectrometer was designed to measure the full 3D distribution functions of low-energy ions. In the high-altitude portion of Interball AP orbits, H+ is usually the major ion, total ion densities ranging between about a few 10±2 and a few tens of cm±3. The investigation of such dilute plasmas requires a very high sensitivity in order to detect dierentialuxes down to about a few 103 (cm s sr eV)±1. The conditions encountered at lower altitudes close to perigee dier drastically, with O+ being the major ion and densities reaching several thousands of cm±3 This imposed a further constraint of large dynamical range in the conception of the detectors in order to avoid saturation or even destructive eects The various modes of operation reect the dierent compromises which have been chosen with respect to the scienti®c objectives (cf. Sect. 2. 6.)
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