HYPERBOLOID is a plasma
spectrometer aimed at the investigation of the low energy ions
dynamics in the polar ionosphere and magnetosphere on the INTERBALL
Auroral Probe. Scientific objectives of this experiment include
studies of:
- thermal plasma transport by magnetospheric
convection and escape along high latitude magnetic field lines,
- ion heating and acceleration by electrostatic
and electromagnetic turbulence,
- generation and propagation of magnetic field
aligned small-scale structures in the auroral plasma.
The instrument is basically a multi-directional ion
mass spectrometer measuring ion distribution functions over an
energy range from ~0.1 to 80 eV. In normal modes of operation,
the four major species H+, He+, O+, O++ can be measured simultaneously;
special modes may be used to detect minor species as N+ or the
molecular ions.
The ion angular and energy distribution are obtained
by means of an electrostatic optics defining 2 planes with several
entrance windows. In the main plane, there are 16 windows at 10
intervals with individual FOV ~ 12 and effective entrance area
~ 0.8 cm2. This plane cotains the spacecraft spin axis so that
the instrument is able to sample practically the full space in
a complete spin period (120 s). Depending on the number of windows
simultaneously opened, the angular resolution can be varied from
10 to 40, thus providing an adjustable trade off between angular
and time resolution. The second plane of measurement is almost
orthogonal to the first one with 10 windows at 15 intervals, FOV
~ 5 and effective entrance area ~ 5 to 8 times less than in the
main plane. Again, from 1 to 4 windows can be activated simultaneously.
This second plane of measurement is meant at obtaining a better
time resolution (~ 1 s) for a full distribution function when
the instrument is facing the rammed plasma, thus enabling to look
at fast plasma variations.
Each window is followed by a spherical electrostatic
energy analyzer. In normal modes of operation the energy resolution
is ~ +/- 10% for the main 16 windows and +/- 5% for the 10 secondary
ones. By pre-accelerating or decelerating the ions at the entrance
of the analyzer, one can vary this energy resolution by a factor
between ~ 0.5 and 2.
An arrangement of 2 toroidal concentrators is used
to focus ions at the entrance of a magnet (which provides the mass
separation), irrespective of their direction of arrival. There
are 4 identical detectors in the focal plane, one for each of
the major ions, which depending on the ion flux can be operated
either in digital (pulse counting) or in analogical (charge integration)
mode, thus providing a total dynamic range of 1e11.
The temporal resolution of the instrument for a complete
angular and energy scan depends on the telemetry rate and varies
from ~ 1 to 16 s in most modes of operation.
To date, the instrument is working nominally.
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