... 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 . ... The instrument is basically a multi-directional ion mass spectrometer measuring ion distribution functions over an energy range from ~0.1 to 80 eV. ... The ion angular and energy distribution are obtained by means of an electrostatic optics defining 2 planes with several entrance windows. ...