Problems of cold plasma measurements on the Regatta-A spacecraft

General physical processes and models which govern the thermal plasma number density distribution, field aligned flows and perpendicular drifts in the magnetosphere are outlined. The decisive role of the spacecraft electric potential and the Debye length for the thermal ion measurements onboard are considered. An approach to evaluate experimentally the necessary corrections to the three dimensional ion measurements is described. Cold electron distributions in the auroral environment are considered and their relation to the spacecraft potential in rarified plasmas is discussed. Advantages of the proposed Regatta A instrumentation are presented.