CURRENf COU.ECTION AND CURRENT CLOSURE IN THE TETHERED SATELLITE SYSTEM

In this paper we address two aspects of the Tethered Satellite System: i) the current collection in the vicinity of the tethered satellite and il) the appropriate physics for modeling the closure path of the tether current in the ionosphere. Our approach to the issue of current collection has been based on numerical simulations performed with one and two dimensional codes. We have found that during transient current collection a positively charged body will attract an electron current which is greater than the steady state value by a factor > 40. The mechanism responsible for this is the influence of plasma ions while they are being evacuated from the sheath region. The duration of the enhancement for ionospheric conditions at 300km altitudes is on the order of a millisecond. During this transient phase some of the electrons in the sheath are accelerated to energies of twice the applied potential. The structure of the evolved sheaths in the simulations indicates that electron collection is predominantly field aligned. The extent of the sheath transverse to the magnetic field is an electron Larmor radius, and is based on the attribution of the full potential drop to the cross field electron velocity. In the magnetic field aligned direction the sheath adjusts itself in extent to absorb the thermal ionospheric electron flux. We find that even in "steady state" there is a conSiderable fluctuation level in the 1