AN EXPERIMENT TO STUDY AND CONTROL THE LANGMUIR SHEATH AROUND INTERBALL-2

The satellite INTERBALL-2 has an orbit with high inclination (62.8 deg rees), covering the altitude range between a few hundred and about 200 00 km. The ambient plasma conditions along this orbit are highly varia ble, and the interactions of this plasma with the spacecraft body as w ell as the photo-electron sheath around it are considered to be intere sting topics for detailed studies. The electric potential of the space craft with respect to the ambient plasma that develops as a result of the current equilibrium reacts sensitively to variations of the bounda ry conditions. The measurement and eventual control of this potential is a prerequisite for accurate measurements of the thermal plasma. We describe the purpose and technical implementation of an ion emitter in strument on-board INTERBALL-2 utilising ion beams at energies of sever al thousand electron volts in order to reduce and stabilise the positi ve spacecraft potential. First results of the active ion beam experime nts, and other measures taken on INTERBALL-2 to reduce charging are pr esented. Furthermore, the approach and initial steps of modelling effo rts of the sheath in the vicinity of the INTERBALL-2 spacecraft are de scribed together with some estimates on the resulting spacecraft poten tial, and effects on thermal ion measurements. It is concluded that ev en moderate spacecraft potentials as are commonly observed on-board IN TERBALL-2 can significantly distort the measurements of ion distributi on functions, especially in the presence of strongly anisotropic distr ibutions.