Electrostatic interaction between Interball-2 and the ambient plasma. 1. Determination of the spacecraft potential from current calculations

Abstract. The Interball-2 spacecraft travels at altitudes extending up to 20 000 km, and becomes positively charged due to the low-plasma densities encountered and the photoemission on its sunlit surface. Therefore, a knowledge of the spacecraft potential F_s is required for correcting accurately thermal ion measurements on Interball-2. The determination of F_s  is based on the balance of currents between escaping photoelectrons and incoming plasma electrons. A three-dimensional model of the potential structure surrounding Interball-2, including a realistic geometry and neglecting the space-charge densities, is used to find, through particle simulations, current-voltage relations of impacting plasma electrons I_e (F_s ) and escaping photoelectrons I_ph (F_s ). The inferred relations are compared to analytic relationships in order to quantify the effects of the spacecraft geometry, the ambient magnetic field B₀ and the electron temperature T_e . We found that the complex geometry has a weak effect on the inferred currents, while the presence of B₀ tends to decrease their values. Providing that the photoemission saturation current density J_ph0 is known, a relation between F_s and the plasma density N_e can be derived by using the current balance. Since J_ph0 is critical to this process, simultaneous measurements of N_e from Z-mode observations in the plasmapause, and data on the potential difference F_s  - F_p  between the spacecraft and an electric probe (p) are used in order to reverse the process. A value J_ph0 ~ = 32 µAm^-2 is estimated, close to laboratory tests, but less than typical measurements in space. Using this value, N_e and F_s  can be derived systematically from electric field measurements without any additional calculation. These values are needed for correcting the distributions of low-energy ions measured by the Hyperboloid experiment on Interball-2. The effects of the potential structure on ion trajectories reaching Hyperboloid are discussed quantitatively in a companion paper.

Key words. Space plasma physics (charged particle motion and acceleration; numerical simulation studies; spacecraft sheaths, wakes, charging)