Enhanced current collection by a positively charged spacecraft

Abstract

The collection of electrons by a conducting spherical body at a high voltage Φo from a magnetized plasma is studied by means of a fully three‐dimensional particle‐in‐cell code. A relative motion between the plasma and the body is included to simulate the orbital motion of a spacecraft. The current‐voltage (I‐V) characteristic of the body as seen from the simulations is compared with that predicted by the theory giving the upper‐bound current Ipm [Parker and Murphy, 1967]. In agreement with measurements in the tethered satellite experiments TSS‐1 and TSS‐1(R), the simulations give currents I(Φo), which are higher than the corresponding currents Ipm(Φo). For sufficiently large values of Φo, Ipm∝Φo0.5, while from the simulations we find that Ipm∝Φo0.62, giving Isim/Ipm∝Φoδ with δ∼0.12. Thus our simulations predict a voltage‐dependent enhancement of the current although the dependence is weak. Despite this enhancement simulations show that the dependence of Isim on the body radius rs is the same as that given by Ipm(rs). In order to better understand these behaviors of the current collection, the distributions of potential and current in the plasma around the body are examined. When eΦo > Wo, the ram ion energy, the reflection of ram ions forms a virtual anode in front of the body in the ram region. The potential structure extends from the virtual anode along the magnetic field B0 and not along the magnetic shadow of the body. The current pattern in the plasma contributing to the collection of electrons reveal that when eΦo > Wo, the current flow is approximately magnetic‐field aligned at parallel distances |z|>rs in a cylindrical volume aligned with the magnetic field lines passing through the virtual anode. When |z|<rs, the parallel currents give way to azimuthal and radial currents in the ram volume in front of the body. The latter currents predominately contribute to the electron collection, which maximizes near the equatorial (z∼0) ram surface of the body. The current patterns are further studied by tracing electron trajectories in the potential distributions in the sheath of the body, showing that the E×B drifts of the electrons in the self‐consistently determined poteial distribution do indeed enable the collection of the electrons near the equatorial ram surface.