Enhanced current collection by a positively charged spacecraft

The collection of electrons by a conducting spherical body at a high voltag e Phi(0) from a magnetized plasma is studied by means of a fully three-dime nsional particle-in-cell code. A relative motion between the plasma and the body is included to simulate the orbital motion of a spacecraft. The curre nt-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 I,, [Parker and Murphy, 1967]. In agreement with measurements in the tether ed satellite experiments TSS-I and TSS-1(R), the simulations give currents I(Phi(0)), which are higher than the corresponding currents I(Phi(0)). For sufficiently large values of Phi(0), I(pm)proportional to Phi(o)(0.5), whil e from the simulations we find that I(sim)proportional to Phi(o)(0.62) givi ng I-sim/I(sim)proportional to Phi(0)(delta) with delta similar to 0.12 Thu s our simulations predict a voltage-dependent enhancement of the current al though the dependence is weak. Despite this enhancement simulations show th at the dependence of I-sim on the body radius r(s) is the same as that give n by I-pm(r(s)) In order to better understand these behaviors of the curren t collection, the distributions of potential and current in the plasma arou nd the body are examined. When e Phi(0) > W-0, the ram ion energy, the refl ection of ram ions forms a virtual anode in front of the body in the ram re gion. The potential structure extends from the virtual anode along the magn etic field Bo and not along the magnetic shadow of the body. The current pa ttern in the plasma contributing to the collection of electrons reveal that when e Phi(0) > W-0, the current flow is approximately magnetic-field alig ned at parallel distances \z\ > r(s), in a cylindrical volume aligned with the magnetic field lines passing through the virtual anode. When \z\ < r(s) , the parallel currents give way to azimuthal and radial currents in the ra m volume in front of the body. The latter currents predominately contribute to the electron collection, which maximizes near the equatorial (z similar to 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 ExB drifts of the electrons in the self-cons istently determined potential distribution do indeed enable the collection of the electrons near the equatorial ram surface.