The electrical stability of the electric field antennas in the plasmasphere
Résumé
The long electric dipole antennas mounted on the POLAR satellite are unstable in the plamasphere, at L-shell values of about 2–3, where the Debye length is shorter than 30 cm; the frequency of the instability lies in the vicinity of the ambient electron plasma frequency. Each antenna consists of two spherical sensors, 8 cm in diameter, which are installed at the ends of wire-booms deployed in the spin plane and separated by distances longer than 100 m. Each sphere is electrically decoupled from the wire-boom by a double-stub, a symmetrical arrangement of two cables of 3 m each, whose ac potential is controlled by the output of a preamplifier with unit gain. A passive antenna is stable under all circumstances and the oscillation is necessarily induced by the active elements of the system. The instability is triggered when the Debye length is significantly shorter than the stubs; this phenomenon is due to a capacitive coupling between the sensor and the stubs, when the self-impedance of the latter electrode is inductive. This problem is analysed using a numerical approach based on the surface charge distribution technique developed for an isotropic thermal plasma in the quasi-static approximation.