The electrical stability of the electric field antennas in the plasmasphere

The long electric dipole antennas mounted on the POLAR satellite are unstab le in the plamasphere, at L-shell values of about 2-3, where the Debye leng th is shorter than 30 cm; the frequency of the instability lies in the vici nity of the ambient electron plasma frequency. Each antenna consists of two spherical sensors, 8 cm in diameter, which are installed at the ends of wi re-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 potent ial is controlled by the output of a preamplifier with unit gain. A passive antenna is stable under all circumstances and the oscillation is necessari ly induced by the active elements of the system. The instability is trigger ed when the Debye length is significantly shorter than the stubs; this phen omenon is due to a capacitive coupling between the sensor and the stubs, wh en the self-impedance of the latter electrode is inductive. This problem is analysed using a numerical approach based on the surface charge distributi on technique developed for an isotropic thermal plasma in the quasi-static approximation. (C) 2001 Elsevier Science Ltd. All rights reserved.