Instability problem of the electric field antennas on the Polar spacecraft

This paper is an application of the surface charge distribution method to t he modeling of the electric antennas installed on board the Polar spacecraf t in order to identify the plasma conditions leading to the instability of the sphere-preamplifier-stub-guard system. First we present an analytic app roach, which allows us to understand the physical mechanism and to define t he conditions of the instability. We then show the results of the numerical modeling for the more common types of instabilities observed in flight. Th e latter, which we call type-1 oscillations, are observed solely in the low -L plasmasphere region. The modeling predicts that the oscillations can occ ur in a weakly magnetized Maxwellian plasma in the upper hybrid range (f(p) < f < f(t)) when the Debye length lies between well-defined limits. The fr equency modulation of the oscillations in this range versus the spin angle of the antennas with respect to the Earth's magnetic field is well explaine d by the model. The type-2 oscillations are observed occasionally and occur always in the exterior cusp, at large L values. They are most likely to be associated with high-density clouds of solar wind streaming plasma enterin g into that region. Our analytic modeling indeed predicts that the instabil ity conditions can be satisfied when the antennas are crossing a 10 eV elec tron flow, with a density of similar to 100 cm(-3) and a bulk velocity of t he order of 200 km s(-1).