Impedance characteristics of an active antenna at whistler mode frequencies

Abstract

We use the radio plasma imager (RPI) on the IMAGE satellite to investigate the impedance characteristics of an active electric antenna in space plasma at whistler mode frequencies. A dedicated experiment was carried out on 21-22 September 2005 for two orbits in the plasmasphere. The input impedance characteristics of the dipole antenna submerged in plasma is determined at whistler frequencies. The results are consistent with a physical model in which the antenna is negatively charged to a large voltage and the plasma around each antenna element forms an ion sheath that varies with time in its radius. Within the plasmasphere, these sheaths are a part of the antenna-plasma system and represent a capacitive component of the tuned antenna circuit. It is shown that, inside the plasmasphere, the RPI antenna capacitance varied from 430 to 480 pF. The plasma sheath formed around the antenna in the plasmasphere increases its capacitance by 20%-30% with respect to the near-free space capacitance (364 pF). Comparison of these values to model calculations shows good agreement with a difference smaller than 5%. Measurements of the antenna input resistance showed that, inside the plasmasphere, its value was between 200 and 500 ω, varying considerably with changes in the ambient electron density and cyclotron frequency. The measured antenna input resistance is compared to model calculations. Copyright 2010 by the American Geophysical Union.