Impedance of a Spacecraft-Borne Antenna in the Magnetospheric Plasma and Quasi-Equilibrium Noise EMF in the Lower-Hybrid Frequency Band

We present analytical and numerical estimations of the value and frequency dependence of the impedance and noise electromotive force (EMF) in the context of the conditions which correspond to the trajectories and parameters of the antennas borne by geophysical monitoring satellites. The estimations were obtained for two circular orbits at altitudes of 600 and 1200 km over the Earth’s surface in the frequency range from 20 to 120 kHz, which corresponded to the area of the lower-hybrid resonance, where a higher level of noise emissions is observed at the altitudes under consideration. It is shown that near the lower-hybrid resonance frequency, the real part of the antenna impedance is determined by the resonant “monopole” loss by radiation of quasipotential waves. In the nonresonant frequency band (at the frequencies below the frequency of the lowerhybrid resonance), the antenna reactance is determined by the transit loss, which is, however, low as compared with the resonant loss. When the noise was calculated, the medium was assumed to be a two-temperature plasma. The spectral density of the power of the noise EMF lies in the range $V_{\omega}^2\approx \left( {2-4} \right)\cdot {10^{-12 }}-{10^{-13 }}\;{V^2}\cdot H{z^{-1 }}$ and is determined mainly by suprathermal electrons. In the nonresonant frequency band, the efficient temperature of noise radiation is equal to the temperature of the “cold” plasma component, and the antenna reactance is determined by the transit loss, i.e., the level of the noise EMF is low as compared with the EMF in the resonant frequency band.