A new approach to the problem of electromagnetic wave propagation through a vacuum-plasma interface is formulated and used to solve two simple antenna problems. First, the impedance of a spherical body covered by a vacuum sheath and immersed in a plasma is calculated. The result is found to coincide with the empirical formula commonly used so far. Second, a cylindrical antenna also isolated from the plasma by a sheath is studied. The antenna can support guided modes whose dispersion relation is derived. An experiment has been performed in order to verify this dispersion relation. Experimental results are found to be in favor of the present approach.
The impedance of a short electric dipole antenna has been measured from rockets in the ionosphere. The EIDI I experiment was launched in conditions where the plasma was expected to be nearly isotropic (operating frequencies 4.48 and 6.09 MHz, f0F2 ⋍ 9 MHz, fH ⋍ 1 MHz). It was designed to detect and measure or to eliminate most of the unwanted effects which complicated the data reduction of previous experiments: outgassing, telemetry radiation, etc. The number of parameters measured simultaneously on board (impedances with variable bias and RF excitation level, resonance frequencies, Te, dc current drawn by the antennas, vehicle potential) and from the ground or an overhead satellite was large enough to provide numerous cross checks and to determine the sheath dimensions. The impedance is very weakly spin‐modulated over most of the flight; the collision frequency is very low (v/ω ⋍ 10−5) unlike some of the previous experiments where collision losses were dominant. The measured values of the impedance are given as a function of X=f2p/f2 where the other parameters are known.
The impedance of a short dipole antenna has been computed using the quasistatic approximation, a triangular current distribution, and the full adiabatic hydrodynamic description of the plasma. Some typical results are presented which show that for parameters typical of the ionosphere the temperature affects mainly the radiation resistance. 1.for the impedance in the case of an antenna in vacuum or in a cold magnetoplasma [Balrnain, 1964].The validity of these hypotheses is by no means a simple matter; if we accept one, then it is a little easier to discuss the other. For example, we may Copyright (•) 1974 by the American Geophysical Union. DESCRIPTION OF THE WARM MAGNETOPLASMAA warm magnetoplasma is a medium whose response to a plane wave is described by a dielectric tensor •(o,, K) depending on the parameters .o, and K of the plane wave [Allis et al., 1963]. The expression for •(o,, K) can be obtained in the following ways. (a) A hydrodynamic description of the plasma using a scalar pressure [Allis et al., 1963]. This is the simplest approach and has been widely used [Deschamps and Kesler, 1967; Chen, 1969; Wang and Bell, 1972]. (b) A more exact formulation uses a tensor pressure [Quemada, 1961] which can be ob-409
The interpretation of the experiment EIDI I, described in a companion paper, is undertaken. A relatively complete calculation of the impedance is done including the effect of the temperature of the plasma by using the Vlasov equation and hydrodynamic equations, the structure of the sheath using a simple model of it, the collision frequency computed for an equilibrium plasma, and an estimation of various other small effects (wake, ionospheric gradient, motion of ions, …). The result so obtained is in very good agreement with the experiment for the most part but a strong discrepancy appears in the resistance when X > 1. This indicates that the main features of the theory are correct except that a loss mechanism is missing; some hypothetical ones are discussed.
The density of ions in the wake of a cylinder moving in an arbitrary direction is calculated: the simple expression obtained generalizes the perpendicular velocity case and seems more satisfying than the one derived by a previous author. In spite of the crudeness of the model our simple formula can be useful in giving an estimate of the wake in rocket experiments when the velocity is not perpendicular to the axis.
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