Electrodynamic tethers are wires deployed across the earth's geomagnetic field through which a current is flowing. The radiation impedance of a tether with end connectors carrying an AC current is computed from classical antenna theory. This simulates the use of a tether on a space structure. It is shown that the current flow pattern at the tether connector is critical to determining the overall radiation impedance. If the tether makes direct electrical contact with the ionosphere then radiation impedances of the order of several thousand Ohms can be expected. If the only electrical contact is through the end connectors then the impedance is only a few Ohms for a DC current rising to several tens of Ohms for an AC current with frequencies in the whistler range.
Current collecting systems moving in the ionosphere will induce electromagnetic wave radiation. The commonly used static analysis is incapable of studying the situation when such systems undergo transient processes. A dynamic analysis has been developed, and the radiation excitation processes are studied. This dynamic analysis is applied to study the temporal wave radiation from the activation of current collecting systems in space. The global scale electrodynamic interactions between a space‐station‐like structure and the ionospheric plasma are studied. The temporal evolution and spatial propagation of the electric wave field after the activation are described. The wave excitations by tethered systems are also studied. The dependencies of the temporal Alfvén wave and lower hybrid wave radiation on the activation time and the space system structure are discussed. It is shown that the characteristics of wave radiation are determined by the matching of two sets of characteristic frequencies, and a rapid change in the current collection can give rise to substantial transient radiation interference. The limitations of the static and linear analysis are examined, and the condition under which the static assumption is valid is obtained.
Large conducting structures in the ionosphere may have currents flowing through them which close in the ionospheric plasma. These currents can arise either from current leakage from an onboard power distribution system or by being induced by the motional electric field. Associated with these currents will be broadband electromagnetic radiation in the Alfven and lower hybrid bands. The radiation impedance of this electromagnetic radiation is explored for a structure of space-station-like dimensions as a function of the geometry of the structure and the composition of the ionic environment. It is shown that modification of the collecting area of the structure and environment can be used to minimize the radiation impedance. For a space station, the radiated power will at most be of the order of watts, which does not represent a significant power loss. However, the radiation field will give rise to a substantial pollution of the electromagnetic spectrum in the vicinity of the space station. Design choices to minimize this interference are suggested.
A general analysis of the electrodynamic interactions between a space station with two exposed charged platforms and the ionospheric plasma is presented. We show that this problem can be separated into a far-field problem, concerned with the electromagnetic interference surrounding the entire space station, and a near-field problem, concentrated on the interactions in the vicinity of the biased platforms. Computer particle simulations as well as approximate analysis were carried out in the near field of the charged platform. Results of the plasma flowfield, the presheath and sheath structure, and current collection characteristics are obtained. The near-field solution is used to construct the perturbation current source in the far-field problem, which is solved by application of plasma fluid theory. It is found that the space station will generate a radiation field composed of the Alfven waves forming a "wing" structure. Based on our analysis, a global description of the space station's electrodynamic environment is obtained. Nomenclaturespeed of light, Alfven speed, and ion sound speed d sh = sheath thickness E = electric field E A = Alfven wave electric field e , = electron charge / = current /, j = current density K = Boltzmann constant k -wave vector M• -ion flow Mach number m e , m t = electron and ion mass «o»«/, n e -ambient plasma, ion and electron density «o» n f = plasma and ion density at the sheath boundary P sh ,P p resh = sheath and presheath power near ,^rad = near-field and far-field power R ce , RCJ = electron and ion Lamor radius /? near , j? far = near-field zone and far-field zone dimension T e -electron temperature u = velocity Vo>Vti>v te -orbital, ion thermal and electron thermal velocity Xi,x 2 ,X3 = coordinates in the rest frame of the plasma x 1 9 xi 9 x£ = moving coordinates with origin at the center box of station x'y' = moving coordinates with origin at the center of the station projected on the plane 5,4 x, z = moving coordinates for a (x{ 9 x^) plane cutting through the platform Z/,Z /7 = radiation impedance in band I and band II Z rad = total radiation impedance Presented as Paper 91-0114 at a. = angle of attack F = ion flux \ d = Debye length Xmfp = mean free patĥ wave = wavelength X X ,X|| = wavelength component in x and z direction $ = electric potential $ w >$sh -potential at the plate surface and at the sheath boundary 0 0 > OA -Mach angle and Alfen angle Upe>Upi -electron and ion plasma frequency -lower and upper hybrid frequency = electron and ion gyro frequency
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