Electric dipole radiation at vlf in a uniform warm magneto-plasma

Abstract

Reçu le 20 septembre 1971) Résumé. 2014 Cet article traite des caractéristiques de rayonnement à très basse fréquence d'une antenne dipolaire mince dans un magnétoplasma tiède. Les solutions formelles sont obtenues pour la partie de la résistance de rayonnement due aux modes thermiques (Rt) et au mode sifflement (Rw). Dans ce calcul, les ions sont supposés au repos, et les effets de température non nulle sont introduits au moyen d'un terme de pression scalaire. Deux orientations particulières du dipôle sont prises en considération (orientations parallèle et perpendiculaire à la ligne de force du champ magnétique statique). Une analyse détaillée est effectuée pour les fréquences comprises entre la fréquence gyromagnétique électronique (fHe) et la fréquence de résonance hybride inférieure (fLHR). Plusieurs graphes numériques de Rt~, et Rw~, sont présentés pour des valeurs typiques des paramètres du plasma magnétosphérique aux très basses fréquences. Pour des fréquences (f) éloignées de fLHR, Rt~, est généralement inférieure à 2 x 10-1 Rw~, tandis que pour f ~ fLHR, Rt~, ~ 10-1 R et Rt~ ~ Rw~. Utilisant ces valeurs numériques de Rt~, et Rw~, le rendement du rayonnement dipolairẽ w pour le mode sifflement a été évalué pour un certain nombre de cas particuliers. Dans la plupart des cas, ~w ~ 70 %, mais décroît à ~ 38 % pour f ~ fLHR.

Terminal Impedance and Antenna Current Distribution of a VLF Electric Dipole in the Inner Magnetosphere

“…Thus, increasing the cell resolution beyond 30 cells does not adversely affect the impedance values since the waves dominating the energy flow are well resolved. This observation is supported by [12] and [13] where it was found that dipole antennas operating in a magnetized plasma environment preferentially radiate waves whose wavelength is on the order of the antenna length.…”

“…FDFD simulations are carried out for dipole antennas oriented perpendicular with respect to the ambient static magnetic field. This orientation is chosen since the antenna pattern and power delivery are optimal for launching waves parallel to the static magnetic field [12], [13]. Antennas considered for the purpose of our application are on the order of 100 m in length and up to 20 cm in diameter.…”

“…The authors of [12] conclude in this work that the refractive index surface that governs wavelength and propagation direction dominates the focusing of the radiation which changes from the resonance cone direction (propagation direction in a cold magnetized plasma for which the refractive index approaches infinity), for frequencies that are a factor of 0.75 the electron gyrofrequency to a pencil beam pattern focused along the static magnetic field for lower frequencies that lie within the whistler-mode regime. In [13], Wang and Bell examine the radiation characteristics of an electric dipole at VLF frequencies in a warm magnetoplasma by adding a finite electron temperature effect incorporated through the addition of a scalar pressure term in the cold plasma equations, a commonly used practice at this time [8]. They assert that for frequencies above , propagation characteristics may be significantly altered since the thermally modified whistler mode can propagate at angles beyond the resonance cone, however, for frequencies below , the refractive index surface is basically unmodified while noting radiation efficiencies for the perpendicular antenna greater than at least 10% over the parallel antenna for most cases.…”

“…They assert that for frequencies above , propagation characteristics may be significantly altered since the thermally modified whistler mode can propagate at angles beyond the resonance cone, however, for frequencies below , the refractive index surface is basically unmodified while noting radiation efficiencies for the perpendicular antenna greater than at least 10% over the parallel antenna for most cases. In addition, the authors of [13] omit the nonlinear sheath (region of nonneutrality that is quasielectrostatic in nature surrounding a conductor immersed in a plasma [15]) problem assuming low-voltage antenna operation and use a Fourier decomposition of the wave and plasma equations of motion to solve the system of equations. During the same period, studies of whistler-mode radiation patterns of electric dipole antennas in a laboratory setting were performed [16] providing some reassurance to the findings in [12].…”

“…Their model assumes an incompressible Maxwellian fluid for the electrons using the first two moments of the Boltzmann equation for electrons only while the ions and neutrals remain stationary. As in [13], a scalar pressure is assumed for the electrons. Current distributions and impedance values are determined for a 1-m linear antenna with results compared once again to Balmain's electrostatic model [6] with good agreement.…”