Geomagnetic fluctuations due to impulse sources with applications to high-altitude nuclear bursts

Abstract

The response of the ionosphere-exosphere system to a bomb-like hydromagnetic source and the resultant time-dependent ground-level magnetic fluctuations are examined theoretically in this paper, with emphasis placed upon gaining an understanding of the time behavior of the ground-level signal. The methods of calculation are also applicable to certain magnetic fluctuations of natural origin. An analytic treatment is used, whereby the problem is reduced essentially to finding the poles and residues of previously derived hydromagnetic transmission coefficients. Conditions corresponding to high magnetic latitudes are used, but some conclusions corresponding to more general conditions are drawn. Bomb-induced ground-level magnetic fluctuations are interpreted in terms of a broadband excitation of ionospheric and exospheric resonant oscillations. Even for broadband sources, which excite all of the many resonances, the signal rapidly becomes dominated by the lowest characteristic frequencies. Thus, for a wide range of source functions, the frequency content of the transmitted signal would be determined largely by the resonant properties of the propagation medium. Two examples of this are the magnetic fluctuations measured after the Argus and Starfish high-altitude nuclear tests. For hypothetical high-latitude bursts detonated in or above the lower ionosphere, the ground-level signal is predicted to be nearly left-hand circularly polarized in the daytime and to exhibit nearly the polarization of the source at night. For many situations of practical interest, the enhancement of the lower ionospheric ionization density by bomb X rays is a very important consideration.