FM pulses in stratified isotropic plasma

Abstract

A study of signals excited by FM pulses in an infinite inhomogeneous cold isotropic plasma is presented. The medium is assumed to vary arbitrarily with position, though sufficiently slowly with respect to all wavelengths contained in the source spectrum. For simplicity only one spatial dimension is considered, but the development may be extended to include more space variables. After deducing integral representations that are evaluated asymptotically, the theory is applied to a particular plasma inhomogeneity characterized by an exponential variation of plasma frequency. Space-time rays associated with an impulsive source are shown to justify the use of time-inversion in establishing the required phase of an FM pulse capable of producing an optimally compressed signal at a particular space-time point. Since ordinary asymptotic methods fail in the neighborhood of such a focal point, a theory that in a prior study described signals in the focal region of a homogeneous plasma is applied to signals in a plasma with exponentially varying plasma frequency. Computed results based on this theory display the character of optimally compressed signals in focal regions.