A Discussion on the early days of ionospheric research and the theory of electric and magnetic waves in the ionosphere and magnetosphere - Experiments with plasma waves

Abstract

Plasma waves play an important role in planetary plasma envelopes and in astrophysical plasmas. They occur naturally, for example, in the Earth’s ionosphere and magnetosphere where they have been detected by rockets and satellites. Historically, the kinds of modes predicted increased both in number and in diversity as cold plasma theory was extended to include the effects of external magnetic fields and of finite plasma temperatures. Attention is mainly concentrated in the paper on waves propagating in a narrow band of frequencies near the plasma frequency,,^, and on those often referred to as the electron Bernstein modes. The latter propagate with wavevectors in a direction closely perpendicular to an applied magnetic field. Their existence is not predicted by cold plasma theory. They are, however, easy to excite in relatively simple laboratory experiments and provide an instructive laboratory demonstration of effects depending on thermal motions in a plasma. An interference experiment designed to detect and to make measurements on such waves is described. Dispersion properties are in good agreement with the predictions of the kinetic theory of hot magnetoplasmas. Three dimensional ray tracing in hot magnetoplasmas in a narrow band of frequencies near the plasma frequency, including Doppler shifts, has confirmed the feasibility of a plasma wave radar which can provide a good technique for determining electron temperatures and densities and their gradients. Computational and analytical techniques have been proposed to determine these quantities. The paper concludes with a brief assessment of the use of plasma waves in space and laboratory plasma diagnostics.