CXVIII. The four possible waves in a magneto-ionic medium

Abstract

Summary It is well known that several different types of waves may be transmitted through a magneto-ionic medium ; that is, ionized gas permeated by a steady magnetic field. Foremost is the pair of radio waves whose properties are so well described by the ‘ magneto-ionic theory ’ of Appleton and others. More recently several hydromagnetic (or magneto-hydrodynamic) waves have been studied. A ‘ plasma ’ or electron-pressure wave has received little attention except in the simple case of zero magnetic field. Finally there are sound waves which may propagate in any gas. The relationship between these various waves is disclosed: the magnetoionic medium is quadruply refracting, transmitting four, and only four, different waves. The hydromagnetie ordinary and extraordinary waves are identical with the radio O and E waves, the latter being the simple limiting ease of the former when heavy ion motion is negligible. The other two waves comprise a ‘magnetic sound’ wave (a type of hydromagnetic wave) and a ‘magnetic ...