Note on waves through gases at pressures small compared with the magnetic pressure, with applications to upper-atmosphere aerodynamics

Abstract

Most treatments of magnetohydrodynamics waves have confined physical interpretation to cases when the Alfvén velocity a₁ is small compared with the sound velocity a₀. Here we consider the ‘low-beta situation’, in which a₁, the only possible waves are longitudinal ones, propagated unidirectionally along lines of magnetic force with velocity a₀. These can be interpreted as sound waves, confined to effectively rigid magnetic tubes of force. Hall-current effects do not alter these conclusions (in contrast to the high-beta situation), and finite conductivity introduces only small dissipation.An application is made to the flow pattern around a body moving through the F₂ layer of the ionosphere, where, although neutral particles have a very large mean free path, charged particles interact electrostatically and, it is argued, may be regarded as forming a continuous fluid whose movement is independent of that of the neutral particles. A body moving at satellite speed or below would then excite the above-mentioned unidirectional sound waves, but no waves at much faster Alfvén velocity. These considerations suggest that its movement would be accompanied by a [xvee ]-shaped pattern of electron density (figure 2), which might be in part responsible for some anomalous radar echoes that have been reported.