Waves

Abstract

In the last chapter we discussed the small-amplitude waves which are predicted by the equations of magnetohydrodynamics. The behaviour found there is, however, applicable only to low frequencies since the waves involve bulk motions of the whole plasma, with electrons and ions moving together and no net charge density appearing. At high frequencies we might expect oscillations in which the electrons oscillate, while the ions, which are much heavier, remain almost stationary. In this case charge separation may occur giving rise to electric fields, a phenomenon which the magnetohydrodynamic approximation, by its very nature, is incapable of describing. In section 4.2 we introduce a description of the plasma in which electrons and ions are treated as two distinct fluids, allowing just this sort of effect to be treated.