Modulational instability of ion acoustic waves in a magnetised plasma

A theoretical study of the modulational instability of ion acoustic waves in the presence of a dc magnetic field is presented. It is shown that the instability sets in for wave numbers greater than 1.47 k sub D in the absence of the magnetic field, but that the switching-on of the magnetic field can produce instability earlier. There are two regions where the waves can become modulationally unstable; the relative sizes of these regions change as the magnetic field and the propagation angle are varied.