Nyquist diagrams for low-frequency oscillations in a non-uniform magnetoplasma

Abstract

A modification of the Penrose criterion is applied to the local dispersion relation for low-frequency, electrostatic instabilities in a low-density, slightly inhomogeneous magnetoplasma. The Nyquist diagrams obtained can be classified according to their behaviour near the origin; this is sufficient in some cases to conclude that the equilibrium is unstable. With this method, in the limit of negligible ion Larmor radius, we find that the Langmuir waves propagating in a direction orthogonal to the external magnetic field are driven unstable by a temperature gradient.