On the enhancement of the whistler mode instability in the magnetosphere by cold plasma injection

Abstract

The modifications in the electron cyclotron electromagnetic (whistler) instability produced by the addition of cold plasma to an infinite uniform anisotropic plasma with parameters similar to those observed in the equatorial magnetosphere at 6.6 RE are discussed. The discussion is restricted to waves propagating parallel to the static magnetic field. We show that cold plasma greatly increases the growth rate if P ≡ β · A(1 + A)² ≪ 1, where β = 4πnκT∥/B² and A = (T⊥/T∥) - 1. For small values of P the enhancement of the maximum rate of growth is proportional to α−1/2 · exp (½ αP), where α = 1 + (nc/nw) and nc and nw are the particle densities of the cold and warm electron populations, respectively. There is an optimum value of nc/nw for maximum growth rate, i.e., ≃ 1/P. The addition of cold plasma also increases the instability range in k space by a factor approximately proportional to (nc/nw)1/2. The recent confirmation of the analytical results above by numerical solution of the dispersion equation as well as by computer simulation experiments is indicated.