Origin of Low-Frequency Oscillations in the Ionosphere

Abstract

The threshold current for the ion-acoustic branch is shown to be significantly lower than the ion-cyclotron branch and insensitive to the ion/electron temperature ratio if there is a transverse gradient in the relative magnetic field aligned drift, ${V}_{d}$, and $|({k}_{y}{/k}_{z})(1/{\ensuremath{\Omega}}_{i})({\mathrm{dV}}_{d}/dx)|$ is sufficiently large. The effect persists even when $|{\mathrm{dV}}_{d}/dx|\ensuremath{\rightarrow}0$ provided $({k}_{z}{/k}_{y})\ensuremath{\rightarrow}0$, where ${k}_{z}$ and ${k}_{y}$ are wave vectors along and across the magnetic field and ${\ensuremath{\Omega}}_{i}$ is the ion gyrofrequency. Therefore, the ion-acoustic branch is more central to the plasma processes in the ionosphere than is currently believed.