Origin of Low-Frequency Oscillations in the Ionosphere

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, Vd, and |(ky/kz)(1/Ωᵢ)(dVd/dx)| is sufficiently large. The effect persists even when |dVd/dx|→0 provided (kz/ky)→0, where kz and ky are wave vectors along and across the magnetic field and Ωᵢ is the ion gyrofrequency. Therefore, the ion-acoustic branch is more central to the plasma processes in the ionosphere than is currently believed.