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, V-d, and \(k(y)/k(z))(1/Omega(i))(dV(d)/dx)\ is sufficiently large. The effect persists even when \dV(d)/dx\ --> 0 provided (k(z)/k(y)) --> 0, where k(z) and k(y) are wave vectors along and across the magnetic field and 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. [S0031-9007(97)05043-6].