The Farley-Buneman instability occurs when electrons and ions differ in velocity by more than the ion-acoustic speed defined by
where and are the ion and electron temperatures and is the ion mass. For this reason, the instability is also often called two-stream instability. The waves produced by this instability propagate nearly perpendicular to the magnetic field within a cone of angle given by
is the relative drift speed between electrons and ions, is the angle between the wave and the magnetic field and , and , are the collision and gyro-frequencies for electrons and ions respectively. This instability grows more rapidly at shorter wavelengths. The importance of this instability is greatest in the E-region at low and high latitudes [Har92, pp. 41-41,]. In the E-region, the electrons are ``magnetized'' ( ) above and exhibit Hall drift perpendicular to , while ions are ``unmagnetized'' ( ) and exhibit Pedersen drift parallel to . Consequently, the electrons ``stream'' through the ions, exciting the instability. In the F-region, the ions and electrons both drift with the Hall velocity, so their relative drift speed is essentially zero, and the instability is inoperative.