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
where
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.