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Farley-Buneman Instability

The Farley-Buneman instability occurs when electrons and ions differ in velocity by more than the ion-acoustic speed tex2html_wrap_inline4648 defined by


where tex2html_wrap_inline4650 and tex2html_wrap_inline4652 are the ion and electron temperatures and tex2html_wrap_inline4654 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 tex2html_wrap_inline4582 given by




tex2html_wrap_inline4658 is the relative drift speed between electrons and ions, tex2html_wrap_inline4660 is the angle between the wave and the magnetic field and tex2html_wrap_inline4662 , tex2html_wrap_inline4664 and tex2html_wrap_inline4666 , tex2html_wrap_inline4668 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'' ( tex2html_wrap_inline4670 ) above tex2html_wrap_inline4672 and exhibit Hall drift perpendicular to tex2html_wrap_inline4512 , while ions are ``unmagnetized'' ( tex2html_wrap_inline4676 ) and exhibit Pedersen drift parallel to tex2html_wrap_inline4512 . 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.

Andreas Schiffler
Wed Oct 9 10:05:17 CST 1996