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Farley–Buneman instability
The Farley–Buneman instability, or FB instability, is a microscopic plasma instability named after Donald T. Farley〔Farley, D. T., "(Two-stream plasma instability as a source of irregularities in the ionosphere )" (1963) ''Physical Review Letters'', Vol. 10, Issue 7, pp. 279-282〕 and Oscar Buneman.〔Buneman, O., "(Excitation of field aligned sound waves by electron streams )" (1963) ''Physical Review Letters'', Vol. 10, Issue 7, pp. 285-287〕 It is similar to the ionospheric Rayleigh-Taylor instability.
It occurs in collisional plasma with neutral component, and is driven by drift currents. It can be thought of as a modified two-stream instability arising from the difference in drifts of electrons and ions exceeding the ion acoustic speed.
It is present in the equatorial and polar ionospheric E-regions. Since the FB fluctuations can scatter the electromagnetic waves, the instability can be used to diagnose the state of ionosphere by the use of electromagnetic pulses.
==Conditions==
To derive the dispersion relation below, we must make the following assumptions. We consider the plasma to be quasi-neutral (ni~ne), which is appropriate since the wavelengths of interest are much larger than the Debye length. Additionally, we take the ions to be unmagnetized due to the high collisions frequency with neutrals compared to the ion gyro-frequency. So the ions, in the equilibrium state, are assumed to be at rest. Additionally we must neglect refraction and propagation effects. We take the electric field to be electrostatic since the plasma pressure is much smaller than the ambient magnetic energy density. We ignore gravity and assume the species are isothermal.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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