Farley–Buneman instability

The Farley–Buneman instability, or FB instability, is a microscopic plasma instability named after Donald T. Farley[1] and Oscar Buneman.[2] 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 make the following assumptions. First, quasi-neutrality is assumed. This is appropriate if we restrict ourselves to wavelengths longer than the Debye length. Second, the collision frequency between ions and background neutrals is assumed to be much greater than the ion cyclotron frequency, allowing the ions to be treated as unmagnetized. Because the Buneman instability is electrostatic in nature, only electrostatic perturbations are considered.

Dispersion relation

We use linearized fluid equations (equation of motion, equation of continuity) for electrons and ions with Lorenz force and collisional term. This additional term describes the collisions of charged particles with neutral particles in the plasma by frequency of collisions. We denote \nu_{en} as the frequency of collisions between electrons and neutrals, and \nu_{in} as the frequency of collisions between ions and neutrals. We also assume exponential type of behaviour, meaning that all physical quantities f will behave as an exponential function of time t and position x (where k is the wave number) :

f \sim \exp(i\omega t + ikx).

This can lead to oscillations if the frequency \omega is a real number, or to either exponential growth or exponential decay if \omega is complex. Dispersion relation takes the form of:

\omega\left( 1 + i \psi_0 \frac{\omega - i \nu_{in}}{\nu_{in}}\right) = k v_E + i \psi_0 \frac{k^2 c_i^2}{\nu_{in}} ,

where v_E is the E\times B drift and c_i is the acoustic speed of ions. The coefficient \psi_0 described the combined effect of electron and ion collisions as well as their cyclotron frequencies \Omega_i and \Omega_e:

\psi_0=\frac{\nu_{in}\nu_{en}}{\Omega_i \Omega_e}.

Growth rate

Solving the dispersion we arrive at frequency given as:

\omega = \omega_r + i \gamma,

where \gamma describes the growth rate of the instability. For FB we have the following:

\omega_r = \frac{k v_E}{1+ \psi_0}
\gamma =\frac{\psi_0}{\nu_{in}} \frac{\omega_r^2-k^2 c_i^2 }{1+ \psi_0}.

See also

References

  1. ↑ 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
  2. ↑ Buneman, O., "Excitation of field aligned sound waves by electron streams" (1963) Physical Review Letters, Vol. 10, Issue 7, pp. 285-287


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