Farley-Buneman Waves Research Articles

Ion thermal effects on E-region instabilities: linear theory

Linear instabilities and turbulent processes in ionospheric electrojets lead to the formation of plasma density irregularities which create strong radar reflections. Studies of these irregularities have focused on the role of the Farley–Buneman (FB) and gradient drift instabilities with the assumption that the plasma behaves isothermally or adiabatically. In the last decade, this restriction has been relaxed, resulting in the prediction of D and E-region thermally driven instabilities, which now have some supporting observational evidence. Even more recently, fully kinetic particle-in-cell (PIC) simulations show that ion thermal effects strongly modify the nonlinear behavior of the FB instability, especially at the top of the electrojet. This paper describes the linear theory of the combined FB and thermal instabilities. The theory predicts that instabilities will develop over a wider range of altitudes than predicted for the adiabatic or isothermal FB instability. It also predicts that preferred directions of thermally modified FB waves may differ significantly from that of standard Farley–Buneman waves. These theoretical developments have important observational consequences.

On the Influence of Electron Drift Velocity Shear on the Farley–Buneman Instability in the Ionospheric E-Region

The influence of electron drift velocity shear on the Farley–Buneman instability in the ionospheric E-region is considered. First, we consider evolution with time of small-amplitude Farley–Buneman waves in the sheared plasma using Laplace transform in time and the exact solution for the model linear layer. As a result we obtain the Farley–Buneman waves with the component of the wave vector along the non-uniformity increasing with time. Second, we consider development of the Farley–Buneman instability in the sheared plasma using an approximate analytical method. We find that the electron drift velocity shear has a stabilizing influence on the Farley–Buneman instability. Also we find that the Farley–Buneman waves with the maximum amplitude have phase velocities close to the ion-acoustic velocity.

The role played by thermal feedback in heated Farley-Buneman waves at high latitudes

Abstract. It is becoming increasingly clear that electron thermal effects have to be taken into account when dealing with the theory of ionospheric instabilities in the high-latitude ionosphere. Unfortunately, the mathematical complexity often hides the physical processes at work. We follow the limiting cases of a complex but systematic generalized fluid approach to get to the heart of the thermal processes that affect the stability of E region waves during electron heating events. We try to show as simply as possible under what conditions thermal effects contribute to the destabilization of strongly field-aligned (zero aspect angle) Farley-Buneman modes. We show that destabilization can arise from a combination of (1) a reduction in pressure gradients associated with temperature fluctuations that are out of phase with density fluctuations, and (2) thermal diffusion, which takes the electrons from regions of enhanced temperatures to regions of negative temperature fluctuations, and therefore enhanced densities. However, we also show that, contrary to what has been suggested in the past, for modes excited along the E0×B direction thermal feedback decreases the growth rate and raises the threshold speed of the Farley-Buneman instability. The increase in threshold speed appears to be important enough to explain the generation of `Type IV' waves in the high-latitude ionosphere.Key words: Ionosphere (auroral ionosphere; iono- spheric irregularities; plasma waves and instabilities)

The role played by thermal feedback in heated Farley-Buneman waves at high latitudes

It is becoming increasingly clear that electron thermal effects have to be taken into account when dealing with the theory of ionospheric instabilities in the high-latitude ionosphere. Unfortunately, the mathematical complexity often hides the physical processes at work. We follow the limiting cases of a complex but systematic generalized fluid approach to get to the heart of the thermal processes that affect the stability of E region waves during electron heating events. We try to show as simply as possible under what conditions thermal effects contribute to the destabilization of strongly field-aligned (zero aspect angle) Farley-Buneman modes. We show that destabilization can arise from a combination of (1) a reduction in pressure gradients associated with temperature fluctuations that are out of phase with density fluctuations, and (2) thermal diffusion, which takes the electrons from regions of enhanced temperatures to regions of negative temperature fluctuations, and therefore enhanced densities. However, we also show that, contrary to what has been suggested in the past, for modes excited along the E0×B direction thermal feedback decreases the growth rate and raises the threshold speed of the Farley-Buneman instability. The increase in threshold speed appears to be important enough to explain the generation of `Type IV' waves in the high-latitude ionosphere.Key words: Ionosphere (auroral ionosphere; iono- spheric irregularities; plasma waves and instabilities)

On the generation of modified low-frequency Farley-Buneman waves in the solar atmosphere

The possibility of the excitation of Farley-Buneman turbulence in the solar atmosphere is examined. It is found that the conditions for the generation of the modified Farley-Buneman instability can be realized in the chromosphere of the Sun 1000 km above the photosphere. While usual Farley-Buneman waves studied in relation to the Earth's ionosphere are almost electrostatic, the modified Farley-Buneman waves in the solar atmosphere are electromagnetic ones. This means, that not only the potential electric field caused by the charge distribution, but also the perturbations of the magnetic field and the circularly-polarized electric field are essential. Although the physical pictures of usual and modified Farley-Buneman waves are different, their dispersion equations are almost the same. However, the increment of the modified Farley-Buneman waves is varied by additional electromagnetic effects. It is demonstrated that electromagnetic effects hinder a Farley-Buneman instability in occurring while ξ < 1, where ξ is the square of the ratio of ion plasma frequency times ion-neutral frequency to ion-cyclotron frequency times wave number times speed of light in vacuum. Under the condition ξ > 1, no Farley-Buneman disturbances appear at all. In weakly-ionized solar regions, the modified (ξ < 1) and also the usual (ξ ≪ 1) Farley-Buneman turbulence could make “electromagnetic” contributions to the process of energy dissipation of nonstationary streams of neutral gases. Besides, they may modify the low-frequency acoustic noise. It seems that the modified Farley-Buneman turbulence contributes to the sporadic radiation of the Sun. It is possible, that such an effect takes not only place in the chromosphere of the Sun, but also in the atmospheres of other stars.

On the generation of modified low-frequency Farley-Buneman waves in the solar atmosphere

On the generation of modified low-frequency Farley-Buneman waves in the solar atmosphere

Stabilization of the Farley-Buneman instability by three-wave interaction as consequence of the modification of the speed of energy transfer from an external electric field

The energetics of the stabilization of Farley-Buneman (FB) waves by three-wave interaction is studied. Considering decay processes of three FB waves in an open system, which are supposed to be called “quasi-decay processesŔ, it is shown, that during wave stabilization the direct energy transfer between the waves, which is essential for the stabilization of instabilities by normal decay processes in closed plasma systems, is less effective. According to the estimates, a decreasing growth rate of a FB wave, and thus a possible stabilization of the FB instability, may occur as a consequence of the change of the speed of energy transfer from an external electric field, which is caused by the action of two other waves.

The effects of small scale field aligned irregularities on E-region conductivities: Implications for electron thermal processes

Unstable Farley-Buneman waves propagate preferentially in a direction close to that of the E×B drift velocity. These waves cause anomalous heating which is equivilent to an increase in the effective electron conductivity of the E-region. Recently a new type of instability has been identified, as a result of detailed kinetic theory approach to E-region electron transport. This new instability favours waves propagating in a direction which bisects that of the E×B drift and the electric field, and originates in the usual electron-neutral Pedersen conductivity. In this paper, the effects of wave enhanced conductivities on this new instability are examined.

Currents and turbulence in and near mid-latitude sporadic &amp;lt;i&amp;gt;E&amp;lt;/i&amp;gt;-layers caused by strong acoustic impulses

Abstract. The generation of Hall and field-aligned currents in and in the vicinity of nighttime mid-latitude sporadic E-layers moving under the action of strong acoustic impulses of seismic, anthropogenic, or meteorological nature is considered in a model presented in this paper. The influence of the electrical polarization fields caused by charges at the horizontal edges of the sporadic layers and the finite conductivity of the external circuits are also taken into account. The theoretical model is applicable for ionospheric altitudes between 95 and 130 km. The estimates show that under certain conditions in a system with two sporadic E-layers, one of which is the current generator and the other is situated in the external circuit, the Farley-Buneman instability could be generated. On the other hand, observations show that Farley-Buneman waves are likely responsible for the infrequent echoes of mid-latitude 50-MHz backscatter with Doppler velocities near 300 m s–1. The possibility exists that the proposed current-generator model is at the origin of the observed mid-latitude Farley-Buneman waves.

Influence of collisional heating on the current generation in moving mid-latitude sporadic E-layers

A theoretical analysis of local heating processes above the borders of night-time midlatitude dense sporadic E-layers moving under the action of strong acoustic impulses in the ambient plasma of lower density at altitudes of 90 km to 130 km is done. It is seen, that a large velocity of motion of the layer generates a strong Hall current in the layer closed by an external circuit. The field-aligned currents of the external circuit can cause remarkable heating of the electron component by collisions with the neutral particles. As a consequence the field-aligned conductivity in the external circuit is considerably reduced and the Hall current in the sporadic layer becomes smaller. But the estimates show, that the Hall current can nevertheless reache values above the threshold of the excitation of Farley-Buneman waves. Besides it can be possible, that the field-aligned currents generate ion-acoustic and drift waves near the borders of the sporadic layers.

Electron thermal effects on the Farley–Buneman fluid dispersion relation

The traditional linear fluid dispersion relation of Farley–Buneman waves has been generalized by including, for the electron gas, the effects of collisional energy exchange, as well as thermal force and thermoelectric effects associated with heat flow. The formalism used is that of Schunk [Rev. Geophys. Space Phys. 15, 429 (1977)] for Grad’s 8-moment approximation, to which inelastic energy exchange has been added phenomenologically. The resulting dispersion relation recovers both the traditional isothermal and adiabatic limits, as well as the dispersion relation of Pécseli et al. [J. Geophys. Res. 94, 5337 (1989)] as a special case. Owing to the fact that the electron–neutral interaction is far from being of the Maxwell molecule type, it is found that, contrary to suggestions in the literature, adiabaticity does not hold at the larger wavelengths of the instability. In the small wave-number limit, the linear instability threshold speed of the waves takes the form [(γeTe0+Ti0)/mi]1/2, with the effective γe being a sensitive function of aspect angle. Its value can be as small as 0.28 or as large as 3.4 depending on conditions.

The role of natural E-region plasma turbulence in the enhanced absorption of HF radio waves in the auroral ionosphere:Implications for RF heating of the auroral electrojet

Abstract. Physical processes which affect the absorption of radio waves passing through the auroral E-region when Farley-Buneman irregularities are present are examined. In particular, the question of whether or not it is legitimate to include the anomalous wave-enhanced collision frequency, which has been used successfully to account for the heating effects of Farley-Buneman waves in the auroral E-region, in the usual expression for the radio-wave absorption coefficient is addressed. Effects also considered are those due to wave coupling between electromagnetic waves and high-frequency electrostatic waves in the presence of Farley-Buneman irregularities. The implications for radio-wave heating of the auroral electrojet of these processes are also discussed. In particular, a new theoretical model for calculating the effects of high-power radio-wave heating on the electron temperature in an electrojet containing Farley-Buneman turbulence is presented.

The role of natural E-region plasma turbulence in the enhanced absorption of HF radio waves in the auroral ionosphere:Implications for RF heating of the auroral electrojet

<strong class="journal-contentHeaderColor">Abstract.</strong> Physical processes which affect the absorption of radio waves passing through the auroral E-region when Farley-Buneman irregularities are present are examined. In particular, the question of whether or not it is legitimate to include the anomalous wave-enhanced collision frequency, which has been used successfully to account for the heating effects of Farley-Buneman waves in the auroral E-region, in the usual expression for the radio-wave absorption coefficient is addressed. Effects also considered are those due to wave coupling between electromagnetic waves and high-frequency electrostatic waves in the presence of Farley-Buneman irregularities. The implications for radio-wave heating of the auroral electrojet of these processes are also discussed. In particular, a new theoretical model for calculating the effects of high-power radio-wave heating on the electron temperature in an electrojet containing Farley-Buneman turbulence is presented.

Adiabatic and isothermal ion-acoustic speeds of stabilized Farley-Buneman waves in the auroral E-region

The influence of ion and electron energetics on the propagation speeds of stable Parley Buneman waves which are excited by E × B drifts in the auroral E-region is studied theoretically in the fluid limit, with the effects of anomalous collisions on electron thermal conduction included for the first time. In particular, the ratio of the phase speed of waves, stabilized by enhanced diffusion effects, to the isothermal ion-acoustic speed are calculated for realistically modelled E-region ion and electron temperatures, as functions of altitude, flow velocity and wavelength. It is found that the phase speeds of these stabilized waves begin to increase above isothermal ion-acoustic speeds as wave frequencies increase to values where they are comparable with the electron inelastic collision frequency. However, at still higher frequencies their phase speeds tend to fall back towards their isothermal values due to the increasing effects, with increasing wavenumber, of electron thermal conductivity. It is also found that the phase speeds are not always isotropic with respect to flow angle. The relationship between the predictions of the present fluid theory and a previous kinetic theory calculation is also briefly discussed.

The effects of the resonance broadening of Farley-Buneman waves on electron dynamics and heating in the auroral E-region

Several theories have now been developed which deal with the saturation mechanism of the Farley-Buneman instability and also the heating effect of Farley-Buneman waves on E-region electrons. Various related phenomena, such as anomalous collisions, anomalous cross-field diffusion and non-zero aspect-angle propagation have been treated. In this paper, it is demonstrated how several of these features may be included in a single resonance broadening formalism. Using this approach, the effects of Farley-Buneman waves on the heating and dynamics of background ionospheric electrons is discussed. In addition the effects of anomalous parallel diffusion in the context of E-region Farley-Buneman waves is treated for the first time.

Nonlinear interaction of electrostatic waves in the E region of the ionosphere

The potential for nonlinear three-dimensional interaction of Farley-Buneman waves in the E region of the ionosphere is studied. It is shown that, as a result of this interaction, waves are formed that propagate at a certain angle relative to the plane orthogonal to the direction of the magnetic field. These waves can be analyzed using radar.

Note on the parallel propagation effects of unstable Farley-Buneman waves at high latitudes

We have studied the effect that substantially enhanced electron temperatures produce on the higher-frequency shorter-wavelength modes of the Farley-Buneman instability at high latitudes. In all cases the value of the parallel wave vector is reduced and the results are closer to those obtained from fluid theory than previous calculations had revealed. The large electron temperatures are themselves the product of heating by unstable plasma waves so that the corrections described in the present work become increasingly important as the linear growth rate increases.