Farley-Buneman Instability Research Articles

Factors influencing the heating of the auroral electrojet by high power radio waves

Results are presented from recent ionospheric modification experiments in which the EISCAT UHF radar measured the E-region temperature and density response to high power RF heating above Tromsø. A variety of electrojet conditions were encountered during these experiments. In particular, the electron drift velocity varied considerably allowing the heating efficiency of the RF heater to be investigated as a function of electron flow velocity. These observations constitute the first direct investigation of electrojet temperature modifications by high power radio waves and provide a test of a recent theoretical model in which the combined effects of RF heating and of natural plasma turbulence associated with the Farley-Buneman instability have been considered.

Polarkation electric field and Farley-Buneman instability during a precipitation event

An anomalous polarization electric field may be set-up in the lower auroral E-region in response to an electron precipitation event during unstable electrojet conditions. For instance, observations during post-midnight to early morning hours on 06–07 June 1990, using the EISCAT radar facility in Scandinavia, show that the in-situ dynamics of the E-region ionization may be radically affected by the presence of the Farley-Buneman instability. In this case, the measured ion drifts at 105 km height are exceptionally strong and comparable in magnitude with the E × B- drift in the F-region, mapped along the same magnetic fieldline. In this paper we present a model to explain the main features of these observations. We assume a simple relaxation model for the E-region ionization generated by an instantaneous electron precipitation event during diffuse aurora conditions and in the presence of the Farley-Buneman instability. In these conditions and for times smaller than the ionization lifetime (tens of seconds to a few minutes), the induced polarization electric field to restore charge quasi-neutrality may radically increase the ion drift velocity, and effectively decouple the ion motion from the dynamics of the neutral atmosphere.

Possible evidence for partial demagnetization of electrons in the auroral E-region plasma during electron gas heating

Abstract. A previous study, based on incoherent and coherent radar measurements, suggested that during auroral E-region electron heating conditions, the electron flow in the auroral electrojet undergoes a systematic counterclockwise rotation of several degrees relative to the E×B direction. The observational evidence is re-examined here in the light of theoretical predictions concerning E-region electron demagnetization caused by enhanced anomalous cross-field diffusion during strongly-driven Farley-Buneman instability. It is shown that the observations are in good agreement with this theory. This apparently endorses the concept of wave-induced diffusion and anomalous electron collision frequency, and consequently electron demagnetization, under circumstances of strong heating of the electron gas in the auroral electrojet plasma. We recognize, however, that the evidence for electron demagnetization presented in this report cannot be regarded as definitive because it is based on a limited set of data. More experimental research in this direction is thus needed.

Dual-beam EISCAT radar observations of the dynamics of the disturbed D- and E-regions in the early morning sector

We have used the two EISCAT UHF and VHF radar facilities in Scandinavia simultaneously to measure, in two different directions, the electron density profiles associated with three electron precipitation events in the early morning sector. They may be classified by: 1. (1) diffuse precipitation during the 12 August 1988 observations, associated with weak precipitation fluxes, low magnetic activity and slowly varying absorption. 2. (2) A discrete absorption event during the 6–7 June 1990 experiment, associated with strong fluxes, a moderately high magnetic activity and electric field strengths above the threshold for the Farley-Buneman instability in the electrojet region. 3. (3) A mixed event during the 8–9 June 1990 experiment, showing similar features to cases (1) and (2) and associated with moderately strong fluxes, stronger absorption and higher magnetic activity. Absorption peaks in this period do not seem to correlate with electric fields above the Farley-Buneman instability threshold. During both June 1990 periods we may be observing a pulsating radio-aurora which modulates (over 10–15 min) the precipitation of electrons into the ionosphere. This would explain the discrete absorption peaks on 8–9 June. On 6–7 June, moreover, the Farley-Buneman instability may be screening the absorption in the D-region. From the cross-correlation of the two simultaneous electron density time-series (from 80 to 120 km height at separations of ≈20 km) we obtain time delays of a few tens of seconds for both the June 1990 periods and of a few minutes for the August 1988 period and, thus, apparent N-S sizes of the induced ionization patches ranging between 20 and 100 km, and 6 and 60 km, respectively. Also, the ionization lifetime inferred from the auto-correlation time (from 30 s to 2 min in June 1990 and from 2 to 10 min in August 1988), seems to decrease with increasing precipitation flux. In the June experiments, the implied drift velocities from both the cross-correlation analysis and the triangulation of absorption data agree well (within the limits of our comparisons), and are mainly to the north-east (along the E × B direction). On 6–7 June 1990 the triangulation velocities also agree with the measured E × B -drift. The gradient-curvature motion of the precipitating electrons may only be detectable from cross-correlation analysis in the E-W plane. On 12 August 1988 the observed structure may not correspond to the same ionization patch (the estimated lifetime is greater than the recombination time), but to sustained ionization from slowly varying precipitation, and carried by the neutral wind. On the other hand, the 6–7 June 1990 data show that the in-situ dynamics of the ions may also be radically affected by the onset of the Farley-Buneman instability. In this case, the measured ion-drifts at 105 km height are exceptionally strong and comparable with the E × B -drift.

Low-frequency current instabilities in nonisothermal magnetoactive plasma

The excitation of current instabilities in the lower ionosphere is studied for magnetic-field-aligned inhomogeneities of arbitrary length with allowance for electron-temperature perturbations and the dependence of electron collision frequency on electron temperature. It is found that the threshold velocity of Farley-Buneman instability has a minimum for a finite degree of inhomogeneity elongation when the temperature dependence of the electron collision frequency is sufficiently strong.

Three-dimensional stabilization mechanism for the auroral Farley-Buneman instability

The auroral ionospheric E-region supports at least two types of plasma irregularities: the gradient drift instability and the Farley-Buneman instability. These are driven by the electron density gradient and the electric field, respectively. In this paper we study the effect of the three-dimensional auroral geometry on the saturation of the instabilities. For that purpose, an exact stationary solution to the two-fluid plasma model equations is derived. The stationary state thus obtained is used as an initial condition of a three-dimensional nonlinear fluid simulation. The algorithm is also described. In the auroral zone, the collision frequencies have gradients along the magnetic field. This implies that the phase velocity of the plasmons depends on the altitude, which in turns causes wave propagation at a nonzero aspect angle. These waves are more damped than exactly perpendicular ones. From this it is no surprise that our three-dimensional code can be used even when the electric field is near the Farley-Buneman threshold value. On the contrary, two-dimensional fluid algorithms blow up when the electric field is that large. We also conclude that the height profiles of the electric field, the electron density and the collision frequencies all affect coherent radar spectra, alongside with the local value of the electric field.

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.

High-frequency coherent plasma waves observed in the lower auroral ionosphere

Probe measurements of plasma irregularities in the lower E region during an auroral event showed the presence of coherent wave forms with a wavelength extending down to 35 cm that can be explained by the Farley–Buneman instability. Our results can be interpreted in the frame of recent theories of electron heating by unstable waves. Our data provide experimental evidence that coherent short-wavelength waves can be excited as is assumed in these theories. The measurements were made from rocket AAD-VIIIC-09 (ARIES-A) that formed part of the ARIES campaign for auroral modelling.

Space-time approach to the analysis of the Farley-Buneman instability. I. Evolution of plasma nonuniformities in crossed magnetic and electric fields

Space-time approach to the analysis of the Farley-Buneman instability. I. Evolution of plasma nonuniformities in crossed magnetic and electric fields

The mean fractional electron density fluctuation amplitude derived from auroral backscatter data

The mean fractional electron density fluctuation amplitude in the auroral E-region has been inferred from data of four auroral radars, obtained in situations where the ionospheric E-field was well above the threshold for the Farley-Buneman instability excitation. Two spatial power spectra of electron density variation have been tested; they give values which are in fairly good agreement with radar results from the equatorial ionosphere and with in situ rocket results from the auroral ionosphere, as well as with several theoretical predictions.

On SEC disturbances in the polar cap ionosphere and their relation to the solar wind parameters

Based on the analysis of the occurrence ofSEC disturbances in the polar cap ionosphere under varying solar wind parameters, the relation between the generation ofSEC-type ionospheric disturbances and interplanetary and magnetospheric conditions is discussed. It is emphasized that the Farley-Buneman instability in the E-layer of the ionosphere, reflected in ionograms as anSEC disturbance, depends on the complex effect of the solar wind parameters, the orientation of the interplanetary magnetic field playing an important role.

Rocket-borne and groundbased observations of coincident field-aligned currents, electron beams, and plasma density enhancements in the afternoon auroral oval

Results are reported from a rocket experiment conducted at Søndre Strømfjord, Greenland, on 22 August 1976, at 16.00 M.L.T. A series of plasma, particles, and fields and wave experiments were carried on board the payload, and the venture was supported by data from the AE- C satellite and by groundbased ionosondes and magnetometers at the launch site and at Godhavn. Two regions of field-aligned electron precipitation, electron density and temperature enhancements, and field-aligned upflowing current sheets were intercepted by the rocket. The density enhancements were also observed by groundbased ionosondes. Significant discrepancies were found between the currents carried by the streaming electrons in the 0.15–10 keV range and the upflowing currents seen by the on board magnetometer, suggesting that the upflowing current could not be the primary driver of the electron acceleration mechanism. The E-region was unstable to the combined Gradient-Drift and Farley-Buneman instability, and plasma turbulence was observed in situ, but the absolute density fluctuations were too small to return detectable HF-radar power to the ground.

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.

Further evidence for the Farley-Buneman instability in the polar cap ionosphere

Ionospheric dc electric fields are correlated with ionospheric backscatter results, providing further evidence for the occurrence of the Farley-Buneman instability in the polar cap.

On the relation of the ‘Slant E condition’ in polar cap ionograms to the solar wind sector structure

The ‘slant E condition’ (SEC) in polar cap ionograms has been shown previously to indicate excitation of the Farley-Buneman instability in the E region. The SEC at Godhavn (Greenland) is shown here to correlate with the solar wind sector structure, the occurrence rate of SEC being large a few days after passage of a sector boundary and declining in the trailing part of the sector. Analysis of SEC occurrence in terms of the solar wind velocity Vsw measured on board Heos 2 indicates that no SEC occurs when Vsw < 350–400 km/s (in apparent agreement with the interpretation of SEC in terms of the Farley-Buneman instability).

Parallel propagation effects on the type 1 electrojet instability

The Farley-Buneman instability has been extended to consider higher-frequency shorter-wavelength modes (thus including finite Debye length effects), and these modes are allowed to propagate with a com- ponent parallel to the magnetic field (k  0). When the current is driven sufficiently hard (drift speeds in the range 2-3 times the ion thermal velocity o), the growth rates of these modes maximize slightly away from the perpendicular to the magnetic field, and thus the importance of k  0 is shown. Although the wavelengths of these maximum growing modes are in the regime of tens of centimeters, the phase velocities are closer to the ion thermal ¾elocity than those modes propagating at 90 o (k = 0). Maximum growth rates of off-angle p{opagation for different densities and collision frequencies are sh6wn. Also, growth rates o'f unstable waves in the radar regime (1-10 m) are shown for drift velocities 1.5v, and 3v. These also maximize with k  0 and have phase velocities closer tothan they have for purely perpen- dicular propagation. In all cases considered the phase velocity of the waves is a rapidly decreasing func- tion of angle as one moves away from pure perpendicular propagation. Observations of backscattered radar signals from the equatorial electrojet have provided the impetus for substantial theoretical research on the source of the density fluctuations that can provide the observed enhancements in the received signals. There is general agreement that the source of the den- sity fluctuations is an electron current across the magnetic field produced by E x B type particle drifts due to the disparity in the electron-neutral and ion-neutral collision frequencies (Farley, 1963a, b). Since the relative drifts produced are of the order of the ion sound speed vand since the electron and ion temperatures Te, are equal, the system would have been linearly stable to ion sound on collisionless time scales. However, for collisional time scales the system becomes un- stable to low-frequency long-wavelength modes (Bunernan, 1963). The nature of this instability is purely resistive. That is, the resistive medium extracts energy from the ion drift and transfers it to the negative energy wave (slow wave) associated with the drifting ions. This instability has been examined first by Farley (1963a, b) for a kinetic plasma and subsequently by Bunernan (1963) in the fluid approximation. In Farley's kinetic description, which neglected finite Debye length effects (kXo - 0), it was shown that the important modes were perpendicular,to the magnetic field. Lee et al. (1971) have extended Farley's results by in- cluding Debye length effects and found higher-frequency shorter-wavelength instabilities. Their calculation was restricted to modes propagating exactly perpendicular to the magnetic field (k = 0). Very recently, Lee and Kennel (1973) have considered a simple parallel propagation effect analysis on type 1 instabilities in the fluid limit, and they note that these modes may be more unstable than those that propagate across the magnetic field. Recent theoretical results (Krall and Liewer, 1971; McBride et al., 1972) have shown that even for a collisionless plasma with TeT, a current perpendicular to the magnetic field produces an instability with small but finite k (k/k  (rne/m)/'). This instability, usually called the modified two- stream instability, is a strong reactive type instability, and non- linear considerations show that one has to go to the strong- turbulence regime for saturation. We therefore feel that an examination of whether and when such modes (k,  0) be- come unstable in the electrojet is necessary before going to the

Backscatter from a postulated plasma instability in the polar cap ionosphere and the direct measurement of a horizontalEregion current

Previously published data by L. J. Cahill, Jr., from a rocket measurement of a horizontal polar E region sheet current have been further analyzed by using nearby ground-based magnetograms and ionograms. The combined data indicate that the requirements of the Farley-Buneman two-stream instability were fulfilled and give further support to the hypothesis that a plasma instability is revealed by the so-called ‘slant E condition’ on the ionograms previously described.

Density gradients and the Farley-Buneman instability

The linear kinetic theory of the Farley-Buneman instability is generalized to include the effect of density gradients on the electrons. Near threshold the growth rates are substantially enhanced.

Nonlinear theory of ‘type I’ irregularities in the equatorial electrojet

This paper is concerned with the nonlinear development of the Farley-Buneman instability in the equatorial electrojet. The stabilization mechanism is as follows. A turbulent (negative) vertical electron flux 〈δυe,xδn〉 develops in the turbulent layer; to preserve charge neutrality, the (positive) vertical ion flux decreases accordingly. Hence the secondary electricfield and the electrojet electron current also decreases, thereby quenching the instability. The predictions of the theory agree quite well with available experimental evidence.