Cross-field Instability Research Articles

SpreadEcaused by cross-field instability

Analysis of spread E, the spreading frequently observed above the sporadic E layer, shows that it can be caused by the cross-field plasma instability mechanism proposed by Reid [1968] and Chen et al. [1972]. The analysis was based on the determination of the extent of spreading relative to the blanketing frequency of the sporadic E layer, and the results of the analysis were such that they conformed with predictions based on the cross-field instability mechanism.

Movement of irregularities in the equatorial E-region—A review

There are various kinds of irregularities in the equatorial electrojet. Small irregularities consist of type I and type II. The type I irregularities are produced by the two-stream instability and the type II by the cross-field instability. Theory can explain the basic characteristics of these irregularities, but detailed agreement between theory and observation is still not confirmed. For large irregularities, wind shear might be important. Observational checking is necessary.

Some characteristics of the ionospheric irregularities associated withEsqlayers

Spaced fading records at Thumba for times when Esq disappears following large changes in the geomagnetic H component are studied. The results for the periods when Esq is present are known to show fast fading, westward drift, and highly elongated ground diffraction patterns with a small semiminor axis. During periods when there is no Esq the fadings are found to be very slow, the drift direction is reversed to the east, and the ground diffraction pattern becomes almost isotropic with a large semiminor axis, whose alignment is a few degrees off the magnetic N-S direction. The characteristics of the fading and those of the ground diffraction pattern when Esq disappears are similar to the characteristics usually obtained at latitudes well away from the equator. The Esq is thus suggested to be a superposition of additional irregularities over the normal E region irregularities (similar to those in the E region at nonequatorial latitudes). Close dependence of the direction of the electric field with the occurrence of Esq favors the conclusion that Esq is due to the cross-field instability created by the action of the H field with the Hall polarization field and the plasma density gradient.

Unified theory of type I and II irregularities in the equatorial electrojet

Turbulent structure of the electrojet is discussed in detail, and a unified interpretation of the electrojet irregularities (type I and type II) is given in terms of nonlinear two-stream and cross field instabilities. It is emphasized that small-scale type II irregularities (3∼15 meters) can be interpreted as resulting from nonlinear decay (collapse) of large-scale irregularities (∼150 meters), which are directly excited by the cross field instability. It is shown that polarization fields associated with the large irregularities modify the straight (horizontal) electron flow substantially; electrons are constrained to make periodic excursions vertically upward and downward in accord with the period of the irregularities. Consequently, the type I irregularities arising from the two-stream instability are excited obliquely and vertically, as well as horizontally. It is also shown that the observed constant phase velocity of the type I irregularities can be explained in terms of the quasilinear stabilization. In addition, it is emphasized that one should not regard the mean Doppler velocity of the type II (3-meter) echoes Vph3II as representing the electron drift velocity when type I echoes coexist, i.e., when Vph3II > Cs, where Cs is the ion sound velocity.

E s-q layer at huancayo during the March 1970 geomagnetic storm

The paper describes a comparison of vertical electron drift in the F-region ( V z ) measured by VHP incoherent scatter radar at Jicamarca with the corresponding variations of geomagnetic horizontal field ( H) and the maximum frequency reflected from The E s layer ( E s ) at Huancayo during the geomagnetic storm period 7–9 March, 1970. The V z is generally upward during the daytime at the equator, but during 7–9, March, 1970, V z was negative for brief periods associated with negative bays in H. These periods of abnormally low or of downward V z correspond closely with the period of complete disappearance of the q type of E s layer. The magnetic bays associated with the intensification of ring current do not affect the equatorial E s - q and it is only the negative bays in H at the equator due to the ionospheric current flowing westward, that cause sudden disappearance of E s − q. It is suggested that the q type of E s is due to cross-field instability created in the electrojet region due to interaction of northward magnetic field and vertical upward Hall polarization electric field when the plasma density gradient is upward. The sudden disappearances of E s − q are due to the reversal of the horizontal electric field in the equatorial ionosphere and thereby due to the reversal of the equatorial electrojet currents. These reversals of electric field may be due to the imposition on the normal S q field of another westward electric field.

Equatorial sporadic E and cross-field instability

The occurrence of sporadic E at an equatorial station during magnetically quiet daytime conditions corresponds almost exactly to the time during which the horizontal component of the earth’s magnetic field is above the mean night time level. Any large decrease of H below the night time level is accompanied by the disappearance of equatorial Es−q reflections precisely for the period when the value of H remains below its night time level.

Nonlinear theory of cross-field instability with application to the equatorial electrojet

Nonlinear theory of cross-field instability with application to the equatorial electrojet

New mechanism of auroral arcs

A new mechanism of quiet auroral arcs is presented in which the leading part of arc formation is played by the ionosphere. The theory basically consists of two stages: Firstly, it is shown that the ionosphere is subject to the macroscopic cross-field instability resulting in a wavy irregularity of ionization whose spatial characteristics are coincident with those of auroral arcs. Secondly, a model is presented that such an irregularity (to-be-arc) develops into auroral arcs under the influence of field-aligned current triggered by an induced potential across a to-be-arc. The model is numerically examined for a number of parameter sets to find that the ionospheric irregularity can certainly result in auroral arcs. The calculated results are compared with recent experiments of auroral arcs. Deformation of an arc is also discussed in terms of the cross-field instability.

Electrostatic Plasma Instabilities in the Equatorial Electrojet

TWO Nike Apache rockets were launched with Langmuir probes and other aeronomy payloads from Thumba, India (dip≃0° 47′), on January 28, 1971. The first rocket was launched at 10.40 1ST and the second at 11.10 IST. During the flights large scale irregularities in electron density in the scale sizes 30 to 150 m and with amplitudes of about 10% were observed in the 87 to 98 km region. Small scale irregularities with scale sizes 1 to 15 m were also observed in this region though with much smaller amplitudes. These irregularities are similar to cross-field instabilities observed earlier and add support to the previous measurements carried out during evening and night hours1,2. Above 100 km, the amplitude of the large scale irregularities decreases whereas the amplitude of the small scale irregularities increases. In this communication we shall present only the results for the small scale irregularities.

Cross Field Instability and Ionization Irregularities in the Equatorial E Region

FOUR Nike Apache rockets with Langmuir probe and plasma noise probe payloads were flown from Thumba (dip latitude 0° 47′ S) to measure the electron density, electron temperature and ionization irregularities in the E region. The measurements revealed the presence of irregularities in electron density with amplitudes of 5 to 20% in the scale size range of 30–300 m, and with amplitudes of 1 to 2% in the scale size range 1–15 m (refs. 1–3). The irregularities were observed in several discrete height ranges. The observations near rocket apogee of the irregularities with scale sizes of a few metres or less were found to be due to rocket motion in the medium1,2. The rest of the irregularities observed on these flights represent genuine ionization irregularities actually present in the ambient medium. Most of these were found to occur in regions where the background electron density exhibits a definite type of gradient. Their positions of occurrence lead us to believe that, depending on the time of the day at which observations are made, electron density gradients of a given sign promote the generation of these irregularities while gradients with the opposite sign inhibit their growth. Results from all the four flights lead us to the same conclusion. For the sake of brevity, only results from two representative flights are presented here. These are typical results and full details are being published elsewhere.

Strong turbulence theory of ionospheric cross-field instability

In the presence of external dc electric and magnetic fields that are perpendicular to each other, a weakly ionized plasma is subject to the so-called cross-field instability if there is an appreciable gradient of plasma density. The strong plasma turbulence theory of Dupree and Weinstock is used to develop the nonlinear theory of this cross-field instability, and the theory is applied to F-region irregularities in the ionosphere. As the waves grow, the nonlinearity is manifest in a growing enhanced ion diffusion coefficient proportional to an integral of the wave spectrum. As the wave spectrum grows, the ordered motion of the ions, necessary to sustain the waves, is offset by the turbulent diffusion, so that eventually the wave growth stops and a stationary turbulent state is reached. The nonlinear dispersion relation, the stabilization time, and the asymptotic spectrum are calculated.

A study of the irregularities in the night time equatorial E-region using a Langmuir probe and plasma noise probe

A rocket borne Langmuir probe and plasma noise probe experiment conducted at 2300 hr IST on 29 August 1968 from Thumba near the magnetic equator, revealed the existence of electron densities varying between 10 3 and 10 4 per cc in the height range of 95–120 km and a deep valley in the 120–140 km region. The electron density in the valley was of the order of a few hundreds per cc. Large scale structures in the electron density profile in the 95–120 km region were observed with vertical scale sizes of a few kms and a horizontal extent of the order of 50 km. In these structures the electron density was observed to vary by factors lying between 4 and 25. Small scale irregularities with scale sizes lying in the ranges 30–300 m and 1–15 m that were present in the ambient medium were detected during the flight. While these irregularities were found to occur randomly and in the form of bursts most of them were located only in regions where the background electron density exhibits large negative gradients. The positions of occurrence of the 30–300 m irregularities were found to confirm qualitatively that these irregularities are generated by a cross-field instability mechanism which needs a background electron density gradient with a specific sign. While the smaller scale size irregularities (1–15 m) also occur in regions where the cross-field instability mechanism can operate they cannot be directly explained on the basis of the present theories of the cross field instability. Hence either the present theories need modification, or there exists another mechanism which is responsible for these irregularities. However, even if a separate mechanism were to exist, the present data indicates that depending on the relative orientation of the ambient electric field and the electron density gradients, the cross-field instability mechanism can promote or inhibit the production of these irregularities.

Ionospheric irregularities and the cross-field plasma instability

Results of an extensive study on the cross-field instability in weakly ionized gases are presented. They are responsible for some charge density irregularities of the midlatitude and equatorial ionospheric E region. Both a linear one-dimensional case and a nonlinear two-dimensional case are formulated and numerically investigated with computers. All these cases strongly support the formation of ionospheric irregularities by the mechanism of cross-field instability. Whether or not the model irregularities are bounded versus height, weak perturbations rapidly grow to large-amplitude plasma turbulence with fluid-like charge density fluctuations of very low frequency (typically, of 10 Hz) and long wavelength (of the order of 100 meters).

Brillouin scattering in plasmas as applied to ionospheric irregularities

Theory of Brillouin scattering in plasmas, i.e., nonlinear interaction of electromagnetic waves with longitudinal (plasma) waves, is developed and applied to the ionospheric irregularities produced by a plasma instability. As a result, characteristic features appearing in VHF radar echoes from the equatorial E region seem to be plausibly explained. Above all, it is found that the main peak echoes and the small image echoes are well interpreted as arising from the phase-matched and mismatched scatterings by unstable ion-acoustic waves, respectively. Furthermore, Dougherty and Farley's interpretation of broad spectra at low frequencies is criticized, and an alternative interpretation is made by introducing another instability, i.e., cross-field instability.

Structure of Plasma Turbulence due to Nonlinear Cross-Field Instability

Evolution of spectral distribution of charge-density fluctuations in a strong plasma turbulence is studied by computer. It is demonstrated that components of sufficiently short wavelengths are strongly modulated by long wavelength components, but not the other way.

Computer Study of Nonlinear Cross-Field Instability

Evolution of a macroscopic cross-field instability in a weakly ionized plasma, first investigated by Simon on a linearized version, is studied numerically by solving a set of system equations. The dependent variables are replaced by spatial Fourier components. Calculations were carried out for two initial disturbances; i.e., a monochromatic disturbance and a white-noise-like disturbance. Results show that this instability explosively develops into a strong turbulence through wave-wave interactions between the excited Fourier modes. As far as the early stage of instability growth is concerned, it is found that machine calculations are in good agreement with linear analysis.