Brillouin scattering in plasmas as applied to ionospheric irregularities

Radio frequency scattering experiments have indicated that irregular fluid motions of tens of meters spatial scale and tenths of seconds temporal scale occur in the E region. It has been suggested that there can also occur in this region a dissipative plasma instability (similar to what has been termed the gradient‐drift or E×B instability in previous work) that displays these spatial and temporal scales and that may be of importance in the evolution of sporadic E. This paper describes an attempt to study the circumstances under which such an instability might appear by injecting alkali plasma clouds into the lower E region and observing them with decameter and meter wavelength, coherent‐pulse‐Doppler radar. After an initial blast‐wave‐driven expansion phase, an alkali plasma cloud undergoes a period of irregular behavior, in which a number of discrete echoes from seemingly coherent, large, short‐lived reflectors appear superimposed on the generally diffuse plasma cloud radar echo. Their large apparent size and spectral discreteness indicate that they may arise from an ordered, possibly wavelike structure, which must involve the entire plasma cloud. Comparison of the observed growth rates and probable spatial scales of the instabilities in the alkali plasma clouds with order‐of‐magnitude calculations of these quantities based on linear instability analysis confirms that the postulated plasma instability can indeed explain the observed behavior. In one phase of plasma cloud evolution a reflecting layer similar to sporadic E is formed, which permits decameter wavelength radiation, incident obliquely on the cloud, to be reflected back to earth in much the same manner as sky‐wave transmission is achieved via sporadic E. The similarity in behavior of an artificial sporadic E patch to the natural variety is evident. Also, a nonlinear, two‐dimensional plasma instability analysis is capable of explaining the spectral appearance and temporal variation of the decameter radar data from the alkali plasma cloud. An association is suggested between these results and the mechanism by which temperate latitude sporadic E is formed.

Section: The Example Illustrated In Figures 3 and 4 Is Typical In Appmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Decameter and meter wavelength radar studies of artificial plasma clouds in the lower ionosphere, 2. Unstable evolution in the lowerElayer and some implications, regarding sporadicE

Davis¹

1971

“…In this paper we extend our previous model [Tsuda et al, 1966;Salo et al, 1968] and further confirm that the irregularity-type sporadic E of midlatitudes can be caused by an instability in a weakly ionized plasma that we call the cross-field instability. The basic mechanism to be studied in detail later operates even in the equatorial situations, and indeed the secondary irregularities as observed by Cohen and Bowles [1967] may very probably be an indication of the cross-field instability occurring in the equatorial E region [Sato, 1968].…”

Section: Introductionmentioning

confidence: 98%

Ionospheric irregularities and the cross-field plasma instability

Tsuda¹,

Sato²,

Matsushita³

1969

Self Cite

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).

“…The question that naturally arises is whether or not such enhancement of the linear growth has any effect on the nonlinear evolution of the instability and, in particular, on the saturated amplitude of the density fluctuations. In this paper we answer this question by investigating the nonlinear evolution of the collisional Rayleigh-Taylor instability with the help of a two-dimensional nonlinear theory that was developed by Rognlien and Weinstock [1974] in order to calculate the saturated amplitudes and the spectra of density and electric field fluctuations associated with the gradient drift instability [Sato, 1968;Rogister and DMngelo, 1970;Sudan et al, 1973;Sudan, 1983]. The theory uses local approximation and solves a set of coupled nonlinear equations for the time evolution of the spectrum of modes that are generated through the dominant nonlinearity in the fluid equations.…”

Section: Introductionmentioning

confidence: 99%

Nonlinear saturation of Rayleigh‐Taylor instability in the presence of time‐dependent equilibrium

Basu¹

1999

Abstract. The nonlinear evolution and the saturation of the collisional Rayleigh-Taylor instability in the presence of a time-dependent ambient density gradient are studied by using a local, two-dimensional nonlinear theory. A set of nonlinear equations that describes coupling of the initial unstable mode with its various harmonics, which are selfconsistently generated by the dominant nonlinearity, is derived and then solved to find the saturated amplitudes and the power spectrum of the density fluctuations. In the saturated state the density spectrum is dominated by the fluctuations that are linearly damped but are nonlinearly excited and that have spatial variations only along the direction of the ambient density gradient. It is shown that the time-dependent (increasing) density gradient enhances the saturated amplitude of the dominant modes by a factor of 2 or more, depending on the altitude of the bottomside F layer at sunset. Analysis also shows that the collisional diffusion damping leads to a two-component density fluctuation spectrum with a shallow slope at long scalelengths (> 1 km) and a steep slope at shorter scalelengths. The calculated values of the spectral index p, for a power law of the form k• p, are p • 2.0 for the shallow slope and p •-3.0 for the steep slope, and they are close to the values determined from the one-dimensional measurements.