S. Hu

Gradient drift instabilities and turbulence in the nighttime equatorial electrojet

Linear and nonlinear evolution of gradient drift instabilities, responsible for type II irregularities in the equatorial electrojet, are discussed for the nighttime density profile obtained by rocket flights at Alcantara, Brazil, during the 1994 Guara Campaign. In contrast with the daytime case, the nighttime density profile is quite jagged. The jaggedness implies strong density gradients which excite short-wavelength instabilities with large growth rates. The most unstable waves have horizontal wavelengths in the range 10 – 20 m. Linear instabilities with much larger horizontal wavelengths, equal to and greater than 1 km, also occur, but their growth rates are smaller. In contrast with short waves that tend to localize themselves in narrow vertical regions where the density gradient is favorable for instability, the long waves have broad vertical extent, spanning regions that have density gradients favorable as well as unfavorable for instability. Despite the larger linear growth rates of the short waves (10 – 20 m), nonlinear numerical simulations exhibit significant kilometer-scale structures in the turbulently saturated state. The saturated power spectrum is anisotropic in the plane perpendicular to the ambient magnetic field. Kilometer-scale structures are realized by a balance between the linear instability drive of long waves and the diffusive damping of short waves produced by mode coupling. The ability of the long waves to fill out the vertical layer, spanning regions in which the density gradients are favorable as well as unfavorable for stability, makes them survive the mode competition in the nonlinear regime. Thus the saturated fluctuations in the nighttime show a preponderance 2- to 4-km structures (which are somewhat longer than those seen in the daytime), qualitatively consistent with observations.

Two‐dimensional simulations of gradient‐drift turbulence in the daytime equatorial electrojet

Two-dimensional numerical simulations of type II irregularities, attributed to gradient-drift instabilities, are carried out with observed profiles in the daytime equatorial electrojet. Earlier work has shown that these profiles are linearly unstable to gradient-drift modes with peak growth rates at wavelengths of the order of 1 km. The nonlinear evolution of these instabilities leads to saturated turbulent structures, with the linear drive at kilometer-scale wavelengths quenched by a direct energy cascade to short wavelengths that are effectively damped by velocity shear and diffusion. The saturated turbulent structures show quasi-steady 1 – 2 km horizontal waves, and appear to be quite isotropic in the plane perpendicular to the ambient magnetic field. The electron vertical velocity spectra, constructed from the simulation output, show 1–2 km vertical structures which are qualitatively in accord with recent high-resolution radar observations at the Jicamarca Observatory in Peru.

Linear gradient drift instabilities in the daytime equatorial electrojet

Linear gradient drift instabilities, which are associated with type II irregularities in the equatorial electrojet, are obtained for daytime profiles using initial-value and eigenmode methods in a nonlocal theory. The methods are shown to be in agreement, thus resolving a recent discrepancy between the theories of Ronchi et al. [1989] and Wang and Bhattacharjee [1994]. A set of equilibrium profiles that are consistent with observed daytime electric and current density profiles are given and analyzed for nonlocal stability. Instabilities with horizontal wavelengths of the order of 1 km are found to be dominant over most of the electrojet, with instabilities of shorter wavelengths (of the order of 100 m) localized in regions of weak velocity shear. The predictions of linear theory are in accord with the observed preponderance of kilometer-scale irregularities in the daytime electrojet.

Acoustic modes in dense dusty plasmas

Properties of acoustic modes in high dust density dusty plasmas are studied. The solutions of fluid equations for electrons, ions, and dust grains with collisional and ionization effects are solved along with an equation for grain charging. The high dust density effects on the acoustic modes are interpreted in terms of a change in the screening properties of the grain charge. At low dust density, the grain charge is screened due to electrons and ions. However, at high dust density, the screening of the grain charge due to other grains also becomes important. This leads to a reduction of the phase-velocity, which in turn is shown to make the plasma more unstable at high dust density. In this regime the role of the ion acoustic mode is replaced by the charging mode. The relevance of these results to earlier theoretical studies and experimental results are discussed.

Ionospheric storm forecast for high‐frequency communications

An improved physical model for ionospheric storm propagation is presented. The model uses the Dst index to monitor storm commencement. It also employs a latitude- and storm-strength-dependent propagation speed for the pressure disturbance that propagates from the auroral source region, where energy is deposited at storm commencement, to lower latitudes. The new storm forecast code is embedded in PropMan, an ionospheric program developed at Rockwell Avionics and Communications. The predictions of the new code for maximum usable frequencies during storms are compared with radio sounding data. Improvements are reported in the capability of PropMan to predict maximum usable frequencies in the 24 hours following a storm.

Observation of Naturally‐Occurring Waves in a Strongly Coupled Plasma

The Fourier spectra of longitudinal and transverse waves corresponding to random particle motion were measured in a two‐dimensional strongly coupled plasma. As a model of a strongly coupled plasma, a plasma crystal, i.e., lattice composed of negatively charged microspheres immersed in a plasma, is used. The phonons were found to obey a dispersion relation that assumes a Yukawa inter‐particle potential. The crystal was in a non‐thermal equilibrium, nevertheless phonon energies were almost equally distributed with respect to wavenumber over the entire first Brillouin zone.

A Nonlinear Theory of Void Formation in Colloidal Plasmas

A nonlinear time-dependent model for void formation in colloidal plasmas is proposed. For experimentally relevant initial conditions, the model describes the nonlinear evolution of a zero-frequency linear instability that grows rapidly in the nonlinear regime and subsequently saturates to form a void. A number of features of the model are consistent with experimental observations under laboratory and microgravity conditions.