S. L. Ossakow

Spread-F theories—a review

Our understanding of equatorial spread-F (ESF) phenomena has progressed significantly in the past several years. ESF phenomena involves ionospheric irregularities spanning some 5–6 orders of magnitude in scale size. The largest scale sizes, on the order of many kilometers, are thought to be caused by a plasma fluid type Rayleigh-Taylor instability mechanism on the bottomside of the night-time equatorial-F region. Plasma density bubbles (depletions) are formed on the bottomside by these ionospheric irregularities and then they rise nonlinearly to the topside, by E × B polarization motion, causing irregularities above the F-peak. The power spectral density (PSD) of these long wavelength plasma density fluctuations is proportional to k−2, where k is perpendicular to the geomagnetic field and equal to 2μλ. Current theories of the very small scale, ≲ 10 m, radar backscatter observed irregularities (specifically at 3 m, 1 m, 36 cm, and 11 cm), during ESF, indicate that these irregularities are due to various kinetic type plasma drift wave instabilities. These instabilities are formed by a two step process in which they are driven by the steep plasma density gradients created by the primary long wavelength Rayleigh-Taylor type plasma fluid instabilities. Recent theoretical and numerical simulation studies supporting these ideas on ESF phenomena will be presented.

The thermal self-focusing instability near the critical surface in the high-latitude ionosphere

A fully nonlinear development of the thermal self-focusing instability of high power radio waves in the ionosphere in the region near the critical surface is the subject of the present study. In the simulation model studied, a high-powered radio wave in the frequency range 5 - 10 MHz, with a 1% amplitude modulation, is launched vertically. In the high latitude geometry this represents a direction antiparallel to the magnetic field which is almost vertically downwards. The modulated wave undergoes strong self-focusing at the critical surface, where the group velocity of the wave goes to zero. The scale size of the structures transverse to the magnetic field is controlled by the wave intensity and the diffraction effects. The large parallel thermal conduction leads to the diffusion of these irregularities into the underdense and overdense plasma in narrow filaments. The depletion in the density in the overdense plasma allows propagation of the wave to higher altitude above the original critical surface and hence into the overdense plasma.

Nonlinear evolution of whistler instabilities.

Theoretical and computer simulation studies of whistler instabilities in anisotropic collisionless plasmas are presented. Initial bi‐Maxwellian, Maxwellian with loss cone, and hot Maxwellian superimposed on a more dense cold isotropic background electron distributions were used. Many of the observed features are common to all cases. Initially, the total wave magnetic energy grows; the average growth rate is in good agreement with linear theory; and the electron distribution isotropizes rapidly with a concomitant switching off of high k number, initially unstable waves. Then, the total wave magnetic energy saturates and at this time there is a residual kinetic energy anisotropy. This anisotropy persists after saturation, although there is a further tendency toward isotropy, together with a further switching off of the higher mode numbers for the remainder of the simulation. Throughout the simulation experiments the perpendicular and parallel energy constants are conserved to good accuracy.

Simulation of Whistler Instabilities in Anisotropic Plasmas

Computer simulations are used to study whistler turbulence in plasmas driven by initially anisotropic bi‐Maxwellian and loss‐cone electron distributions. Linear growth saturates when the growth rate is of the order of the particle trapping frequency.

Ionospheric plasma cloud dynamics via regularized contour dynamics. I. Stability and nonlinear evolution of one-contour models

The linear stability and nonlinear evolution of a regularized contour dynamical model of an ionospheric plasma cloud (or deformable dielectric) is examined. That is, the cloud is modeled by piecewise‐constant ion density regions; and the regularization is accomplished with a tangential diffusion operator that models aspects of the diffusion operator in two dimensions. A complete linear stability analysis of a circular cloud shows that a single‐mode excitation ‘‘cascades downward’’ in wavenumber as it grows in amplitude, a process that results from the symmetry‐breaking electric field. Approximate formulas are derived for the amplitude growth and cascade‐down phenomena and verified with precise numerical calculations. A simple rescaling shows that clouds with large λ (=cloud‐ion density/ambient‐ion density) evolve more slowly and appear more dissipative. The regularized contour‐dynamical algorithm for computations in the nonlinear regime is validated against the linear analysis and truncation errors are as...

The Rayleigh‐Taylor instability is not damped by recombination in the F region

The collisional Rayleigh-Taylor instability plays a crucial role in explaining the onset and development of equatorial spread F. A number of linear and nonlinear analyses of the instability have been developed for the F region that include recombinative damping. It has been argued that recombination damps the unstable density perturbations and provides a mechanism to suppress the onset of instability or saturate the instability via mode coupling. We show that F region recombination, in fact, does not damp unstable density perturbations relative to the background ionosphere. Recombination acts to reduce the total electron density, both the ambient background and the perturbation, at a rate VR.

Destruction of Cyclotron Resonances in Weakly Collisional, Inhomogeneous Plasmas.

It is shown, both analytically and numerically, that cyclotron resonances can be destroyed in dense (..omega../sub rho/ > ..cap omega.. where ..omega../sub rho/ is the plasma frequency and ..cap omega.. is the cyclotron frequency), weakly collisional, inhomogeneous plasmas when (..nu../..cap omega..) k/sup 2/ (r/sub L/)/sup 2/ is somewhat > 1, where ..nu.. is the collision frequency and r/sub L/ is the mean Larmor radius. The theory is based upon a model Fokker-Planck equation. It is found that the particles make a transition from magnetized to unmagnetized behavior. This is an important result since it indicates that the ion- and electron-cyclotron-drift instabilities transform into their unmagnetized counterparts, the lower-hybrid-drift instability and the ion acoustic instability, respectively. The ion-cyclotron-drift instability (or drift-cyclotron instability) is examined in detail and is found to become the lower-hybrid-drift-instability in the region of maximum growth when (..sqrt..(m/sub e//m/sub i/) ..omega../..cap omega../sub i/ somewhat > ..nu../sub ii//..cap omega../sub i/ somewhat > m/sub e//m/sub i/ for T/sub e/ approximately equal T/sub i/ plasmas. The first inequality is required to overcome electron viscous damping, while the second allows the ions to become unmagnetized. Applications to the equatorial F region of the ionosphere and the Tandem Mirror Experiment (TMX) are discussed.

The self-focusing instability in the presence of density irregularities in the ionosphere

The self-focusing instability of high-power radio waves in the ionosphere in the presence of density irregularities is presented. The present study addresses the role of preexisting density irregularities which are always present in the instability region. The presence of ambient irregularities results in the excitation of wave numbers, which, on the basis of homogeneous theory of the self-focusing instability, should be stable. This effect can explain the puzzling observations that indicate growth of the medium-scale (on the order of hundreds of meters) irregularities during ionospheric heating. The computational results are compared with available experimental data and will be used to plan experiments using the upcoming High-Frequency Active Auroral Research Program heater.

Physical mechanism of the lower-hybrid-drift instability in a collisional plasma

Abstract We present a physical discussion of the lower-hybrid-drift instability in both collisionless and collisional plasmas. The instability is important since it is the most promising explanation of small-scale irregularities (i.e. ≲ 1 m) observed during equatorial spread F .

Computer Studies of Collionless Damping of a Large Amplitude Whistler Wave

Computer simulation of a single large amplitude whistler wave propagating in a plasma has been carried out. Such a wave shows an initial linear damping, then a regrowth and amplitude oscillation, and finally settles to a constant amplitude value; all in good agreement with theory.