[1] Results from the three-dimensional nonlinear simulations of the gradient drift instability to study structuring in high-latitude plasma patches are presented. Simulations demonstrate that the existence of the mesoscale structures (10 km to 100 m) on both leading and trailing edges and inside a patch is due to (1) the nonlinear development of the gradient drift instability (GDI) or due to (2) occasional reversal of the direction of convection. The high-resolution simulation data have a dynamic range that allows us to make detailed comparisons with the observed density and velocity fluctuation spectra. We use the measured ion density and horizontal velocity data from the Dynamic Explorer 2 (DE 2) spacecraft examined by Kivanç and Heelis [1997] to compare them with our simulations. Density structures in the leading and trailing edges of patches have been examined, and comparison of statistical characteristics of the average density gradient on these edges with those in the work by Coley and Heelis [1998] has been performed. The dynamics and evolution of the density fluctuations over the realistic time scale are shown, thereby providing a significant evidence of the existence of the GDI at high latitudes as a structure-generating mechanism and a first principle understanding of the hierarchy of the instabilities involved in the observed structuring in high-latitude plasma patches.
[1] Simulations of the generation and evolution of ionospheric density irregularities due to thermal self-focusing are presented. A nonlinear, self-consistent development of the self-focusing instabilities (SFI) is studied with a two-dimensional model of HF radio wave propagation in inhomogeneous magnetized plasmas. The full-wave model is coupled with the electron temperature and plasma density equations. In these studies, the pump beam is incident vertically or tilted to the magnetic zenith direction. Our results demonstrate that irregularities generated and developed due to a local heating of the anisotropic ionosphere plasma are density depletions/striations with a strong enhancement of electron temperature inside depletions elongated along the magnetic field lines. For the ordinary mode (O mode) heating the reflection point is shifted upward with decreasing electron concentration. During the nonlinear evolution of the SFI instability the bunching of striations is observed. A number of the bunch-scale irregularities increase when the beam is tilted to the magnetic zenith direction. The presented results are consistent with the experiments carried out at the midlatitude SURA HF heating facility.Citation: Gondarenko, N. A., S. L. Ossakow, and G. M. Milikh (2005), Generation and evolution of density irregularities due to selffocusing in ionospheric modifications,
A dynamic model of multi-MA current commutation in a double wire array Z-pinch load is proposed and studied theoretically. Initially, the load is configured as nested concentric wire arrays, with the current driven through the outer array and imploding it. Once the outer array or the annular plasma shell formed from it approaches the inner array, the imploded plasma might penetrate through the gaps between the wires, but the azimuthal magnetic field is trapped due to both the high conductivity of the inner wires and the inductive coupling between the two parts of the array, causing a rapid switching of the total current to the inner part of the array.
[1] The magnitudes of the density and electric field fluctuations and their spectral characteristics were studied with a three-dimensional nonlinear model of the structuring in high-latitude patches caused by the primary gradient drift instability. The simulations were compared with the simultaneous density and electric field spectra obtained from noon-midnight and dawn-dusk orbits of the Dynamic Explorer 2 (DE 2) satellite [Basu et al., 1990]. The magnitudes of the simulated density and electric field fluctuations were found to be in very good agreement with the measured data. In the directions parallel and perpendicular to convection, the similarities in the spectral slope distributions of both simulated and observed density and electric field fluctuations were demonstrated. These findings together with our earlier results [Gondarenko and Guzdar, 1999, 2001] provide a significant evidence for coexistence of the primary gradient drift instability and subsequent secondary Kelvin-Helmholtz and tertiary shear-flow instabilities.
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