NASA-CR-204564 Structures in ionospheric number density and velocity associated with polar cap ionization patches C )
Spectral characteristics of polar cap F region irregularities on large density gradients associated with polar ionization patches are studied using in situ measurements made by the Dynamics Explorer 2 (DE 2) spacecraft. The • 18 patches studied in this paper were identified by the algorithm introduced by Coley and Heelis [1995], and they were encountered during midnight-noon passes of the spacecraft. Density and velocity spectra associated with these a_tisunward convecting patches are analyzed in detail. Observations indicate the presence of structure on most patches regardless of the distance between the patch and the cusp where they are believed to develop. Existence of structure on both leading and trailing edges is established when such edges exist. Results, which show no large dependence of AN/N power on the sign of the edge gradient VN, do not allow the identification of leading and trailing edges of the patch. The AN/N is an increasing function of VN regardless of the sign of the gradient. The correlation between AN/N and AV is generally poor, but for a given intensity in AV, AN/N maximizes in regions of large gradients in N. There is evidence for the presence of unstructured patches that seem to coexist with unstructured horizontal velocities. Slightly smaller spectral indices for trailing edges support the presence of the E×B drift instability. Although this instability is found to be operating in some cases, results suggest that stirring may be a significant contributor to kilometer-size structures in the polar cap. C/) "D .//' "" "--"
The status of observations and theory of high latitude ionospheric and magnetospheric plasma turbulence
A review is given for the current status of both observations and theory of high latitude ionospheric plasma turbulence. The principal purpose of this review is to draw connections between the existing body of experimental evidence of fluid-like plasma turbulence in the ionosphere and the predictions of various fluid plasma models which have been proposed to describe the dynamics of the turbulent ionosphere. To place the ionospheric problem in perspective, a tutorial summary of 3-D and 2-D turbulent cascade theory is included along with citations of its applications in neutral fluid turbulence and the supportive experimental evidence found mainly in atmospheric flows. The high latitude observational evidence for low frequency (ω < Ωi) macroscale (L > ϱi) turbulence is summarized. The evidence includes observations of irregularities which occur over an altitude range of 400 km to 8000 km or possibly higher, and vary over scale sizes of 5 meters (ϱi) to about 2000 km. At the shorter wavelengths the irregularities are known to be nonpropagating hence convective nonlinear processes are important and this suggests that turbulence is the rule. The mathematical description of the turbulent dynamics of much of the ionosphere is suggested to be embodied in a low frequency fluid model, the essential features of which have been discussed in the literature. Application of standard cascade theory to this model leads to testable predictions of the power spectra for the density and electric field. A comparison of the spectral theory and the observations indicates significant agreement in some cases and ambiguity in others, but no apparent contradictions. Suggestions are made for future experimental diagnostics which could resolve ambiguities.
Simulation study of high power microwave generation by virtual cathode oscillation in coaxial vircator
Summary form only given, as follows. We have numerically simulated the high power microwave generation by virtual cathode oscillation in a coaxial vircator with 3 dimensional PIC code Magic. It is noted that the coaxial vircator has the advantage of an enhancement of power efficiency and gaining of narrow bandwidth frequency. The study is focused on the design of a diode structure for a coaxial vircator suitable for Chundoong IREB pulser (Max. 600 kV, 85 kA, 60 ns). It consists of a center annular cathode, a cylindrical meshed-anode and a reflector. The simulation results showed that the coaxial vircator can indeed be operated at the selected frequency with the narrow bandwidth and the microwave output power strongly depends on the position and geometry of the reflector. From this simulation results we expect that the maximum microwave output power will be obtained by the reflector of 4 cm in width and also 4 cm in distance away from the IREB, where the resonant frequency of microwave from coaxial vircator is simulated to be 6.577 GHz.