W. A. Scales and E. Yaniv
Bradley Department of Electrical Engineering
Sponsor: Office of Naval Research
Work in Progress: Irregularities in the earth's upper atmosphere may have significant effects on communication systems that use radiowaves. An important source for these irregularites is sheared flow velocities in the high latitude auroral regions of the ionosphere. The ionosphere extends from altitudes of about 80 to 1000 km. The upper atmosphere is composed of a charged particle gas referred to as a plasma which is produced primarily by photoionization. Crossed electric and magnetic fields in the upper atmosphere lead to velocity flows in the ionospheric plasma. When the electric field is nonuniform in space, this flow velocity is sheared. The object of this work is to study the physical process by which irregularities are produced by sheared flows. In the longwavelength regime, the mechanism is the well known Kelvin Helmholtz Instability which has been extensively studied in fluid dynamics.
A two-dimensional theoretical model is used to describe the Kelvin Helmholtz Instability in the ionosphere. This model was numerically solved by using finite difference and Fourier pseudo-spectral methods. This animation (MPEG) / (QuickTime) shows the temporal evolution of the electron density in the ionosphere subject to a sheared flow produced by crossed electric and magnetic fields. Note the development of vortices during the nonlinear phase of the instability. These vortices lead to irregularites which structure the ionosphere. This may adversely effect radiowave propagation. This animation (MPEG) / (QuickTime) shows the development of the electrostatic potential for the same case as the previous animation (MPEG) / (QuickTime)showing electron density. This potential indicates a radial electric field orientation inside the vortices. As the system progresses into the nonlinear stage, it becomes highly turbulent. Much information about the development of turbulence can be gained from considering the wavenumber power spectrum. This animation (MPEG) / (QuickTime) shows the temporal evolution of the power spectrum for this case. It can be seen that as time evolves, energy is transferred from small wavenumbers to large wavenumbers. This is equivalent to a transfer of energy to short wavelengths. The power spectrum in exhibits the classical power law dependence associated with turbulence in the saturated state.
Ongoing work is to refine the model by incorporating other relevant physical processes and to make more direct comparisons with experimental observations for rockets and satellites.