Transverse Ion Acceleration by Lower Hybrid Waves in the Topside Auroral Ionosphere.
This thesis proposes a possible means for ion acceleration in the topside auroral ionosphere. This model is consistent with the results from the Marie and TOPAZ II and III sounding rockets which measured a strong correlation between transverse ion acceleration, depletions in the background density, and lower hybrid waves. Specifically, it postulates that the interaction of lower hybrid waves with a small-amplitude (1-2%) density cavity leads to an energy transfer to shorter-wavelength, slower phase velocity modes. Such a mechanism is necessary since the original lower hybrid waves in this region possess too high a phase velocity to interact with the cold ionospheric ions. A two-dimensional model perpendicular to the geomagnetic field was utilized to explore the propagation of lower hybrid waves. The fundamental effects arise from the coupling of the electron drift with the density inhomogeneity. The ion effects are modeled in three stages; the least complex is a fluid model for both the electrons and ions. Both analytic and computer simulation results from this fluid model show the development of these slow phase velocity waves. The next level of complexity incorporates test -particle ions into the fluid model. The resulting acceleration of test particle ions is consistent with the theory of quasi-linear diffusion of the test particle distribution function. At the final level of complexity, a hybrid model with fluid electrons and particle ions allows for the self -consistent incorporation of ion kinetic effects such as Landau damping. The self-consistency provides an upper limit on how much ion acceleration can occur once the wave energy has been drained. Inclusion of ion kinetic effects results in an increase in the efficiency of the ion acceleration process due to the presence of ion Bernstein modes between the ion cyclotron and the lower hybrid frequencies. In the final component of this thesis, a linear instability was introduced as a means of incorporating the effects of a continuous influx of lower hybrid energy. The presence of this instability increases the amount of lower hybrid wave energy and thus increases the amount of ion acceleration.
- Pub Date:
- Physics: Fluid and Plasma