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Small-scale E-Region Irregularities in High-Latitude Plasma



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Turbulence is pervasive in natural fluids. This includes rivers, the atmosphere, and the ionosphere, i.e., the plasma above 100 km altitude in the upper atmosphere. This thesis investigates ionospheric turbulence at altitudes between 100 and 120 km. In that region turbulence is triggered by Hall currents introduced by the fact that, given the presence of the geomagnetic field, ions and electrons respond very differently to electric fields of the kind that are found in the auroral regions. The turbulent properties of this region have been rather well characterized through numerous rocket flights and with ground-based radars observations. Turbulence in this case leads to the creation of large amplitude plasma density structures. This thesis focuses on the fact that the structures have been found over the years to move at velocities that are far slower than expected from the notion that they are driven by the electron motion and should therefore be moving at close to the so-called ${\bf E}\times {\bf B}$ drift introduced by the presence of electric fields. Based on fluid theory, this thesis offers an explanation for the slower motion making the point that it has to be due to electric field inside the density structures becoming increasingly small as their amplitude grows through the so-called Farley-Buneman instability mechanism. The electric field changes until the structures are moving so slowly that they can no longer grow in amplitude. Prior to getting into the new theory this thesis presents the context for the work. It starts with a discussion of plasmas, and more specifically ionospheric plasmas. It then goes over some of the salient properties observed by rocket instruments and ground-based radars. It then reviews how Hall currents create an unstable situation that leads to the growth of density structures. This is followed by a review of a subset of nonlinear theories that have attempted to explain the observations through nonlinear particle fluxes. The evolution in the theoretical thinking on this topic has lead to the development of the new theory presented in Chapter 5 of this thesis. The main highlights of the theory are that it offers an explanation for the decrease of the electric field inside structures on the basis of a notion of anomalous diffusion that had later evolved into the evolution of substructures that were described through so-called mode-coupling theories and later on through the notion of an evolving tilt in the structures as they evolved. Based on the new theory I was able to calculate how and by what amount the turbulent structures make the average electric field smaller than the ambient field and how they change the currents in the medium. The theory has also been used here to calculate perturbed density and electric field profiles that look reasonable compared to observations although it does not perform as well in the lower part of the unstable region as it does near the top. A possible cause for this discrepancy could be that I did not take into account the fact that the lower part of the unstable region may well be affected by ambient density gradients associated with the bottom-side of the ionosphere.



electron density fluctuations, irregularities



Doctor of Philosophy (Ph.D.)


Physics and Engineering Physics




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