Radar and satellite characterization of the ionosphere under strong electric field conditions
It is now well established that strong electric fields can distort high-latitude ion velocity distributions to the point that this affects Incoherent Scatter Radar (ISR) observations, and therefore the ion and electron temperatures inferred through those observations. Until now, studies of this topic have focused on first order, semi-empirical ion velocity distribution descriptions. However, a precise description has been lacking, notably along directions parallel or near-parallel to the magnetic field. To remedy these shortcomings and provide the best possible tools to analyze ISR observations, this thesis uses a state-of-the-art Monte-Carlo (MC) simulation to retrieve accurate ion velocity distributions for any electric field, ion-neutral particle interaction, and direction relative to the magnetic field. Through these improvements, a number of important points have been made, such as: 1) for the most part simulated NO+ ISR observations can be modeled using Maxwellian velocity distributions having the same line-of-sight ion temperature as the simulated MC distribution, 2) although simulated O+ ISR observations parallel to the magnetic field are similar to those produced from Maxwellian velocity distributions they reflect an erroneous increase in electron temperature due to a wide O+ velocity distribution, and 3) signatures of toroidal ion velocity distributions in IS spectra are possibly the easiest to identify near 20 degrees with respect to the magnetic field. Based on these results, accurate distorted ion velocity distributions are currently being incorporated into IS spectral fitting routines. In the logical next step, this thesis turns to radar observations to characterize the ion temperature anisotropy, which is particularly important for Joule heating studies. Using ISR observations from a particularly strong heating event reported by Clauer et al. (2016), it is found that the O+-O collision cross-section from Knof et al. (1964) represents the anisotropy of the ionosphere fairly accurately, but still suggests the ionosphere to be slightly more anisotropic than expected. Knowing this allows for the preliminary determination of the effective electric field (the electric field in the neutral frame of reference). To obtain the electric field vector at a given latitude and longitude this thesis has explored a novel technique that employs multi-altitude measurements. This method combined with a knowledge of the effective electric field from the ion temperature studies opens up the possibility of a determination of the neutral wind in future work. Finally, to study the impact electric field strength has on Swarm satellites observations and on the upper ionosphere in general, a time-dependent gyro-kinetic O+ model of the motion of ions above a discontinuous boundary between fully collisional and collisionless plasmas has been revisited. This upgraded model uses descriptions of the ion velocity distribution provided by the MC simulation for the boundary velocity distribution as a function of electric field. As well, it incorporates a variable boundary plasma density and can describe any temporal variation of the ion velocity distribution at the boundary without complications. The results agree with the observations of highly distorted ion velocity distributions at high altitudes, as well as explain heretofore unpredictable anisotropic ion temperatures, attributing them to changing boundary conditions propagating upwards along a given flux tube, away from strongly-collisional regions.
High-latitude ionosphere, Ion velocity distributions, Ion frictional heating, Monte-Carlo simulations, Incoherent scatter spectra, Electric field determinations, Time-dependent gyro-kinetic model, Plasma stability
Doctor of Philosophy (Ph.D.)
Physics and Engineering Physics