Non-Linear Magnetosphere-Ionosphere Interactions at High Latitudes

Non-linear, 3D electromagnetic coupling between the ionosphere and magnetosphere is investigated in this dissertation. The study is based on a non-linear, 3D, reduced magnetohydrodynamic model describing interaction between dispersive Alfven waves and the nightside high-latitude ionosphere. Results are presented from a numerical study of small-scale, intense magnetic field-aligned currents observed in the vicinity of the discrete auroral arc by the Magnetosphere-Ionosphere Coupling in the Alfven Resonator (MICA) sounding rocket launched from Poker Flat, Alaska, on 19 February 2012. The goal of the MICA project was to investigate the hypothesis that such currents can be produced inside the ionospheric Alfven resonator by the ionospheric feedback instability (IFI) driven by the system of large-scale magnetic field-aligned currents interacting with the ionosphere. Simulations of the reduced 2D MHD model with realistic background parameters confirm that IFI indeed generates small-scale ULF waves inside the IAR with frequency, scale-size, and amplitude showing a good, quantitative agreement with the observations. The 3D model was used to verify the results from the ionospheric heating experiment conducted at the High Frequency Active Auroral Research Program (HAARP) facility, Alaska, on March 12, 2013. During the experiment, HAARP transmitted in the direction of the magnetic zenith X-mode 4.57 MHz wave. The transmitted power was modulated with a frequency of 0.9 mHz, and it was pointed on a 20 km spot at the altitude of 120 km. It was observed that this artificially initiated heating 1) generated disturbances in the magnetic field detected with the fluxgate magnetometer on the ground, and 2) produced bright luminous spots in the ionosphere, detected with the HAARP telescope. Numerical simulations of the 3D reduced MHD model reveal that these effects can be related to the magnetic field-aligned currents, excited in the ionosphere by changing the conductivity in the E-region when the large-scale electric field exists in the heating region. The importance of the Hall currents in magnetosphere-ionosphere interactions, carried by ULF waves and field-aligned currents, has been consistently overlooked in studies devoted to the active experiments. Simulations of the 3D two-fluid MHD model, presented in this study, demonstrate that the Hall conductivity changes 1) the growth rate and the amplitude of ULF waves generated by the heating and 2) the orientation and the direction of propagation of the generated waves. These findings provide insight into the experiments where the waves were generated with a geometric modulation technique, and suggest a new and more efficient approach for conducting such experiments in the future