Effects of the active auroral ionosphere on magnetosphere - ionosphere coupling

The thesis is devoted to the effects of electromagnetic coupling between the Earth's magnetosphere and the active auroral ionosphere. The research has been focused, in particular, on the concept of ionospheric feedback instability. The feedback instability arises when localized perturbations in ionospheric conductivity become polarized in the presence of background electric field. Under favorable conditions of low ionospheric conductivity and strong electric convection the development of ionospheric feedback instability leads to the generation of magnetospheric ULF pulsations and precipitation of energetic particles into the auroral ionosphere that produce visible glow in the sky known as aurora. Numerical studies of magnetosphere-ionosphere coupling have been performed using a model that includes active ionospheric feedback and shear Alfvén wave dynamics of the magnetospheric response. Strong parallel inhomogeneities of the magnetospheric parameters included in the numerical model permit the simultaneous development of local ionospheric resonator modes (fast feedback) trapped between the ionosphere and Alfvén speed maximum above it and field line eigenmodes (slow feedback) that stand along the entire magnetic field line between two conjugate ionospheres. Effects of plasma anomalous resistivity in the large field-aligned currents of the feedback-driven Alfvén waves generate fluxes of energetic electrons that precipitate into the ionosphere producing auroral luminosity. The results of these simulations provide insights into the magnetospheric processes that cause the precipitation and allow us to estimate the precipitating electron energy flux for given ambient conditions. Numerical analysis of the effects of seasonal asymmetry in the ionospheric conductivity suggest that the feedback instability can be responsible for higher occurrence of auroral arcs in dark winter hemisphere as demonstrated by satellite observations. Simulations of the heating effects imposed on the auroral ionosphere by powerful radio waves suggest that the feedback instability can be excited artificially by HF radars. The results of numerical modeling demonstrate an agreement with satellite observations of the Alfvén waves and electron fluxes registered during experiments of modulated heating of the auroral electrojet.