On the Regulation of the Geospace System by Solar-Wind/IMF Discontinuities and Ionospheric Outflow

The terrestrial environment is controlled by many external factors, some of which can potentially drive strong geomagnetic storms. In this dissertation, how the terrestrial magnetosphere-ionosphere (MI) system responds to solar wind variations and the subsequent self-modulation within the MI system are investigated by utilizing the Space Weather Modeling Framework (SWMF).;Studies of an interplanetary magnetic field (IMF) southward turning demonstrate that the arrival of the IMF discontinuity at the bow shock does not immediately disturb the ionosphere. With a solar wind speed of 400 km/s, the delay is about 10 minutes later. In addition, we find that a sudden increase in the solar wind density produces a two-phase response observed in ground-based magnetic perturbations and ionospheric potentials. The two phases are ascribed to the emergence of two successive pairs of field-aligned currents (FACs), which are caused respectively by dusk-to-dawn inductive electric fields due to a sudden compression of the magnetosphere and by magnetospheric vortices associated with the high-pressure gradient under northward IMF conditions or associated with large shear flows under southward IMF conditions. Although actual observations suggest that not all the solar wind density increase events are associated with a two-phase pattern, numerical simulations confirm the existence of the two-phase response. In addition to the two-phase response, the sudden compression launches Earthward-propagating waves that are further reflected multiple times between the Earth and magnetopause boundaries, resulting in multiple sets of the two-phase response.;Although the solar wind variations create disturbances in the MI system, one of the self-modulation processes within the MI system, the ionospheric heavy ion outflow into the magnetosphere, is equally important. The dayside cusp-origin heavy ion outflow significantly disturbs the magnetotail and provides significant energy to the ring current; while direct leakage of the heavy ions into the inner magnetosphere from the nightside aurora provides little geomagnetic feedback. Sufficient mass loading into the magnetosphere can lower the threshold of the Kelvin-Helmholtz instability, triggering surface waves more easily near the equatorial magnetopause boundary, changing the efficiency of the solar wind-magnetosphere coupling.