The Physics of the Strongly Driven Magnetosphere: Global MHD Modeling

Abstract

This study examines the behavior of the earth's magnetosphere under extreme solar wind conditions using global magnetohydrodynamic (MHD) simulations. Particular emphasis is placed on the phenomenon of the cross polar cap potential (CPCP) saturation. The effect refers to the tendency of the potential to level off instead of growing linearly as the interplanetary electric field (IEF) increases. The CPCP is an important indicator of the coupling in the solar wind-magnetosphere-ionosphere (SW-M-I) system since it is mapped along the equipotential magnetic field lines from the dayside reconnection line. Due to the CPCP saturation a large amount of the solar wind energy is prevented from directly accessing the ionosphere which limits the ionospheric response to the solar wind driver. Global MHD simulations are a natural tool to study the phenomenon of the CPCP saturation. Despite the lack of proper microscopic shock and reconnection physics, such models reproduce many global phenomena in the SW-M-I system and its geometry. As discussed in this dissertation the problem of the CPCP saturation is in many respects a matter of geometry: The reconnection potential is formed in the magnetosheath flow which properties are affected greatly by the geometry of the magnetosphere. A series of simulations with idealized solar wind and ionosphere was conducted to study the dependence of the CPCP on the IEF and ionospheric conductance in a wide range of values. The simulations confirmed the CPCP saturation, but the level of saturation was shown to be strongly dependent on the ionospheric conductance. A mechanism of the ionospheric conductance feedback on the global characteristics of the SW-M-I system leading to the CPCP saturation was proposed. As a result of these studies a phenomenological model of the CPCP saturation was formulated. The main building blocks of this model are the direct amplifying effect of the solar wind electric field and the adverse feedback of the ionospheric conductance on the reconnection potential. Finally, test simulations were conducted with improved ionospheric model which incorporated a parametrized dependence of the ionospheric conductance on the IEF.