Magnetosphere-Ionosphere Coupling During Substorms

Abstract

This thesis concerns the development and evolution of the magnetospheric substorm. In particular this thesis concentrates on the dynamics of Ultra Low Frequency (ULF) waves during the substorm growth and expansion phases, the coupling of the ionosphere and magnetosphere through substorm onset and the substorm expansion phase and the location in the magnetosphere and physical processes leading to substorm onset. Firstly we present a superposed epoch analysis of ULF waves through the substorm growth and expansion phase, characterizing the two dimensional distribution of power spectral density and polarization of these waves with respect to auroral onset. We demonstrate that ULF wave power during substorms is characteristic of a power law and that the distribution of wave power and polarization is consistent with the expected pattern in the presence of a substorm current wedge. Secondly we examine the global field aligned current (FAC) morphology through the substorm growth and expansion phase. We demonstrate, for the first time, that there is a clear reduction in FACs coupling the ionosphere and magnetosphere just prior to substorm onset. We suggest that this change in coupling may play an important role in the destabilization of the magnetosphere at substorm onset. We also present the first two‐dimensional in‐situ observation of the FACs and ionospheric current sheets comprising the substorm current wedge in the substorm expansion phase. We demonstrate that the structure of the substorm current wedge is significantly more complex than the classical view of the substorm current wedge. Finally we present a comprehensive ground and in‐situ analysis of two substorms to determine where in the magnetosphere the substorms initiate. We demonstrate that both substorms initiate on closed field lines in the inner magnetosphere. More significantly we demonstrate that magnetic reconnection can follow substorm onset and that earthward magnetotail flows or auroral streamers, their ionospheric counterparts, are not a necessary condition for substorm onset.