Two solar wind parameters in particular are thought to be responsible for the majority of solar wind-driven ULF waves. These two parameters, solar wind dynamic pressure and solar wind velocity, are studied in this work through the use of global magnetohydrodynamic (MHD) simulations of the solar wind-magnetosphere interaction. We drive the global MHD simulations with idealized solar wind input conditions, chosen to mimic each of the above mechanisms. This allows us to study, in isolation, both of the solar wind parameters and to compare and contrast their effectiveness in the generation of magnetospheric ULF waves. Moreover, the global, three-dimensional nature of the MHD simulations allows us to fully characterize the spectral properties and global distribution of the ULF waves generated. These wave properties are known from a theoretical standpoint to be important for the interaction of ULF waves with radiation belt electrons. From these considerations, we are able to quantify the effect that the simulated ULF waves could have on radiation belt electrons, and thus, how the solar wind ultimately couples its energy into the radiation belts. Though this is the primary goal of our work, these investigations have uncovered two magnetospheric phenomenon, the MHD Kelvin-Helmholtz instability and magnetospheric cavity modes, that have never before been studied in the realistic magnetospheric configuration provided by global MHD simulations.
The MHD Kelvin-Helmholtz instability has long been predicted to occur at the boundary interface between the solar wind and the magnetosphere and is often invoked as an explanation for various magnetospheric phenomenon. For example, several theoretical studies have suggested that the Kelvin-Helmholtz instability at the magnetospheric boundary drives magnetospheric field-line resonances, resonant oscillations of geomagnetic field lines analogous to standing waves on a string. In addition, the nonlinear evolution of the MHD Kelvin-Helmholtz instability leads to large-scale (several times the size of the Earth) vortices in the plasma flow. A number of studies have suggested that these vortical structures are responsible for the entry of solar wind plasma into the magnetosphere. This is an important process in the magnetosphere as solar wind plasma ultimately fuels geomagnetic storms and the auroras. Though the magnetospheric Kelvin-Helmholtz interaction has been studied from a theoretical standpoint in simple flow configurations for many years, it has received little attention in modern, global MHD simulations. Moreover, while many studies have presented surface wave observations that can be easily explained in terms of the Kelvin-Helmholtz theory, others argue that the observations can just as easily be explained by fluctuating upstream solar wind plasma parameters. We circumvent these ambiguities by driving global MHD simulations with idealized solar wind input conditions, where all of the upstream solar wind plasma parameters are held constant. Thus, any surface waves generated at the magnetospheric boundary cannot be due to fluctuations in the upstream solar wind. We show that the Kelvin-Helmholtz instability is excited at both edges of the magnetospheric boundary layer and over a wide range of solar wind speeds (400-800 km/s). The results presented in the first half of this work are the first comprehensive study of the MHD Kelvin-Helmholtz instability in a realistic magnetospheric configuration.
Magnetospheric cavity modes have been studied for many years, both theoretically and through simple numerical simulations. The concept of magnetospheric cavity modes is physically quite appealing; however, such oscillations have proved elusive in magnetospheric observations. Moreover, global MHD simulations have yet to reproduce cavity mode oscillations, further calling into question their existence. The results presented in the second half of this work show the excitation of magnetospheric cavity modes in global MHD simulations of the solar wind-magnetosphere interaction. These resonant cavity modes are excited when the frequency of upstream solar wind dynamic pressure fluctuations matches one of the natural frequencies of the magnetospheric cavity. Furthermore, the results from this study suggest that only even mode number cavity resonances can be excited within the magnetosphere. We also show that monochromatic fluctuations in the upstream solar wind dynamic pressure directly drive magnetospheric pulsations at the same frequency, regardless of whether or not these fluctuations simultaneously energize cavity mode resonances. These results provide substantial support for the existence of magnetospheric cavity modes, and also shed light on why cavity mode observations have thus far proved so elusive. (Abstract shortened by UMI.)