Structure and dynamics of high latitude magnetospheric boundaries.

摘要

Mass, momentum and energy are transferred from the solar wind into the magnetosphere via their interface, the magnetospheric boundaries. High latitude boundaries including the high latitude magnetopause, cusp, entry layer and mantle have been rarely studied since only a few spacecraft have visited there. There are many long standing open questions about high latitude boundaries, e.g., what is the magnetic structure of high latitude boundaries during various interplanetary magnetic field (IMF) conditions? Do the boundaries lose their distinct well-defined edge during southward IMF conditions? How do they respond to outside (solar wind) and inside (magnetic storm and substorm) conditions? What is the behavior of energetic particles in these regions? This dissertation addresses these questions via extensive Cluster data analysis and comparison with global MHD simulations.; First, this dissertation presents a statistical study of energetic particles in the cusp region. It presents the first observation that energetic ions exist in the high latitude magnetospheric boundary regions for 80% of the cusp crossings. The spectra of energetic particles with energies greater than 30 keV become flatter for higher solar wind speeds. Second, the high latitude magnetopause has also been studied. When the IMF is northward, the magnetopause adjacent to the cusp is associated with sharp changes in plasma density, velocity, temperature and magnetic field. However, this interface becomes uncertain when the IMF turns southward. A superposed epoch analysis was applied to study the average variations of key plasma parameters across the magnetopause under different conditions for the first time. This dissertation reports the first in-situ observation of collisionless Hall reconnection at the high latitude magnetopause when the IMF By dominates. Finally, this dissertation compares observations to MHD simulations for a real cusp event. Although the simulated magnetospheres are smaller than the real magnetosphere, the simulated magnetic fields and the amplitude of the model-derived plasma parameters of density, velocity and temperature in the cusp region agree reasonably well with observations. The MHD code qualitatively simulated the responses of the cusp position to the solar wind azimuthal flow for the first time and the formation of the cold dense plasma sheet.