Ultra low frequency waves in the magentosphere-ionosphere system : a joint space- and ground-based investigation
This thesis examines the behaviour of large scale magnetohydrodynamic ultra low frequency (ULF) waves in the coupled magnetosphere-ionosphere system. Wave energy from solar wind driven disturbances at the magnetopause, is carried through the magnetosphere by compressional, fast mode waves, which couple to field guided Alfven mode waves on geomagnetic field lines. Field lines with eigenfrequencies corresponding to the driving frequency become resonant. Energy is dissipated in the ionosphere where partial reflection of the Alfven waves takes place, via Joule heating. The general aim of work presented in this thesis is to combine ground based measurements of the large scale structure of individual ULF waves with in-situ measurements of the small scale structure of the electric field, magnetic field and particle precipitation, made by the Fast Auroral SnapshoT (FAST) satellite. With a more specific aim to investigate the small scale structures which give rise to small regions of high current density, leading to parallel electric fields, particle acceleration and aurora. Details of the mechanisms which result in particle acceleration are not fully understood and are of considerable interest at present.;Four field line resonances (FLRs) with conjugate radar, magnetometer and FAST Satellite observations have been studied and compared. In each case a simple FLR model was created and scaled using the wave's spatial and temporal characteristics inferred from SuperDARN radar and ground magnetometer observations. The model field aligned current is compared with field aligned currents derived from the FAST energetic particle spectra and magnetic field measurements. In all four events downward currents appear to be carried, partially by upgoing electrons below the FAST energy detection threshold (5 eV), but also consist of a mixture of hotter downgoing magnetospheric electrons and upgoing ionospheric electrons of energies 30 eV -- 1 keV. In two of the events downgoing magnetospheric electrons with energies of a few keV, which are associated with upward field aligned currents of ~ 1 microA m --2, are observed. Strong intervals of upward current show that small-scale structuring of ~50 km has been imposed on the current carriers, which is thought to be associated with a mode conversion of an ideal magnetohydrodynamic (MHD) Alfven wave to an inertial Alfven wave.