This thesis examines small scale poloidal mode magnetohydrodynamic waves which have an energy generation mechanism internal to the magnetosphere in the form of unstable particle populations. The energy from these particles is fed into a resonant wave mode and, ultimately, dissipated in the terrestrial ionosphere. By analysing Ion Distribution Functions (IDFs) the statistical occurrence of the driving magnetospheric particle populations is presented. Results indicate that the dominant driving particle populations are those of ~10 - 40 keV protons. The free energy of the particles has been quantified, revealing the dominance of the lower energy particle populations. The majority of unstable low energy protons contain > 1010 J of free energy in comparison to the higher energy protons which contain < 109 J. Using this method, one event, using conjugate ionospheric - magnetospheric data is examined and compared to a similar conjugate event of a rare subset of small scale waves called Giant Pulsations (Pgs). The available free energy is compared to the energy dissipated into the conjugate ionospheres. Estimates of the energy at the source and sink reveal that ~ 1010 J is transferred for the first event. Pgs are shown to transfer ten times this energy. The statistical study reveals that 1010 J is frequently available from unstable IDFs but 1011 J is not, thus providing an exploration for both the rarity of Pgs and the ubiquity of other small scale waves. Observations also suggest that drawn sector waves are driven by the drift-bounce resonance mechanism, while in the dark sector waves are driven by both drift and drift-bounce resonance mechanisms. Additionally, ionospheric observations indicate that the occurrence of small scale pulsations could be more abundant than previously thought. This implies that the quantity of energy being transported round the magnetospheric cavity and into the ionosphere via wave-particle interactions has previously been underestimated.

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.

L'interaction entre le vent solaire (VS) et la magnétosphère (MSP) terrestre se fait par l'intermédiaire de la magnétopause (MP). Le VS éjecté du Soleil, voyage transportant avec lui le champ magnétique interplanétaire (CMI). Ce dernier interagit avec le champ géomagnétique provoquant le phénomène de reconnexion magnétique (RM). La RM permet l'entrée d'une grande quantité de particules du VS dans la MSP. Si le CMI est dirigé vers le nord, la RM peut avoir lieu à haute latitude, mais n'est pas assez efficace pour justifier la quantité de plasma typique du VS, observée par les satellites à l'intérieur de la MSP. En outre, dans les cas où le CMI est dirigé vers le nord, la formation d'une couche de mélange est observée à basse latitude. Les tourbillons de Kelvin-Hemholtz (KH) fournissent un mécanisme efficace pour la formation d'une couche de mélange à la MP. Les simulations numériques montrent que l'évolution temporelle de l'instabilité de KH dépend fortement des profils initiales à grande échelle. La comparaison des données spatiales et des simulations numériques est donc d'une importance fondamentale dans ce contexte. Les principaux résultats obtenus au cours de ce travail sont la caractérisation de la turbulence à l'intérieur des tourbillons de KH, ainsi que des événements de RM à petite échelle; la sélection d'un événement où nous avons une combinaison des données des satellites avant et après KHI se développe; l'observation d'un décalage entre les profils de densité et de vitesse et constat que ce décalage initial entraîne une évolution différente de la simulations numériques qui est en accord avec les observations satellites. / Solar Wind (SW) and the Earth's magnetosphere interaction is mediated by the magnetopause. The SW carries with it the Interplanetary Magnetic Field (IMF) which interacts with northwards geomagnetic field lines causing magnetic reconnection (MR) events that make SW particles to be tranferred into the Earth's magnetosphere. If the IMF is directed northward, MR takes place at high latitude, but it is not efficient enough to justify the amount of SW plasma observed by satellites inside the magnetosphere. During northwards conditions one observe the formation of a wide boundary layer (BL) at the low latitude. This BL is thought to be driven by the the Kelvin-Helmholtz instability (KHI) , originating from the velocity shear between SW and the almost static near-Earth plasma. Numerical simulations (NS) have shown that the long time evolution of the KHI depends strongly on the initial large scale field profiles used as initial conditions. In order to make a further step towards the comprehension of this complex system, it is imperative to combine satellite data and NS. The idea here is to initialize NS by using in-situ observations of the main field profiles since only a correct initialization can reproduce the correct dynamics. The main results achieved in this work are: characterize the turbulence inside KH vortices and the small scale MR; select one event where there is a combination of a satellite measurements before and after KH develops, find that density and velocity profiles are shifted by a distance comparable to their shear lengths and that this initial shift cause a different evolution of the KHI leading to a final state in agreement with satellites observations.