No description available.

ABSTRACT This Thesis investigates ionospheric plasma flows (commonly referred to as “convection”) at high latitudes with the objectives to assess seasonal and solar cycle variations in the shape of the flow patterns and the flow intensity in terms of external drivers of the flow, first of all the magnitude and orientation of the interplanetary magnetic field (IMF). Multi-year (2001-1011) line-of-sight Doppler velocity data collected by the Super Dual Auroral Network (SuperDARN) HF radars are considered. Two approaches are used: 1) analysis of monthly-averaged 2-dimentional patterns inferred from data of all SuperDARN radars operated and 2) analysis of near magnetic noon data from only two SuperDARN radars, Rankin Inlet and Inuvik monitoring meridional component of the flow in the near North Pole areas (polar cap). We show and discuss seasonal and solar cycle variations of three characteristics of the flows: magnetic latitudes of the region where plasma flow direction changes from toward the noon to away from the noon (convection reversal boundary), the magnetic local time location of the near noon region with stagnated flow (throat region) and, finally, the magnitude of the flow. All three parameters show trends, although not strong and consistent all the time, which agrees with previous publications where different analysis approaches and more limited data sets were used. For two specific points, one at the magnetic latitude of 72 degrees, representing the auroral oval latitudes (region where optical arcs occur most frequently) and the other one at 82 degrees, representing the polar cap latitudes we demonstrate that the average flow magnitude increases with the IMF intensity, and the effect is much stronger for the negative vertical component of the IMF Bz. In our second approach we demonstrate that the flow velocity increases almost linearly with an increase of the reconnection electric fields characterizing processes of interaction between the solar wind/IMF and the Earth`s magnetic dipole. Saturation effect is seen for strongest electric field. More clear seasonal effects are noticeable in these data; the velocity response to the reconnection electric field enhancement is stronger summer (winter) time for positive (negative) IMF Bz. The data are consistent with previous reports, where highly smoothed velocity data were considered.

Une étude théorique et expérimentale de la convection de Rayleigh-Bénard pour un fluide non-Newtonien rhéofluidifiant a été effectuée. L’approche théorique consiste en une analyse linéaire et faiblement non linéaire de l’instabilité thermo-convective d’une couche horizontale d’un fluide non-Newtonien, d’étendue supposée infinie dans le plan horizontal, chauffée par le bas et refroidie par le haut. Le comportement rhéofluidifiant est décrit par le modèle de Carreau. Pour ce modèle, les conditions critiques d’instabilité du régime conductif sont les mêmes que pour un fluide Newtonien. L’objectif de l’analyse faiblement non linéaire consiste à déterminer d’une part la valeur critique du degré de rhéofluidification à partir duquel la bifurcation primaire devient sous critique et d’autre part l’influence de rhéofluidification sur la sélection du motif de convection au voisinage des conditions critiques, en tenant compte d’un éventuel glissement à la paroi, d’une conductivité thermique finie de celle-ci et de la thermodépendance de la viscosité. Les conséquences sur le champ de viscosité et l’évolution du nombre de Nusselt sont caractérisées. L’approche expérimentale consiste à visualiser par ombroscopie les motifs de convection qui se développent dans une cellule cylindrique. Deux rapports d’aspect ont été considérés : AR = 3 et AR = 4. Les fluides utilisés sont des solutions aqueuses de Xanthan à différentes concentrations. L’influence du degré de rhéofluidification combiné avec la thermodépendance de la viscosité sur le domaine de stabilité des rouleaux et des hexagones ainsi que sur la zone de transitions rouleaux hexagones est mise en évidence / Theoretical and experimental study of Rayleigh-Bénard convection in a non-Newtonian shear-thinning fluid was performed. The theoretical approach consists in a linear and a weakly nonlinear of thermo-convective instability in a horizontal layer of a non-Newtonian fluid, assumed infinite in extent, heated from below and cooled from above. The rheological behavior of the fluid is described by the Carreau model. For this rheological model, the critical threshold is the same as for a Newtonian fluid. The objective of the weakly non linear analysis is to determine on one hand the critical value of the shear-thinning degree above which the bifurcation becomes subcritical and on the other hand, the influence of shear-thinning effects on the pattern selection near the onset, taking into account the possibility of wall slip, a finite thermal conductivity of the walls as well as the thermo-dependency of the viscosity. The impact on the viscosity field and on the evolution of the Nusselt number are characterized. The experimental approach consists in visualizing the convection patterns using the shadowgraph method in a cylindrical cell. Two aspect ratios were considered : AR = 3 and AR = 4. The fluids used are aqueous solutions of xanthan-gum at different concentrations. The influence of shear-thinning effects combined with the thermo-dependency of the viscosity on the stability domain of rolls and hexagons as well as on the transition between rolls and hexagons is highlighted