Les satellites en orbite terrestre se déplacent dans le plasma ionosphérique, un mélange de particules chargées, et éventuellement de particules neutres. Des électrons et des ions issus de ce plasma, ainsi que les émissions Ultra-Violets(UV) en provenance du soleil, interagissent avec les surfaces du satellite et modifient sa charge électrostatique. Cette chargement peut induire elle-même des décharges électrostatiques aux conséquences allant de perturbations électromagnétiques (fausses commandes par exemple) à la perte du satellite. En orbites de basse altitude (LEO) l'énergie cinétique et thermique du plasma est généralement faible et donc, les satellites vont rarement présenter des décharges importantes. Néanmoins, les missions scientifiques qui embarquent des instruments très performants et précis peuvent être affectées par cette interaction satellite-plasma-émissions UV. Cette thèse s'intéresse particulièrement à ces phénomènes de charge des structures externes du satellite et à l'impact de ce chargement sur les mesures scientifiques effectuées à bord, i.e. mesures du champ électrique et de la densité du plasma thermique. / Earth-orbiting satellites travel in ionospheric plasma, a mixture of charged particles, and possibly neutral particles. Electrons and ions from this plasma, as well as Ultra-Violet (UV) emissions from the sun, interact with the surfaces of the satellite and modify its electrostatic charge. This loading can itself induce electrostatic discharges to the consequences ranging from electromagnetic disturbances (false commands for example) to the loss of the satellite. In low-Earth orbits (LEO), the kinetic and thermal energy of the plasma is generally low and therefore satellites rarely exhibit large discharges. Nevertheless, scientific missions that carry high-performance and accurate instruments can be affected by this satellite-plasma-UV-emissions interaction. This thesis is particularly interested in these phenomena of charge of the external structures of the satellite and the impact of this load on the scientific measurements carried out on board, i.e. measures of the electric field and the density of the thermal plasma.

This thesis has two major objectives. The first objective is to investigate the seasonal and diurnal variations in occurrence of HF coherent echoes. We assess F-region echo occurrence rates for the PolarDARN HF radars at Inuvik (INV) and Rankin Inlet (RKN) and the auroral zone SuperDARN radars at Saskatoon (SAS) and Prince George (PGR) for the period of 2007-2010. We show that the INV and RKN PolarDARN radars show comparable rates of echo occurrence all the time and they detect 1.5-2.5 times more echoes through ½-hop propagation mode (MLATs=80°-85°) than the SAS and PGR SuperDARN radars through 1½-hope propagation mode (MLATs=75°-80°). For all four radars, the winter occurrence rates are about ~2 times higher than the summer rates. For observations in the dusk, midnight and dawn sectors, equinoctial maxima are evident. The pattern of echo occurrence in terms of MLT/season is about the same for all radars with clear maxima near noon during winters and summers and enhanced (as compared to other time of the day) occurrence rates during equinoctial dusk and dawn hours. Additionally, to investigate the effect of solar cycle on occurrence of F-region echoes, we consider the near noon and near midnight echo occurrence rates for the Saskatoon radar over the period of 1994-2010. We show that there is a strong, by a factor of ~10, increase in SAS night-side echo occurrence towards solar maximum. The effect does not exist for the dayside echoes; moreover, a decrease in number of echoes, by a factor of ~2, was discovered for the declining phase of the solar cycle. The second objective is to evaluate the electron density and the electric field as factors controlling the occurrence of F-region echoes. We use observations of these two ionospheric parameters measured by CADI ionosonde and RKN observations of echo occurrence rates over Resolute Bay (MLAT=83°). We show that there is a correlation in changes of echo occurrence and electron density changes for 3 years of radar-ionosonde joint operation (2008-2010). The comparison of radar-ionosonde data shows that the enhanced echo occurrence at near noon hours during summer months correlate with the enhanced electric field during these periods.