Nowadays our society is extremely dependent on all kinds of satellite systems, which are inherently exposed to geomagnetic phenomena induced by the solar activity. Van Allen radiation belts, toroidal structures populated of energetic particles trapped in the Earth's magnetic field, have been studied extensively since their discovery in the late 1950’s. Radiation belts sources are manifold and the physical processes driving their dynamics are complex. Despite of the theoretical knowledge accumulated over time, uncertainties remain on the physics-based mechanisms underpinning both the temporal and spatial evolution of the distributions of trapped energetic particles. More specifically, the sensitivity of particle distributions in the belt to changes in geomagnetic conditions, such as geomagnetic storms, are still not fully understood. This context constitutes the crux of the motivation for the present thesis, which provides a comprehensive review of the theory behind radiation belt dynamics beforehand, subsequently addressing a few of the aforementioned physical uncertainties. Through the study of a new satellite observation dataset, this work shows how geomagnetic conditions affect the particle fluxes and especially reveals more insight on the presence of high energy electrons in the inner belt, a still-open question in this field of research. As such, this thesis represents a contribution towards both consolidating the radiation belts theory and improving our current space weather forecast capabilities, ultimately vital for all satellite-reliant human activities. (SC - Sciences) -- UCL, 2018