International audience; The radiation belts are a key region located close to the Earth, where the satellites travel. They are located in the centre of the magnetosphere and constitute a region sensitive to the variations of magnetosphere activity. The magnetosphere is in equilibrium in the solar wind. If the solar wind parameters change, then, the magnetospheric balance is upset. Using several processes, particles and energy from the solar wind can enter it, disturbing the magnetosphere and the radiation belts. In this paper, the am index has been used to define a new parameter named Cm, which is indicative of the energy level in the magnetosphere. The impact of CIRs (Corotating Interaction region) and of CMEs (Coronal Mass Ejection) on the magnetosphere has been studied from the Cm point of view, as well as the reaction of the radiation belts to a solar wind disturbance. The results show that the Cm parameter provides a new perspective in space weather studies as it clearly shows that the energy level can be higher for a CIR than for a CME. It also demonstrates that the events with several solar wind structures are much more effective to increase the energy level in the magnetosphere than single ones. Finally, Cm correlates better with the radiation belts fluxes, showing again that Cm is a good indicator of the inner magnetosphere activity. Nevertheless, the energy level in the radiation belts is maximised and the energy level in this population cannot go above a given value which depends on the altitude. The particles coming from the plasmasheet also push the particles from the highest altitudes to the lower ones, allowing the slot filling for Cm> .