The entire solar system including Earth is enveloped in a region of space where the Sun’s magnetic field dominates, this region is called the heliosphere. Due to this position in the heliosphere, a strong coupling exists between the Sun and our planet. The Sun continuously ejects particles, the solar wind, which is composed mainly of protons, electrons as well as some helium and heavier elements. These high energetic particles then hit the Earth and are partly deflected by the Earth’s magnetosphere (the region around Earth governed by the geomagnetic field). Depending on the strength of the solar wind hitting our planet, the magnetosphere is disturbed and perturbations can be seen down to the lower atmosphere.

The upper atmosphere is affected by short wave-length solar radiation that ionise the neutral atoms, this region is referred to as the ionosphere. In the ionosphere, some of the heavier ion populations, such as O+, are heated and accelerated through several processes and flow upward. In the polar regions (polar cap, cusp and plasma mantle) these mechanisms are particularly efficient and when the ions have enough energy to escape the Earth’s gravity, they move outward along open magnetic field lines. These outflowing ions may be lost into interplanetary space.

Another aspect that influences O+ ions are disturbed magnetospheric conditions. They correlate with solar active periods, such as coronal holes or the development of solar active regions. From these regions, strong ejections emerge, called coronal mass ejections (CMEs). When these CMEs interact with Earth, they produce a compression of the magnetosphere as well as reconnection between the terrestrial magnetic field lines and the interplanetary magnetic field (IMF) lines, which very often leads to geomagnetic storms. The energy in the solar wind as well as the coupling to the magnetosphere increase during geomagnetic storms and therefore the energy input to the ionosphere. This in turn increases the O+ outflow. In addition, solar wind parameter variations such as the dynamic pressure or the IMF also influence the outflowing ions.

Our observations are made with the Cluster mission, a constellation of 4 satellites flying around Earth in the key magnetospheric regions where we usually observe ion outflow. In this thesis, we estimated O+ outflow for different solar wind parameters (IMF, solar wind dynamic pressure) and extreme ultraviolet radiations (EUV) as well as for extreme geomagnetic storms. We found that O+ outflow increases exponentially with enhanced geomagnetic activity (Kp index) and about 2 orders of magnitude during extreme geomagnetic storms compared to quiet conditions. Furthermore, our investigations on solar wind parameters showed that O+ outflow increases for high dynamic pressure and southward IMF, as well as with EUV radiations. Finally, the fate of O+ ions from the plasma mantle were studied based on Cluster observations and simulations. These results confirm that ions observed in the plasma mantle have sufficient energy to be lost in the solar wind.