The region of space dominated by the Sun's magnetic field is called the heliosphere. It envelops the entire solar system including Earth. Therefore, a strong coupling exists between the Sun and our planet. The Sun continuously ejects particles, the solar wind, and when these high energy particles hit Earth, the magnetosphere (the region around the Earth governed by the geomagnetic field) is affected. When the solar wind is enhanced this disturbs the magnetosphere and perturbations can be seen also in ground-based observations.

The upper atmosphere is subjected to solar radiation that ionise the neutral atoms and molecules, 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 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 and may be lost into interplanetary space. Ion outflow in general has already been well studied, however, ion outflow under extreme magnetospheric conditions has not been investigated in detail.

Disturbed magnetospheric conditions correlate with solar active periods, such as coronal holes or the development of solar active regions. From these regions, strong ejections called coronal mass ejections (CMEs) emerge. When these extreme events 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 most of the time leads to geomagnetic storms. The amounts of incoming solar particles and energy increase during geomagnetic storms and we also observe an increase in the O⁺ outflow.

Our observations are made with the Cluster mission, a constellation of 4 satellites flying around Earth in the key magnetospheric regions where ion outflow is usually observed. In this thesis, we estimate O⁺ outflow under disturbed magnetospheric conditions and for several extreme geomagnetic storms. We find that O⁺ outflow lost into the solar wind increases exponentially with enhanced geomagnetic activity (Kp index) and increases about 2 orders of magnitude during extreme geomagnetic storms.

Den del av rymden som domineras av solens magnetfält kallas heliosfären. Helios-fären omfattar hela solsystemet inklusive jorden, vilket gör att det finns en starkkoppling mellan solen och jorden. Solen sänder oavbrutet ut laddade partiklar in denså kallade solvinden och när dessa energika partiklar träffar jorden påverkas mag-netosfären (det område kring jorden där det geomagnetiska fältet dominerar). Närsolvinden är starkare än vanligt uppstår störningar. I magnetosfären som ger effektersom kan uppmätas med markbaserade instrument.

Den övre atmosfären utsätts för strålning från solen som joniserar atomer ochmolekyler, och formar det område som kallas jonosfären. Några av de tyngre jonpop-ulationerna i jonosfären, som till exempel syrejoner, kan hettas upp och accelererasgenom flera olika möjliga processer. Detta gör att de flödar uppåt i atmosfären. Ipolarområdena är dessa mekanismer särskilt effektiva och om tillräckligt med energitillförs jonerna kan gravitationen övervinnas, vilket gör att jonerna flödar upp längsöppna magnetfältlinjer och kan gå förlorade ut i den interplanetära rymden. Generelltsett har jonutflöde redan studerats väl, men jonutflöde under extrema magnetosfäriskaförhållanden har inte undersökts i detalj.

Störda magnetosfäriska förhållanden korrelerar med då solen är aktiv, som tillexempel koronahål eller under utvecklingen av aktiva solområden. Från dessa områ-den härstammar koronamassautkastningar. När dessa extrema händelser når jordenkomprimeras magnetosfären och det geomagnetiska och interplanetära magnetiskafältet omkopplas, vilket ofta leder till geomagnetiska stormar. Under dessa införsstora mängder av partiklar i solvinden och energi till magnetosfären, och ett högresyrejonsutflöde är också observerat.

Data från Clustersatelliterna har använts; dessa utgörs av fyra satelliter i for-mation i omloppsbana kring jorden. Plasmaområdena där de befinner sig är därjonutflödet vanligtvis observeras. Denna avhandling behandlar syrejonsutflöde understörda magnetosfäriska förhållanden och flera extrema geomagnetiska stormar. Detvisas att syrejonsutflödet som förloras till solvinden ökar exponentiellt med geomag-netiskt aktivitet (Kp-index) och ökar med upp till 2 storleksordningar under extremageomagnetiska stormar.

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.