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- Schillings, Audrey
(författare)
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How does O+ outflow vary with solar wind conditions?
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 2. |
- Slapak, Rikard
(författare)
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O+ heating, outflow and escape in the high altitude cusp and mantle
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Earth and its atmosphere are embedded in the magnetosphere, a region in space dominated by the geomagnetic field, shielding our planet as it acts to deflect the energetic solar wind. Even though the atmosphere is protected from direct interaction with the solar wind it is indirectly affected by significant magnetosphere-solar wind interaction processes, causing constituents of the upper atmosphere to flow up into the magnetosphere. The fate of the atmospheric originating ions is interesting from a planetary evolution point of view. If the upflowing ions in the magnetosphere are to escape into the solar wind they need to not only overcome gravity, but also the magnetic forces, and therefore need to be energized and accelerated significantly. The subject of this thesis is analysis of oxygen ions (O+) and wave field observations in the high altitude cusp/mantle and in the high latitude dayside magnetosheath of Earth, investigating magnetospheric processes behind ion heating, outflow and escape. Most data analysis is based on observational data from the Cluster satellites, orbiting the Earth and altitudes corresponding to different key regions of the magnetosphere and the immediate solar wind environment. The mechanism behind O+ heating mainly discussed in this thesis is energization through interactions between the ions and low-frequency waves. The average electric spectral densities in the altitude range of 8-15 Earth radii are able to explain the average perpendicular temperatures, using a gyroresonance model and 50% of the observed spectral density at the O+ gyrofrequency. Strong heating is sporadic and spatially limited. The regions of enhanced wave activity are at least one order of magnitude larger than the local gyroradius of the ions, which is a necessary condition for the gyroresonance model to be valid. An analysis indicates that enhanced perpendicular temperatures can be observed over several Earth radii after heating has ceased, suggesting that high perpendicular-to-parallel temperature ratio is not necessarily a sign of local heating. This also explains why we sometimes observe enhanced temperatures and low spectral densities. We also show that the phase velocities derived from the observed low frequency electric and magnetic fields are consistent with Alfvén waves. Outflowing ions flow along magnetic field lines leading downstream in the magnetotail, where the ions may convect into the plasma sheet and be brought back toward Earth. However, the effective heating in the cusp and mantle provides a majority of the O+ enough acceleration to escape into the solar wind and be lost, rather than entering the plasma sheet. The heating can actually be effective enough to allow outflowing cusp O+ to escape immediately from the high altitude cusp and mantle along recently opened magnetic field lines, facilitating a direct coupling between the magnetospheric plasma and interplanetary space. Observations in the shocked and turbulent solar wind (the magnetosheath) reveals hot O+ flowing downstream and approximately tangentially to the magnetopause and often close to it. An estimated total flux of O+ in the high-latitude magnetosheath of 0.7 ·1025 s-1 is significant in relation to the observed cusp outflows at lower altitudes, pointing to that escape of hot O+ from the cusp and mantle into the dayside magnetosheath being an important loss route.
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