Browsing by Author "Lipsanen, Veera"

The constant outflow of solar wind from the Sun and possible larger structures within it influence the Earth's magnetosphere. These large structures include interplanetary coronal mass ejections (ICMEs) and high speed streams (HSSs). They can contain substructures: fast enough ICMEs can have a turbulent sheath region in front of them, while a HSS can interact with the slower ambient solar wind and form a stream interaction region (SIR). Pc5 Ultra-low frequency (ULF) waves have a frequency range of 2–7 mHz and they are important in energy transfer from solar wind to the magnetosphere and they affect energetic electrons in the radiation belts. ULF waves in the magnetosphere are generated by multiple mechanisms. For example fluctuations in solar wind's dynamic pressure create waves on the dayside, Kelvin-Helmholtz instability, often caused by HSSs, on the magnetopause flanks and substorms on the nightside. This makes ULF waves MLT dependent. For this thesis a new ground-based ULF index that is MLT dependent and has a resolution of 1 min is constructed using the wavelet analysis method. The aim of this thesis is to study how this new index correlates with multiple solar wind parameters, geomagnetic indices and an already existing ULF index during substructures of four events: weak HSS and ICME and strong HSS and ICME. The ULF power is found to peak at the sheath–ejecta boundary during ICMEs and the stream interface during HSSs, primarily driven by dawn and night ULF powers. The AE index is found to correlate with ULF power during all of the events, which indicates that even the non-geoeffective event produces some kind of substorm activity. Solar wind speed is found to correlate well with the ULF power during SIRs. It is important to take into account the MLT dependence of ULF waves since their generation mechanisms are different at different parts of the magnetosphere. In addition, we found that the ULF power in the four MLT sectors can behave differently at the same moment.