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A dual-frequency GPS receiver, GSV4004B, has been installed at Alert, Canada since May 2008 to study the ionospheric variability at a high-latitude location. This Ionospheric Scintillation and TEC Monitor (GISTM) receiver is able to measure amplitude and phase scintillation, and also Total Electron Content (TEC). In addition, a High Frequency (HF) link has been established from Qaanaaq to Svalbard since March 2009 to study the channel characteristics, including Doppler spreads. This study covers from end of May 2008 to February 2011 where it was mostly during the minimum state of solar activity. During the period of where both GISTM and HF data were available (i.e. March 2009-July 2010), data from both links are compared to see any relation between TEC variations and scintillation effects and also Doppler spreads. Data from the GISTM receivers at Svalbard were also utilised. Winter months expectedly show lower mean vertical TEC (VTEC) than in other months. Higher mean VTEC was observed in 2010 that could be related to the increase of solar activity. In 2010, both small and large patches of moderate-high intensity were found to be more than twice as common in February and March than in other months. The difference in the occurrences of small patches between months of the same year was about 1-2% (for low intensity patches) and 1-10% (moderate-high intensity). This difference increased to 1-4% (low intensity) and 1-12% (moderate-high intensity) for large patches. UT dependence can be seen where ~80 large and ~300 small patches were identified ~1000 to 2000 UT from June to October in both 2008 and 2009, and they doubled in 2010. Amplitude scintillations index, S4 were very low where at least ~99% were between 0 and 0.1, which is insignificant. Meanwhile, phase scintillations index, σφ were from 0 to 0.1 rad for at least 93% of the time. The relationship of TEC fluctuations/increase and phase scintillations is complex. There are few events where phase scintillations occurred simultaneously with slant TEC increases, but this is not frequent. The magnitude of the irregularities was generally independent of geomagnetic indices, Interplanetary Magnetic Field (IMF) components, and local magnetic fields at Eureka and Resolute Bay. Monthly upper decile of Doppler spreads generally varied from 1 to 5.5 Hz. High upper decile normally occurred from around September/October to March, from ~0000 to 1700 UT on 6.95 and 8.01 MHz. The hourly upper decile reached up to 7-8.5 Hz (6.95 MHz) and 7-9.5 Hz (8.01 MHz). Upper decile on these frequencies was mostly 0.5-4.5 Hz larger in October 2010-February 2011 than in the same months of the previous year which possibly related to the increase in solar activity. However, Doppler spreads were generally independent of independent of geomagnetic indices, IMF components, and local magnetic fields. The relation between the irregularities observed via transionospheric link and HF link is also complex. Large TEC fluctuations/increases and/or phase scintillations observed on satellite(s) close to HF midpoint may correspond to either small or large Doppler spreads.

The Earth ionosphere becomes highly variable during extreme space-weather events. Observations from the Global Positioning System (GPS) in conjunction with tomographic imaging have the potential to characterise its disturbed behaviour. In this project, a four-dimensional tomographic technique is employed to reveal the temporal evolution and spatial distribution of ionisation in the ionosphere on a large scale (over Europe and the USA). The effects of geomagnetic storms on the dynamics of the plasma are investigated throughout the recent solar maximum, when the intensity of geornagnetic storms is greatest. The results presented in this thesis show, for the first time, a wide area view of the ionosphere during storm-time. Electron-density images of the disturbed mid/high latitude ionosphere and Total Electron Content (TEC) maps are produced over North America and Europe for several severe storms. Using both ground-based and Low Earth Orbit satellite-based GPS data as well as available ionosonde data during storm periods, improvements in imaging of ionospheric structures at storm time are made. A case-study for the 29th - 30th October 2003 storm shows the detailed evolution of ionisation in space and in time throughout the entire period of a complex storm. The peak heights of the disturbed F region were redistributed (uplifted) both over Europe and the USA around dusk for several major storm events. This uplift propagated westwards. Investigations into common features of storms indicated that positive or negative storm effects correlated with local time. In addition, the strongest enhancement in TEC and the highest uplift were in the USA sector. The project achievements are twofold. Firstly, abnormal behaviour of the disturbed ionosphere,such as enhancements in electron density and TEC and sudden uplifts of the peak height, is shown using a new GPS imaging technique. Secondly, the results will provide important experimental inputs into physical models of ionospheric storms and also contribute to the further understanding the dynamics of ionisation and underlying mechanisms of severe storms on a global scale.

In this thesis techniques are developed for the simulation of partially ionised plasmas in the fluid approximation. These techniques are used to model the evolution of magnetic fields in the partially ionised regions of the solar atmosphere. Single fluid equations for a partially ionised plasma are derived based on the individual equations for each species. A Lagrangian Remap MHD code is then adapted to simulate a plasma of arbitrary degree of ionisation. The effects of the presence of neutrals on the propagation and damping of Alfv´en waves in the solar atmosphere are investigated. Ion-neutral collisions are shown to be an efficient damping mechanism for outwardly propagating Alfv´en waves of frequencies greater than 0.1 Hz, showing that high frequency waves in the outer solar atmosphere cannot originate at the surface of the Sun. Next simulations to show the effects of neutrals on the emergence of magnetic flux from beneath the solar surface into the outer atmosphere are performed. Results from 2D and 3D numerical experiments show that the presence of neutrals increases the amount of magnetic flux that can emerge into the corona. Furthermore, ion-neutral collisions are strong enough to dissipate currents perpendicular to the magnetic field as it emerges. This shows that ion-neutral collisions are a viable mechanism for the formation of force-free (j ∧B = 0) coronal magnetic field from sub-surface field, which is not the case when the plasma is assumed to be fully ionised.