Ground-based imaging and broad beam riometers are used in conjunction with ionospheric radars and satellite instruments to investigate high-energy precipitation in the auroral zone. There are two dominant precipitation regimes in the auroral zone which lead to enhanced high frequency radio absorption; high energy electrons (> keV) from closed field lines, and protons (> MeV) penetrating from the solar wind following solar flares. Much of the work in this thesis uses data from riometers in Fennoscandia to measure the extent and movement of energetic precipitation from both sources. A case study of dayside absorption combines data from the imaging riometer with radar and satellite observations leading to an estimation of the energy of precipitation based on the riometer data. Two separate precipitation mechanisms were identified in the case study through the use of satellite particle measurements and ground-based observations of geomagnetic pulsations. The riometer showed varying movements of the absorption patches through the case study and a determination of different dominating particle drift regimes was possible through comparison with coherent HF radar. A statistical analysis of absorption in the imaging riometer field of view is carried out. The absorption is linked to both Kp and solar wind velocity using linear and quadratic fits of the data. The daily variation and distribution of absorption is investigated along with seasonal effects which are shown to be reliant on geomagnetic activity. A study of the large number of solar proton events from 1995 to 2001 inclusive is carried out with particular reference to those that produce significant absorption in the northern hemisphere polar cap (polar cap absorption –PCA). The occurrence of the absorption events is investigated and a simple empirical relationship between the integral proton flux and the absorption observed during geomagnetically undisturbed PCA conditions is developed.

High power electromagnetic waves are able to modify the Earth’s ionosphere. A number of plasma waves and instabilities may be excited in this way, including patches of small-scale field-aligned electron density irregularities. The EISCAT heater, situated in northern Norway, has been used to transmit high power, high frequency radio waves into the ionosphere since its construction in the early 1980s. The CUTLASS coherent backscatter radars have been able to measure the consequent electron density perturbations in the ionosphere above the heater since their construction in the mid 1990s. This thesis contains a statistical study of CUTLASS Finland backscatter power measurements of patches of irregularities excited by the EISCAT heater during campaigns carried out between 1996 and 2002. A study of CUTLASS backscatter power measurements made during heater transmissions carried out at a fixed heater beam pointing direction, frequency and power has provided fundamental information on the effect of these heater beam parameters on the irregularities. A study of a selection of backscatter data from above the SPEAR heating facility has shown similar results. Backscatter data corresponding to experiments involving heater beam-sweeping and power-stepping have also been analysed. During beam-sweeping experiments the heater beam direction is steadily changed from northwards- to southwards-pointing. During power-stepping experiments the heater beam power is steadily increased and decreased. The results from these experiments give compelling evidence in support of the upper hybrid theory of irregularity excitation. Comparisons are made between statistical parameters of CUTLASS backscatter power distributions and modelled heater beam power distributions provided by the EZNEC4 software. In general a good agreement between the statistical parameters is observed, indicating the large influence of the heater beam on the irregularities. The influence of the Earth’s magnetic field is also evident and often leads to discrepancies between the statistical parameters of the two sets of distributions.

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Signals broadcast by the Global Navigation Satellite Systems (GNSS) enable global, autonomous, geo-spatial positioning exploited in the areas such as geodesy, surveying, transportation and agriculture. The propagation of these signals is affected as they propagate through the Earth's upper atmosphere, the ionosphere, due to the ionic and electronic structure of the ionosphere. The ionosphere, a highly dynamic and spatially and temporally variable medium, can be the largest error source in Global Navigation Satellite System (Klobuchar 1991) in the absence of the Selective Availability. Propagation effects due to the ionosphere lead to errors in the range measurements, impact on receiver signal tracking performance and influence the GNSS positioning solution. The range error can vary from 1 to 100m depending on time of day, season, receiver location, conditions of the earth's magnetic field and solar activity (Hofmann-Wellenhof et al. 2001). This thesis focuses on modelling, monitoring and mitigating the ionospheric effects in GNSS within the scope of GNSS modernization, which introduces new signals, satellites and constellations. The ionosphere and its effects on GNSS signals, impact of the ionospheric effects at the receiver end, predicted error bounds of these effects under different solar, geomagnetic and ionospheric conditions, how these effects can be modelled and monitored with current and new (possible with GNSS modernization) correction approaches, degradation in the GNSS positioning solution and mitigation techniques to counter such degradation are investigated in this thesis. Field recorded and simulated data are considered for studying the refractive and diffractive effects of the ionosphere on GNSS signals, signal tracking performance and position solution. Data from mid-to-high latitudes is investigated for the refractive effects, which are due to dispersive nature of the ionosphere. With the use of multi-frequency, multi-constellation receivers, modelling of the refractive effects is discussed through elimination and estimation of these effects on the basis of dual and triple frequency approaches, concentrating on the benefit of the new GNSS signals. Data from the low latitudes is considered for studying the diffractive effects of the ionosphere, scintillation in particular, in GNSS positioning, and possible mitigation techniques to counter them. Scintillation can have a considerable impact on the performance of GNSS positioning by, for instance, increasing the probability of losing phase lock with a signal and reducing the accuracy of pseudoranges and phase measurements. In this sense, the impact of scintillation on signal tracking performance and position solution is discussed, where a novel approach is proposed for assessing the variance of the signal tracking error during scintillation. The proposed approach also contributes to the work related with scintillation mitigation, as discussed in this thesis. The timeliness of this PhD due to the recent and increasingly active period of the next Solar Cycle (predicted to reach a peak around 2013) and to the ongoing GNSS modernization give this research an opportunity to enhance the ionospheric knowledge, expertise and data archive at NGI, which is rewarding not only for this PhD but also for future research in this area.