The complex interactions between the Sun and Earth are referred to as Space Weather. Key parameters include magnetic indices which quantitatively describe geomagnetic activity by determining a baseline that removes the background magnetic field and allows quantification of the remaining activity during geomagnetic events. However, most used indices have a low temporal resolution and rely on a sparse and frozen network of ground magnetic observatories. This thesis introduces a novel way of determining the baseline for future high temporal and spatial resolution magnetic indices. Firstly, the main phenomena and effects of Space Weather are outlined, followed by a review of currently used magnetic indices and their derivation. The computation of a novel baseline introduced in this work relies on basic statistical methods which are applied on magnetic data from a dense and flexible network of ground observatories for the period 1991-2016. The focus is on the investigation of geomagnetic quiet periods for which average annual activity at each observatory is determined. A global latitudinal normalisation function with dependency on solar activity for quiet periods is found. The analysis of the newly derived baseline shows that it provides the temporal, spatial and amplitudinal resolution needed to characterise geomagnetic disturbances adequately. The residual signal has the capability of being used as the basis for further quiet period studies. A first attempt of new indices based on the introduced derivation shows a good agreement with already existing high temporal and spatial resolution magnetic indices. Future indices derived with this baseline lay a favourable fundament for the application of articial intelligence methods.

We have been actively involved in the research and management activities of European Co-Operation in the Field of Scientific and Technical Research (EU COST) actions such as COST 238 Prediction and Retrospective Ionospheric Modeling over Europe (PRIME), COST 251 Improved Quality of Service in Ionospheric Telecommunication System Planning and Operation, COST 271 Effects of the Upper Atmosphere on Terrestrial and Earth-Space Communications, COST 296 Mitigation of Ionospheric Effects on Radio Systems (MIERS) and COST 724 Developing the Scientific Basis for Monitoring, Modeling and Predicting Space Weather. In this thesis High Frequency (3-30 MHz) (HF) radar system under ionospheric disturbances has been identified globally and some operational suggestions have been presented. The use of HF radar system is considered from the identification of ionospheric propagation medium point of view. Doppler velocity is considered as the characteristic parameter of the propagation medium. ap index is chosen as the parameter for disturbance characterization due to geomagnetic storms in the ionosphere. The main difficulty is the scarcity of data, which is rare and confidential. Therefore semi-synthetic data are generated. Dependence of Doppler velocity and group range of the echo signal on ap index is examined and some details of dependence are studied and demonstrated. Thus, effects of space weather on the ionosphere and as a result on HF radar wave propagation are displayed. These results are examples of system identification. This can be used in communication system planning and operation.

High Frequency (3-30 MHz) (HF) Ionospheric Channel is used for military, civilian and amateur communications. By using Ionosphere, communication for distances beyond the line of sight is achieved. The main advantage of this type of communication is that it does not to require a satellite to communicate with a point beyond the line of sight. Actually the Ionosphere is used instead of a satellite. To use Ionosphere but not a satellite means independent communication for a country. The disadvantage of HF Ionospheric Communication is that the characteristics of the reflecting media (i.e. channel&rsquo / s transfer function) depends on many variables, e.g. sun spot number, hour of the day, season, solar cycles etc., so that mathematically modeling the channel is very difficult. Since military standards like STANAG 4538, STANAG 4285, STANAG 4415, MIL-STD-188-110A and MIL-STD-188-141A define the required performance of an HF modem in terms of Signal to Noise Ratio (SNR), Doppler Spread and Delay Spread according to desired conditions, a new approach to characterize the channel in terms of these three parameters is presented. In this thesis, HF Channel is considered as a system which involves various physical and chemical processes. A new method to characterize the HF channel to be used for modem performance evaluation is presented. In this study, it is aimed to relate modem/channel availability with the magnetic indices, which may be considered as the disturbances to the system. For this purpose the data taken from an HF communication experiment is used to model the channel to be used for modem availability calculations. The aim of the study is to asses the HF Channel Availability under Ionospheric Disturbances. This new technique will be a useful tool for HF Modem operators to select the optimum data rate or modulation method during HF Communication.