The ability to predict the future behavior of solar activity has become of extreme importance due to its effect on the near-Earth environment. Predictions of both the amplitude and timing of the next solar cycle will assist in estimating the various consequences of Space Weather. Several prediction techniques have been applied and have achieved varying degrees of success in the domain of solar activity prediction. These techniques include, for example, neural networks and geomagnetic precursor methods. In this thesis, various neural network based models were developed and the model considered to be optimum was used to estimate the shape and timing of solar cycle 24. Given the recent success of the geomagnetic precusrsor methods, geomagnetic activity as measured by the aa index is considered among the main inputs to the neural network model. The neural network model developed is also provided with the time input parameters defining the year and the month of a particular solar cycle, in order to characterise the temporal behaviour of sunspot number as observed during the last 10 solar cycles. The structure of input-output patterns to the neural network is constructed in such a way that the network learns the relationship between the aa index values of a particular cycle, and the sunspot number values of the following cycle. Assuming January 2008 as the minimum preceding solar cycle 24, the shape and amplitude of solar cycle 24 is estimated in terms of monthly mean and smoothed monthly sunspot number. This new prediction model estimates an average solar cycle 24, with the maximum occurring around June 2012 [± 11 months], with a smoothed monthly maximum sunspot number of 121 ± 9.

This thesis presents an update to the ionospheric propagation factor, M(3000)F2, global empirical model developed by Oyeyemi et al. (2007) (NNO). An additional aim of this research was to produce the updated model in a form that could be used within the International Reference Ionosphere (IRI) global model without adding to the complexity of the IRI. M(3000)F2 is the highest frequency at which a radio signal can be received over a distance of 3000 km after reflection in the ionosphere. The study employed the artificial neural network (ANN) technique using relevant geophysical input parameters which are known to influence the M(3000)F2 parameter. Ionosonde data from 135 ionospheric stations globally, including a number of equatorial stations, were available for this work. M(3000)F2 hourly values from 1976 to 2008, spanning all periods of low and high solar activity were used for model development and verification. A preliminary investigation was first carried out using a relatively small dataset to determine the appropriate input parameters for global M(3000)F2 parameter modelling. Inputs representing diurnal variation, seasonal variation, solar variation, modified dip latitude, longitude and latitude were found to be the optimum parameters for modelling the diurnal and seasonal variations of the M(3000)F2 parameter both on a temporal and spatial basis. The outcome of the preliminary study was applied to the overall dataset to develop a comprehensive ANN M(3000)F2 model which displays a remarkable improvement over the NNO model as well as the IRI version. The model shows 7.11% and 3.85% improvement over the NNO model as well as 13.04% and 10.05% over the IRI M(3000)F2 model, around high and low solar activity periods respectively. A comparison of the diurnal structure of the ANN and the IRI predicted values reveal that the ANN model is more effective in representing the diurnal structure of the M(3000)F2 values than the IRI M(3000)F2 model. The capability of the ANN model in reproducing the seasonal variation pattern of the M(3000)F2 values at 00h00UT, 06h00UT, 12h00UT, and l8h00UT more appropriately than the IRI version is illustrated in this work. A significant result obtained in this study is the ability of the ANN model in improving the post-sunset predicted values of the M(3000)F2 parameter which is known to be problematic to the IRI M(3000)F2 model in the low-latitude and the equatorial regions. The final M(3000)F2 model provides for an improved equatorial prediction and a simplified input space that allows for easy incorporation into the IRI model.

The South African Global Navigation Satellite System (GNSS) network of dual frequency receivers provide an opportunity to investigate solar cycle effects on ionospheric Total Electron Content (TEC) over the South Africa region by taking advantage of the dispersive nature of the ionospheric medium. For this task, the global University of New Brunswick Ionospheric Modelling Technique (UNB-IMT) was adopted, modified and applied to compute TEC using data from the southern African GNSS Network. TEC values were compared with CODE International GNSS services TEC predictions and Ionosonde-derived TEC (ITEC) measurements to test and validate the UNB-IMT results over South Africa. It was found that the variation trends of GTEC and ITEC over all stations are in good agreement and show pronounced seasonal variations with high TEC values around equinoxes for a year near solar maximum and less pronounced around solar minimum. Signature TEC depletions and enhanced spikes were prevalently evident around equinoxes, particularly for a year near solar maximum. These observations were investigated and further discussed with an analysis of the midday Disturbance Storm Time (DST) index of geomagnetic activity. The residual GTEC – ITEC corresponding to plasmaspheric electron content and equivalent ionospheric foF2 and total slab thickness parameters were computed and comprehensively discussed. The results verified the use of UNB-IMT as one of the tools for ionospheric research over South Africa. The UNB-IMT algorithm was applied to investigate TEC variability during different epochs of solar cycle 23. The results were investigated and further discussed by analyzing the GOES 8 and 10 satellites X-ray flux (0.1 – 0.8 nm) and SOHO Solar Extreme Ultraviolet Monitor higher resolution data. Comparison of UNB-IMT TEC derived from collocated HRAO and HARB GNSS receivers was undertaken for the solar X17 and X9 flare events, which occurred on day 301, 2003 and day 339, 2006. It was found that there exist considerable TEC differences between the two collocated receivers with some evidence of solar cycle dependence. Furthermore, the daytime UNB TEC compared with the International Reference Ionosphere 2001 predicted TEC found both models to show a good agreement. The UNB-IMT TEC was further applied to investigate the capabilities of geodetic Very Long Baseline Interferometry (VLBI) derived TEC using the Vienna TEC Model for space weather monitoring over HartRAO during the CONT02 and CONT05 campaigns conducted during the years 2002 (near solar maximum) and 005 (near solar minimum). The results verified the use of geodetic VLBI as one of the possible instruments for monitoring space weather impacts on the ionosphere over South Africa.

The use of ray tracing in the analysis of certain daytime ionograms recorded at Grahamstown is discussed in this thesis. A computer program has been modified and used to trace rays in the frequency range 1 - 30 MHz. Vertical, short distance oblique, and long distance oblique ionograms have been synthesized from the results and compared with experimental ionograms for Grahamstown, the Alice - Grahamstown transmission path (64 km), and the SANAE - Grahamstown transmission path (4470 km) respectively. Ray paths have been calculated and related in detail to the models of the ionosphere and geomagnetic field. The main features of the vertical and short distance oblique ionograms can, in general, be reproduced using spherically stratified ionosphere models with electron density profiles derived from vertical ionograms. A suitable model for the geomagnetic field is a tilted dipole equivalent to the actual field at Grahamstown. The two-hop mode is shown to be, usually, the lowest on the long distance oblique records. The ionosphere model is the principal limiting factor in reproducing such ionograms, and the most satisfactory results have been those obtained with a model in which electron density is assumed to vary linearly with latitude between the profiles at SANAE and Grahamstown. The promising results obtained by ray tracing with normal ionospheric conditions indicate that the method has further possibilities which could usefully be explored. / KMBT_363 / Adobe Acrobat 9.54 Paper Capture Plug-in

L’objectif de la thèse était d’analyser deux types de perturbations de l’ionosphère supérieure observées sur les mesures de plasma fournies par le satellite DEMETER.Les premières sont des Perturbations Ionosphériques Itinérantes de moyenne échelle engendrées par l’action sur le plasma des ondes de gravité atmosphériques qui se propagent jusqu’à haute altitude. Elles prennent la forme de variations quasi-périodiques de densité du plasma qui peuvent atteindre des amplitudes considérables et sont plus fréquentes dans l’hémisphère Sud avec un maximum au-dessus de l’Océan Pacifique. Elles peuvent modifier l’électrodynamique de l’ionosphère et amplifier le champ électrique d’origine dynamo.Les secondes sont observées dans l’ionosphère équatoriale comme des augmentations à grande échelle de la densité du plasma sous la forme de plateaux. En fonction des conditions d’activité magnétique, des dépressions de densité à moyenne échelle peuvent apparaitre sur les plateaux et conduire à la formation des bulles de plasma équatoriales. / This thesis aims at analyzing two kinds of ionospheric disturbances observed on plasma measurements on-board the DEMETER satellite.The first events are Mid-Scale Traveling Ionospheric Disturbances that develop through the interaction between atmospheric gravity waves and the ionospheric plasma. They are observed as quasi-periodic variations of the plasma density that may reach very large amplitudes and are more frequently observed in the Southern hemisphere with a maximum over the Pacific Ocean. These MSTID may strongly modify the electrodynamics of the mid latitude ionosphere and form structures where the dynamo electric field is significantly enhanced.The second events are detected in the equatorial ionosphere as large scale enhancements of the plasma density under the form of plateau. Depending on the level of magnetic activity these large scale structures may be modified by mid-scale density depletions that, eventually, get instable and led to the formation of depleted plasma bubbles.

In the search for natural orbital angular momentum (OAM) effects, some of the first incoherent scatter experiments with twisted radar beams during aurora were conducted at Poker Flat Incoherent Scatter Radar (PFISR), Alaska, USA, in October 2012. Experimental data of scatter from beam configurations with opposite twists were investigated. By the use of hypothesis tests in combination with Monte Carlo simulations together with traditional estimations of the mean and confidence interval, asymmetries between scatter of radar beams with opposite twists were identified for an integration time of at least 30 minutes. Asymmetries were detected in the internal radar noise too, but not necessarily with the same signs as for the asymmetries from the ionospheric signals. The asymmetries identified could be due to amplified noise for signals coming from intense aurora or perhaps the rectangular-shaped antenna array used at PFISR. These two possible causes need to be ruled out before suggesting that the asymmetries identified are an outcome of OAM effects present in the ionosphere.

The accessibility of the Martian atmosphere to spacecraft provides an opportunity to study an ionosphere that differs from our own. Yet, despite the half century of measurements made at Mars, the current state of the neutral atmosphere and its embedded plasma (ionosphere) remains largely uncharacterized. In situ measurements of the neutral and ionized constituents versus height exist only from the two Viking Landers from the 1970s. Subsequent satellite and remote sensing data offer sparse global coverage of the ionosphere. Thermal characteristics of the plasma environment are not well understood. Patchy crustal magnetic fields interact with the Martian plasma in a way that has not been fully studied. Hence, investigating the coupled compositional, thermal and crustal-field-affected properties of the ionosphere can provide insight into comparative systems at Earth and other planets, as well as to atypical processes such as the solar wind interaction with topside ionospheric plasma and associated pathways to escape. Ionospheric models are fundamental tools that advance our understanding of complex plasma systems. A pre-existing one-dimensional model of the Martian ionosphere has been upgraded to include more comprehensive chemistry and transport physics. This new BU Mars Ionosphere Model has been used to study the composition, thermal structure and dynamics of the Martian ionosphere. Specifically: the sensitivity of the abundance of ions to neutral atmospheric composition has been quantified, diurnal patterns of ion and electron temperatures have been derived self-consistently using supra-thermal electron heating rates, and the behavior of ionospheric plasma in crustal field regions was simulated by constructing a two-dimensional ionospheric model. Results from these studies were compared with measurements and show that (1) ion composition at Mars is highly sensitive to the abundance of neutral molecular and atomic hydrogen, (2) lighter ions heat up more efficiently than heavier ones and provide additional heating sources for cooler plasma, and (3) crustal field morphology affects plasma dynamics and structure at Mars in a way that is consistent with observations. Finally, model predictions of ion composition and plasma temperatures are provided for observations to be made by several instruments on board the upcoming 2013 MAVEN orbiter.

The ionospheric response to solar extreme ultraviolet (EUV) variability during 2011–2014 is shown by simple proxies based on Solar Dynamics Observatory/Extreme Ultraviolet Variability Experiment solar EUV spectra. The daily proxies are compared with global mean total electron content (TEC) computed from global TEC maps derived from Global Navigation Satellite System dual frequency measurements. They describe about 74% of the intra-seasonal TEC variability. At time scales of the solar rotation up to about 40 days there is a time lag between EUV and TEC variability of about one day, with a tendency to increase for longer time scales.

In this work I present a study dedicated to the penetration of whistler- mode waves to the ionosphere. An algorithm of automatic detection of whist- lers in spectrograms computed from the data measured on the DEMETER sa- tellite is described. A method of causative lightning detected by the EUCLID lightning detection network assignment to a detected whistlers is described. Results of statistical study dedicated to relationship between the detected whistlers and assigned causative lightning. Based on the proccessing of data from 364 passes of the DEMETER satellite over monitored area is shown, that mean whistler amplitude decreases with distance between the causative lightning, increses with causative lightning current and in the evening is ap- proximately three times higher than in the morning. A study dedicated to subprotonospheric whistlers is presented. We found, that subprotonospheric whistler causative lightnings currents are very high compared to that of usual 0+ whistlers.

The purpose of this study is two-fold. The first is to determine the dependence of the molecular ion profiles on the various ionospheric and atmospheric parameters that affect their distributions. The second is to demonstrate the correlation of specific ionospheric parameters with 6300 Å nightglow intensity during periods of magnetically quiet and disturbed conditions.