| dc.description.abstracteng | The ionosphere is a conductive layer of the Earth's atmosphere, characterised by the presence of free electrons. Through interaction with electromagnetic waves, it influences a wide spectrum of radio wave communications, including short-wave communication between stations across the globe as well as Ultra-High Frequency (UHF) from ground to satellites. The ionosphere is generated mainly by solar Extreme Ultraviolet (EUV) radiation, but also subject to changes due to a range of different phenomena, most importantly the effects of solar activity and space weather. Changes in the solar EUV output across the solar activity cycle affect its ground state, while transient events such as solar flares and geomagnetic storms introduce strong, short term disturbances that in turn can affect wireless communications and the precision of Global Navigational Satellite Systems (GNSS). The strongest effects are caused by Coronal Mass Ejections (CMEs), large outbursts of solar coronal plasma travelling with velocities of up to 3500 km/s that can be observed and tracked in space using white-light coronagraphs. The continuously growing use of technological systems affected by ionospheric disturbances increases the need to develop space weather forecasting models, as well as dedicated instrumentation to provide the required input data.
In the first part of this dissertation, I investigate the response of the global ionosphere to different phenomena of solar activity, and discuss their application to forecasting models based on my work in the national German research project Operational Tool for ionosphere Mapping and Prediction (OPTIMAP). Global maps of the Vertical Total Electron Content (VTEC) from the Center for Orbit Determination in Europe (CODE) between 2003 and 2018 were processed together with solar EUV irradiance measured by the Solar Dynamics Observatory (SDO), solar X-Ray irradiance measured by the Geostationary Orbital Environmental Satellites (GOES) as well as solar wind data from the OMNI data set. The long duration of the time series allowed me to analyse all effects across a full solar cycle, and include extreme events like the Halloween storms of 2003.
A high correlation around r = 0.88 was found between EUV and global mean VTEC, and r > 0.7 for the medium-term trend of the 27-day solar rotation, both following a linear relationship across the last solar cycle. Due to a seasonal trend, the lowest relative global TEC values were observed in summer, and the highest in spring and autumn. An analysis of regional correlation coefficients revealed the highest correlations just after sunrise and around the dayside magnetic equator. The delayed response of the ionosphere was measured at 20 hours on the global scale, with only weak patterns and a wide range of delays up to several days in the regional and seasonal analysis.
For the X17 flare of 2003-10-28 and the X28-flare of 2003-11-04, regional TEC enhancements of around 10 TECU and 3 TECU were measured, which is lower than values reported by studies using single-station data. A superposed epoch analysis was performed for 7838 C-, 752 M- and 73 X-flares between 2003 and 2014. The results were used to formulate a mean- and worst-case scaling law.
For the study of CMEs and CIRs, two separate selections of a total of 46 events were made, one including events of all magnitudes and one events with strong geomagnetic impact. The VTEC profiles of these events were again studied on global and regional scales. A good correspondence was found between the time of peak global TEC and strong southward turnings of the solar wind v x Bz parameter. Negative storm phases, which were identified for 13 events, were found to be restricted to the southern hemisphere in winter and the northern hemisphere in summer.
A comparison between intra-day fluctuations of TEC and v x Bz during periods of low geomagnetic activity showed only very low correlation coefficients of r <= 0.2.
In the second part of the dissertation, I describe the design and tests of an optical prototype of a new solar coronagraph, developed within the European Space Agency's (ESA) Solar Coronagraph for OPEration (SCOPE) project. SCOPE follows a design known as compact coronagraph, which tries to reduce the complexity of the baffle system required by the classical Lyot design. Optical performance tests of the prototype have been conducted after manufacturing at the University of Göttingen, followed by tests at Centre Spatial de Liege, Belgium, and RAL Space, UK.
The 5-disc external occulter showed levels of diffracted light about a factor of 1.5 lower than numerically predicted in tests with monochromatic light sources.
Strong diffractive stray light originating on circular baffles limited the performance to a relative brightness of 1e-9 Bsun across the field of view. A test with an altered setup at RAL Space achieved values below 1e-11 Bsun, but with remaining differences to a final design.
Rough edges of the spider-arms, which hold the external occulter of the instrument, also produced stray light around 1e-9 Bsun. In a systematic test of 5 different models, I demonstrated that polished edges can deliver a rejection ratio close to 2e-12 Bsun near the inner field of view boundary, below the defined requirements at this location.
The results of this work show that different space weather impacts on the ionosphere can be quantified and implemented in forecasting models. For CMEs, a scheme based on prototypic events was suggested that can be validated and expanded according to operational requirements. Further developments are required to provide accurate forecasts of the input parameters. A new generation of space-borne coronagraphs is required to maintain the capability of detecting and modelling the propagation of CMEs. The development of SCOPE has been an important step towards the development of a European coronagraph. The test results have already demonstrated the performance goals for some of its components, giving confidence that further developments can be concluded successfully. | de |