The master's thesis starts with the investigation of cross-correlation between Wolf's number (Solar activity) and number of solar X-flares, which are responsible for a terrestrial atmospheric phoenomenon called a sudden ionospheric disturbance (SID). This is an abnormally high ionization density in the D layer of the Earth's ionosphere. When a Solar flare occurs on the Sun a blast of intense ultraviolet and X-ray radiation hits the dayside of the Earth. This high energy radiation is absorbed by atmospheric particles, raising them to excited states and knocking electrons free in the process of photoionization. The low-altitude ionospheric layers (D and E layer) immediately increase in electron density over the entire dayside. The ionospheric disturbance enhances very low frequency (VLF) radio propagation. With an instrument called the SID monitor, located at Astronomical Geophysical Observatory on the Golovec hill in Ljubljana, I detect Solar flares by monitoring the signal strength of distant VLF transmitters over the course of time. The recorded SIDs indicate whether Solar flares have taken place. Furthermore I investigate diurnal variation of VLF signals, where I focus on calculation of the reflective height and an estimation of effective recombination coefficient and average mass of constituents at the reflecting D-layer. In the last part of the thesis I observe effects of the August 21, 2017 total solar eclipse caused on the lower part of the Earth's ionosphere, by reducing the total incident flux from the Sun. As the rate of ionization in the ionosphere (D-region) was reduced, the effective reflection height increased. Measurements were done using the LAVNet-Mex VLF radio receiver in Mexico City, Mexico. In this work I present a detailed eclipse model which is based on the equation of the phase delay due to an increase in reflection height. I describe the steps of the model and present the obtained profile of the phase and reflection height variation. Results show a good agreement with the results from previous eclipses ground and rocket measurements.
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