Abstract
An analysis of four solar flare X-ray irradiance effects on VLF signal amplitude and phase delay variations on the NAA/24.0 kHz signal trace during the period from 2005 September to 2006 December was carried out. Solar flare data were taken from the GOES12 satellite one-minute listings. For the VLF data, recordings at the Institute of Physics, Belgrade were used. It was found that solar flare events affect VLF wave propagation in the Earth-ionosphere waveguide lowering the changes of the ionosphere electron density height profiles. This follows from the variation during the solar flare events of the following propagation parameters: the sharpness of the lower edge of the ionosphere and the reflection height.
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