PT Unknown AU Banys, D TI Propagation of LF and VLF waves and their use for monitoring space weather events PY 2019 PU Christian-Albrechts-Universität zu Kiel WP https://macau.uni-kiel.de/receive/diss_mods_00026241 LA en DE Solar flares; ionosphere; space weather; radio wave propagation; very low frequencies; VLF; Sonneneruptionen; Ionosphäre; Weltraumwetter; Radiowellenausbreitung; Langwellen AB Electromagnetic waves with wavelengths from 1 to 100 km, or correspondingly in the frequency range between 3 and 300 kHz, are denoted by low frequencies (LF) and very low frequencies (VLF). Their signal propagation is determined significantly by the state of the ionosphere. Only because of its predominating electrical conductivity it is possible to receive (V)LF signals of powerful transmitters on the whole globe. Hence, VLF and LF measurements contain useful information about the ionosphere, in particular about its bottomside, the D layer in 60 − 90 km altitude. The ionosphere is subject to diurnal and seasonal variations. Moreover, there are geographic dependencies, which are in general ascribed to influences of the Sun. Primarily, extreme ultraviolet (EUV) and X-ray radiation determine the electron density distribution of the ionosphere. Therefore, especially irregular events visible in the X-ray range, e.g. solar flares, can cause a sudden enhancement of the D-region ionisation, and as a result, change the propagation characteristics of VLF signals on the dayside Earth. This manifests in significantly changing amplitudes and phases of propagating signals. Severe events can even interrupt specific signals and can cause a loss of lock. PI Kiel ER