Refraction of ELF Electromagnetic Waves by the Ionospheric Gradients at the Day/Night Terminator Measured at the Hylaty Station

Abstract We perform azimuth time tracking of multiple thunderstorm centers on the globe, which are sources of extremely low frequency (ELF) electromagnetic waves propagating in the spherical Earth‐ionosphere cavity. For observations made in September 2023 we identify azimuths of numerous global emission centers using our data sampled at 3 kHz at the Hylaty station in Poland. We confirm significant and relatively regular thunderstorm azimuth variation during the solar terminator passage over the observation site. The magnitude and duration of the azimuth deviations depend on the observed azimuths but are also varying between successive days and changing detailed thunderstorm activity patterns. The measured maximum positive (preferentially at the sunrise time) and negative (preferentially at the sunset time) azimuth deviations reach even above 20 for waves propagating close to the terminator. We discovered also particular composite deviation structures, with the negative azimuth deviation directly preceding a larger positive one, sometimes occurring near the morning terminator passage 100 km above the surface. It is possibly the first detection of the ELF equivalent of the “greyline” transmission known in the HF radio propagation. At azimuths distant from the terminator one can observe decreasing of the regular deviation magnitude and occasionally lower magnitude deviations with opposite sign. The variations between successive days are expected to result from varying thunderstorm activity on Earth as well variability of ionospheric parameters, in particular of ionization gradients and nonuniformities occurring along the terminator. We postulate that the observed deviations result from a signal refraction at the varying ionospheric gradients.