Diffraction 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, as
occurring during the morning terminator passage. At azimuths distant
from the terminator one can observe decreasing of the regular deviation
magnitude and occasionally lower magnitude deviations with opposite
sign. Variations of the observed azimuth time structures between
successive days are expected to result from varying thunderstorm
activity on Earth as well variability of ionospheric parameters, in
particular of charge gradients and nonuniformities occurring along the
terminator. We postulate that the observed deviations result from a
signal diffraction at the varying ionospheric gradients.