Polarization parameters of ELF-VLF radio waves propagating within the Earth-ionosphere waveguide under night-time conditions have been investigated using experimental records of tweek-atmospherics (tweeks). Tweeks are observed during nighttime and represent a response of the Earth-ionosphere waveguide excited by lightning strokes. Tweeks are formed by series of pulses consecutively reflected from the waveguide boundaries under different incident angles depending on a source-observer distance and effective ionosphere height along a propagation path. Using a source location technique described in our previous works we determined geometrical propagation path parameters such as source range, effective ionosphere height. We used then a time-frequency representation of the analyzed tweek waveforms to infer polarization parameters of the electromagnetic waves propagated under different incident angles as the first waveguide mode. Obtained dependencies show substantially different behavior in depend on a propagation path orientation in refer to the magnetic meridian.
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