Variations of the lower ionosphere height deduced from measurements of tweek-atmospherics

Statistical analysis of experimental records of tweek-atmospherics (tweeks) was produced. Tweeks are observed during nighttime and represent response of the Earth-ionosphere waveguide to excitation 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. The tweek signals demonstrate the propagation of a few Earth-ionosphere waveguide modes. We used a time-frequency representation of analyzed tweek waveforms to infer separate branches, corresponding to different waveguide modes. We determined then propagation path's parameters such as source range, ionosphere height, and azimuthal orientation of a propagation path. Obtained results show the increase of effective height of a reflecting layer in low ionosphere during the night from evening until 3 hour after local midnight. These data also demonstrate the number of branches equal 2...4 in case of effective ionosphere height > 90 km and source range more than 1 Mm. In conjunction with effective height's rise this fact tends to decreasing of average branch quantity in the tweeks observed about few hours after local midnight.