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.