Analysis of a bipolar upward lightning flash based on simultaneous records of currents and 380-km distant electric fields

In this paper, we present and discuss simultaneous records of current and wideband electric field waveforms at 380 km distance from the strike point associated with an upward bipolar flash initiated from the Säntis Tower. The flash contains 23 negative strokes and one positive stroke. The intervals between the groundwave and skywave arrival times are used to estimate ionospheric reflection heights for the negative return strokes using the so-called zero-to-zero and peak-to-peak methods. A full-wave, finite-difference time-domain (FDTD) analysis of the electric field propagation including the effect of the ionospheric reflections is also presented. FDTD simulation results are compared with the measured radiated electric field associated with the studied flash to evaluate the reference reflection height of the conductivity profile. It is also found that the ratio of the peak field to the current peak is about two times smaller for the positive pulse compared to negative pulses. This difference in the amplitudes can be attributed to a lower return stroke speed for the positive stroke compared to that for negative strokes, and also to the fact that the enhancement of the electric field due to the presence of the tower and the mountain might be more significant for negative pulses, which are characterized by faster risetimes compared to the positive one.