New measurements of lightning electric fields in Florida: Waveform characteristics, interaction with the ionosphere, and peak current estimates

We analyzed wideband electric field waveforms of 265 first and 349 subsequent return strokes in negative natural lightning. The distances ranged from 10 to 330 km. Evolution of first-and subsequent-stroke field waveforms as a function of distance is examined. Statistics on the following field waveform parameters are given: initial electric field peak, opposite-polarity overshoot, ratio of the initial electric field peak to the opposite polarity overshoot, zero-to-peak risetime, initial half-cycle duration, and opposite polarity overshoot duration. The overwhelming majority of both first and subsequent return-stroke field waveforms at 50 to 330 km exhibit an opposite polarity overshoot. At distances greater than 100 km, electric field waveforms, recorded under primarily daytime conditions, tend to be oscillatory. Using finite difference time domain modeling, we interpreted the initial positive half-cycle and the opposite-polarity overshoot as the ground wave and the second positive half-cycle as the one-hop ionospheric reflection. The observed difference in arrival times of these two waves for subsequent strokes is considerably smaller than for first strokes, suggesting that the first-stroke electromagnetic field caused a descent of the ionospheric D-layer. We speculate that there may be cumulative effect of multiple strokes in lowering the ionospheric reflection height. Return-stroke peak currents estimated from the empirical formula, I = 1.5-0.037DE (where I is considered negative and in kA, E is the electric field peak considered positive and in V/m, and D is distance in km), are compared to those reported by the NLDN.