Electrostatic field changes and durations of narrow bipolar events

Abstract

Using an array of ten sensors, electric field change measurements of 35 positive narrow bipolar events (NBEs) were obtained at close range (≤10 km). At the closest sensor all 35 NBEs had a net electrostatic change (ΔEfast) associated with the main bipolar pulse with amplitudes of 0.4 to 16.3 V/m (not range normalized). At the closest sensor the bipolar pulse of each of the 35 NBEs was followed by a relatively long, slow electrostatic change (ΔEslow) with amplitudes of 0.1 to 43.4 V/m and durations of 0.7 to 33.7 ms. For ΔEfast, estimated 3-D charge moments for 10 NBEs ranged from 0.46 C kmto 1.81 C km with an average and standard deviation of (1.09 ± 0.36)C km. Seven 3-D charge moments were essentially vertically oriented, and the other three moments were tilted at 10°-20° from vertical. The ten 3-D charge moments were overlaid on vertical radar cross sections; six NBEs occurred in weak reflectivity near the upper reflectivity boundary; the other four NBEs occurred near the top of the high-reflectivity core of the thunderclouds. For ΔEslow, we estimated 3-D charge moments for only 3 NBEs; they ranged from 1.11 C km to 2.69 C km with an average of (1.83 ± 0.80)C km. A two-current transmission line model matched the bipolar pulse and the following slow change (ΔEslow) of one NBE reasonably well. The slow change mechanism may be different from the NBE mechanism and similar to the initial E-change before typical lightning flashes.