Observations of positive narrow bipolar pulses

Abstract

Waveforms of 226 positive narrow bipolar pulses (NBPs) were obtained with five to eight stations of E-change meters covering an area of 70 × 100 km2. The NBPs had typical average parameters: 10-90% rise time of 2.6 µs, full width at half maximum time of 2.8 µs, zero cross time of 9.9 µs, and range-normalized amplitude at 100 km of 11.0 V/m. Four main types of positive NBP waveforms were identified: Type A had a simple bipolar waveform with a positive peak and a negative overshoot peak (1% of NBPs), Type B had extra peak(s) superimposed on the overshoot peak (67%), Type C had extra peak(s) on or just after the main positive peak (13%), and Type D had extra peak(s) before the main positive peak (19%). Regardless of type, each NBP waveform maintained its basic shape across a range of 10 to 130 km from its origin. NBP locations, obtained with a time of arrival technique, seemed unrestricted in their horizontal distribution (except for Type C), while NBP altitudes ranged from 7 to 19 km with an average of 13 km. Estimated peak currents were 2-126 kA with an average of 30 kA. Isolation of NBPs from other lightning events was determined for both temporal (660 ms) and spatial (>10 km) quantities; 37% of NBPs were isolated, 38% occurred within 660 ms before a flash, 19% occurred within flashes, and 11% occurred within 660 ms after a flash. The total RMS power radiated by NBPs within 1 kHz-2.5 MHz bandwidth had a range of 5.0 × 106 -6.1 × 108 W with an average of 7.8 × 107 W.