Electric fields near triggered lightning channels measured with Pockels sensors

Abstract

Electric field waveforms at horizontal distances from the triggered lightning channel attachment point ranging from 0.1 to 1.6 m have been measured with Pockels sensors at the International Center for Lightning Research and Testing at Camp Blanding, Florida. The measuring system had a dynamic range from 20 kV/m to 5 MV/m and a bandwidth from 50 Hz to 1 MHz. The corresponding currents at the channel base and electric fields at 5, 15, and 30 m from the lightning channel were also measured using a current viewing resistor and flat-plate antennas, respectively. Very close vertical electric fields for 36 strokes in nine triggered lightning flashes were obtained using Pockels sensors. For 8 out of the 36 strokes, horizontal electric fields were also measured using Pockels sensors. Electric field waveforms appear as pulses, with the leading edge of the pulse being due to the leader and the trailing edge due to the return stroke. Of the 36 vertical electric field waveforms, six were more or less V-shaped, while 30 exhibited a considerably slower variation during the return-stroke stage than during the leader stage. Vertical electric field pulse peaks are in the range from 176 kV/m to 1.5 MV/m (the median is 577 kV/m), and horizontal electric field pulse peaks are in the range from 495 kV/m to 1.2 MV/m (the median is 821 kV/m). On-site calibration results show that these electric fields measured using Pockels sensors may be underestimated by 40% or so due to the insufficient upper frequency response of 1 MHz of the measuring system. Additionally, vertical electric fields due to M components were measured and compared to electric fields produced by leader/return stroke sequences. For 8 out of 10 M components having channel-base peak currents greater than 500 A, vertical electric fields at 0.1 to 1.6 m were below 20 kV/m, the lower measurement limit. For the remaining 2 of 10 M components, whose current peaks were between 2.3 and 3.2 kA, vertical electric field peaks were about 100 and 48 kV/m at a distance of 0.1 m from the attachment point, apparently unaffected by the upper frequency response of the measuring system. The vertical electric field measured very close to the lightning channel tends to increase with an increase in the previous no-current interval, that is, in the time elapsed from the cessation of current of the preceding stroke (or of the initial-stage current). Copyright 2002 by the American Geophysical Union.