Characteristics of Unconnected Upward Leader Currents Measured at the KSC Industrial Area Tower

Abstract:

We analyzed in detail currents associated with unconnected upward leaders (UULs) initiated from the Kennedy Space Center (KSC) Industrial Area Tower (IAT). Current measurements of UULs in natural lightning are relatively rare and provide important insights into the development and propagation of upward leaders. Eight UULs were initiated from the KSC-IAT between August 1, 2018 and November 15, 2019. All UULs were positive as they occurred in response to downward negative leaders that attached to ground near the tower. The KSC Mesoscale Eastern Range Lightning Information System (MERLIN), located these nearby strokes at distances ranging from 185 to 783 m from the tower, with the median being 538 m. The peak current for these strokes ranged from 13 to 69.3 kA, with the median being 26.7 kA. The durations of the eight UULs ranged from 116 μs to 1.15 ms, with the median being 789 μs. Charge transferred by UULs in the time-interval between the inception of current and current polarity-reversal ranged from 0.5 to 109 mC, with the median being 6.3 mC. UUL-currents consisted of faster (total durations of the order of 10 μs) impulses overlaid on slower (millisecond-scale) “background” current. The faster current pulses are likely due to electric field changes from stepping in the downward leader. The background-to-peak UUL current-pulse amplitudes for 223 pulses ranged from 3.4 – 289.2 A with the median being 30.1 A. The total duration, full-width at half-maximum (FWHM), and risetimes could be measured for 179 pulses. The median pulse total duration and FWHM were 14.1 μs and 4.9 μs, respectively. The median background-to-peak and 10-to-90% risetimes were 3.2 and 2.0 μs, respectively. Interpulse intervals ranged from 4.2 – 132.8 μs, with the median being 20 μs. Generally speaking, pulse amplitudes are larger, background currents are higher, and interpulse intervals are shorter at later times during UULs.