Characterization of Downward Leaders in Cloud-to-ground Lightning Using Correlated Electric Field, High-Speed Video Camera, and VHF Mapping Observations

In this study, we examine the characteristics of cloud-to-ground lightning leaders for which we acquired broadband electric field and high-speed video camera measurements. The measurement systems are a part of the Melbourne Lightning Observatory (MLO) located on the campus of Florida Institute of Technology in Melbourne, Florida. The electric field measurement system had a bandwidth of 0.16 Hz (corresponding to a 1-s integrator decay time constant) to 12 MHz (-3 dB bandwidth upper-limit of the electronic amplifier). Data were digitized at 50 MHz and the record length was 2 s. The high-speed video camera was operated at 792,000 frames per second using a lens with 35 mm focal length. For two strokes recorded in June 2021, the NLDN-estimated return-stroke peak currents were -21 kA and +18 kA, and they occurred at distances of 1.2 and 13.6 km from the MLO, respectively. The negative return stroke was the last one in a three-stroke negative flash and was preceded by a dart-stepped leader. The +18-kA stroke was the only one in the flash. We estimated the arithmetic mean two-dimensional leader speeds from the high-speed video camera records to be and m/s at altitude ranges of 29-76 and 23-710 m above ground for the negative and positive strokes, respectively. Using our electric field records in conjunction with the Kennedy Space Center Lightning Mapping Array (LMA) data we measured the leader durations (time-interval between when the leader starts progressing toward ground and the following return stroke) to be 25 and 211 ms, respectively. We will use our measurements along with the relationship between leader line-charge-density and the vertical component of electric field on perfectly conducting ground (e.g., Thottappillil et al., 1997) to obtain estimates of the charge density for these and other recorded strokes in our dataset. References: Thottappillil, R., Rakov, V. A., & Uman, M. A. (1997). Distribution of charge along the lightning channel: relation to remote electric and magnetic fields and to return-stroke models. Journal of Geophysical Research, 102(D6), 6987-7006. https://doi.org/10.1029/96JD03344.