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FORTE Measurements of Global Optical Lightning Waveforms and Implications for Optical Lightning Detection
  • Michael Jay Peterson
Michael Jay Peterson
ISR-2,Los Alamos National Laboratory

Corresponding Author:michaeljp24@gmail.com

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Lightning processes generate a diverse collection of optical pulses depending on how current traverses the lightning channels. These signals are then broadened spatially and temporally via multiple scattering in the clouds. The resulting optical waveforms measured from space with instruments like the photodiode detector (PDD) on the Fast On-orbit Recording of Transient Events (FORTE) satellite have a variety of shapes. In this study, we use coincident optical and Radio Frequency (RF) measurements to document the properties of optical PDD waveforms associated with different types of lightning, estimate delays from scattering in the clouds, and comment on how pulse shape impacts optical lightning detection. We find that the attributes of optical pulses recorded by the PDD are generally consistent with prior studies, but vary across the globe and with event amplitude. The brightest lightning tends to be single-peaked with faster rise times (median: ~100 µs) and shorter effective widths (median: ~400 µs). Dim events also include cases of broad optical waveforms with sustained optical emission throughout the PDD record, which the pixelated FORTE LLS instrument has difficulty detecting. We propose that this is due to the optical signal being divided between individual pixels that are each, individually, not bright enough to trigger the LLS. We use PDD waveforms and Monte Carlo radiative transfer modeling to demonstrate that increasing the temporal and spatial resolution of a pixelated lightning imager will make it more difficult to detect these broad / dim pulses as their energy becomes divided between additional pixels / integration frames.