FORTE Measurements of Global Optical Lightning Waveforms and
Implications for Optical Lightning Detection
Abstract
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.