Combined Optical and Radio-Frequency Perspectives on the Time Evolution
of Lightning Measured by the FORTE Satellite
Abstract
We use a cluster feature dataset for the Fast On-orbit Recording of
Transient Events (FORTE) satellite that combines detections from its
pixelated lightning imager (Lightning Locating System: LLS), photodiode
detector (PDD) and Radio-Frequency (RF) instrumentation to generate
statistics describing the frequency and timing of lightning events
detected by each instrument during lightning flashes. Coincident
observations from the same vantage point allow us to directly compare
flash details that can be resolved by the wide Field of View (FOV)
instruments relative to the pixelated LLS – whose design is based on
NASA’s Lightning Imaging Sensor (LIS).
We find that both the PDD and RF system typically generate more
detections than the lightning imager (mean: 1.5 PDD events per LLS
group, 2 RF events per LLS group) from pulses that are either not
sufficiently bright in the optical band (in the case of RF) or that lack
the optical energy density (in the case of the PDD) required to trigger
one of the pixels on the LLS imaging array. This includes additional
activity before the first LLS group or after the final LLS group.
These FORTE results demonstrate that certain lightning processes would
be better resolved by wide-FOV optical and RF instruments than lightning
imagers. Current / future space-based missions that use / plan to use
similar instruments will improve our understanding of flash evolution by
resolving details missed by lightning imagers.