Combined Optical and Radio-Frequency Measurements of a Lightning
Megaflash by the FORTE Satellite
Abstract
The optical and VHF instrumentation on the FORTE satellite is used to
document the combined phenomenology evolution of a lightning
“megaflash” - mesoscale lightning that propagates laterally over
exceptional distances. We identify a FORTE flash whose maximum extent
was 82 km and inferred length over multiple distinct branches exceeded
100 km. This flash lasted 1.2 s and produced 250 optical and 591 RF
events. We find that the channel development mapped by FORTE’s pixelated
lightning imager (LLS) occurred at a typical speed of 2.6x10^5 m/s
and was accompanied by sustained periods VHF emission that could
individually exceed 100 ms in duration. The impulsive IC events
generated by the flash indicate that this development occurred at
altitudes between 3 and 8 km. Four +CG strokes were identified in the
VHF waveform data that are responsible for two of the three
highly-radiant LLS groups (the third radiant group came from a possible
-CG while 2 of the +CGs were not as optically bright as the others).
These strokes occurred at different locations throughout the flash
footprint with the most distant strokes separated by approximately 50
km. These space-based observations match previous observations of
megaflashes from space as well as ground-based measurements of slow
negative leader development during “spider” lightning, suggesting that
FORTE is sensing the same phenomena.