Combined Optical And Radio-Frequency Measurements Of A Lightning Megaflash By The Forte Satellite

The optical and VHF instrumentation on the fast on-orbit recording of transient events (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 radio frequency events. We find that the channel development mapped by FORTE's pixelated lightning imager (LLS) occurred at a typical speed of 2.6 x 10(5) m s(-1) and was accompanied by sustained periods of 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 (two 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 as well as ground-based measurements of negative leader development during "spider" lightning, suggesting that FORTE is sensing the same phenomena.