Characteristics of impulsive VHF lightning signals observed by the FORTE satellite

[1] We study very high frequency (VHF) and optical emissions from lightning, observed by the FORTE satellite, differentiating between impulsive (transionospheric pulse pairs (TIPPs)) and nonimpulsive events. TIPPs are seen to constitute 47% of the FORTE VHF data but only 32% of the optically coincident data. The median peak optical irradiance of the optical emission associated with TIPPs is 916 muW/m(2) at FORTE and for non-TIPPs is 195 muW/m(2). The median effective pulse width of the optical signal from TIPPs is 658 mus, and it is 548 mus for non-TIPPs. In the VHF, both event types have similar observed peak powers (0.086 mV(2)/m(2) and 0.089 mV(2)/m(2), for TIPPs and non-TIPPs, respectively). The optically coincident lightning (of either type) is weaker in peak VHF emission than is the lightning that lacks coincident optical signals, although for non-TIPPs, the stronger the VHF peak, the more likely the event is to have a coincident optical signal. For TIPPs, however, this is true only for events with peak E-2 < 0.1 mV(2)/m(2). Above that threshold, TIPPs are increasingly less likely to show coincident optical emission with increasing VHF peak E-2. For both TIPPs and non-TIPPs, the peak current reported by the U. S. National Lightning Detection Network(TM) and peak VHF power reported by FORTE are statistically proportional. The nature of the proportionality appears to depend upon the polarity of the discharge but not upon the event type. We also find that only 11% of TIPPs are associated with negative-polarity discharges, compared to 75% of non-TIPPs. Finally, we find that TIPPs arise from events with altitudes of 6-15 km, although we see optical coincidence only for those TIPPs occurring above similar to 10 km.