FORTE observations of simultaneous VHF and optical emissions from lightning: Basic phenomenology

Preliminary observations of simultaneous VHF and optical emissions from lig htning as seen by the Fast on-Orbit Recording of Transient Events (FORTE) s pacecraft are presented. VHF/optical waveform pairs are routinely collected both as individual lightning events and as sequences of events associated with cloud-to-ground (CG) and intracloud (IC) flashes. CG pulses can be dis tinguished from IC pulses on the basis of the properties of the VHF and opt ical waveforms but mostly on the basis of the associated VHF spectrograms. The VHF spectrograms are very similar to previous ground-based HF and VHF o bservations of lightning and show signatures associated with return strokes , stepped and dart leaders, attachment processes, and intracloud activity. For a typical IC flash, the FORTE-detected VHF is generally characterized b y impulsive broadband bursts of emission, and the associated optical emissi ons are often highly structured. For a typical initial return stroke, the F ORTE-detected VHF is generated by the stepped leader, the attachment proces s, and the actual return stroke. For a typical subsequent return stroke, th e FORTE-detected VHF is mainly generated by dart leader processes. The dete cted optical signal in both return stroke cases is primarily produced by th e in-cloud portion of the discharge and lags the arrival of the correspondi ng VHF emissions at the satellite by a mean value of 243 mu s. This delay i s composed of a transit time delay (mean of 105 mu s) as the return stroke current propagates from the attachment point up into the region of in-cloud activity plus an additional delay due to the scattering of light during it s traversal through the clouds. The broadening of the light pulse during it s propagation through the clouds is measured and used to infer a mean of th is scattering delay of about 138 mu s (41 km additional path length) for CG light. This value for the mean scattering delay is consistent with the Tho mason and Krider [1982] model for light propagation through clouds.