Abstract. Preliminary observations of simultaneous VHF and optical emissions from lightning as seen by the Fast on-Orbit Recording of Transient Events (FORTE) spacecraft 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 distinguished from IC pulses on the basis of the properties of the VHF and optical waveforms but mostly on the basis of the associated VHF spectrograms. The VHF spectrograms are very similar to previous ground-based HF and VHF observations 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 by impulsive broadband bursts of emission, and the associated optical emissions are often highly structured. For a typical initial return stroke, the FORTE-detected VHF is generated by the stepped leader, the attachment process, and the actual return stroke. For a typical subsequent return stroke, the FORTE-detected VHF is mainly generated by dart leader processes. In remedy of this situation the Fast on-Orbit Recording of Transient Events (FORTE) satellite was launched on August 29, 1997. FORTE is a joint Los Alamos National Laboratory and Sandia National Laboratories satellite experiment that was primarily designed to address technology issues associated with treaty verification and the monitoring of nuclear tests from space. The satellite carries VHF broadband radio receivers and an Optical Lightning System (OLS) which are optimally designed for the detection of lightning transients. The design of this instrumentation and its availability for continuous scientific use makes FORTE an ideal space platform from which to monitor and study the simultaneous emission of VHF and optical radiation from lightning. This paper reports on the preliminary phenomenology and analysis of the correlated FORTE VHF and optical data sets.The goals of this study are twofold: (1) to demonstrate the utility of using a dual phenomenology approach for the remote 2191

Abstract. We review data from observations of terrestrial lightning obtained by the FORTE satellite between September 1997 and January 2000. A silicon photodiode detector (PDD) records the intensity-time history of transient optical events occurring within its 80 ø circular field of view. This field of view corresponds to a circle on the Earth's surface having an approximate diameter of 1200 km. We describe the instrument, present examples of the data, explain how the data are screened for false triggers, and review, within the context of previous measurements, the general statistics of peak irradiance, pulse width, and energy associated with the data. We compare the FORTE data with National Lightning Detection Network (NLDN) reported cloud-to-ground (CG) strokes and find that the PDD detection efficiency for these CG strokes is -6%. Moreover, we infer that FORTE preferentially detects the in-cloud portion of optical lightning signals. Events having inferred peak powers between l0 s and 10 TM W and optical 3 9 energy outputs between 10 and 10 J are observed. From a population of nearly 700,000 events we find that the median peak power and median detected optical energy at the source are estimated to be -1 x 109W and 4.5 x l0 s J, respectively. These values of source peak power and energy are comparable to previous space-based measurements and consistent with aircraft-based and ground-based measurements. The observed median effective pulse width is about 590 microseconds. Further, the pulse widths for CG strokes, reported by NLDN, are inversely proportional to pulse peak power.

Abstract. This paper presents an overview of the coordinated observation of optical lightning from space using the photodiode detector (PDD) and CCD-based imager known as the Lightning Location System (LLS) aboard the Fast On-Orbit Recording of Transient Events (FORTE) satellite. PDD/LLS coincidence statistics are presented and show that both the detected energy density and the detected peak irradiance of optical lightning events are proportional to the number of LLS pixels (pixel multiplicity) which are activated during the event. The inference is that LLS pixel multiplicity is more a function of the detected intensity and horizontal extent of the optical event rather than a direct indicator of the degree of scattering. PDD/LLS event coincidence is also used to improve upon traditional recurrence/clustering algorithms that discriminate against false LLS events due to energetic particles and glint. Energy density measurements of coincident events show that about 4% of the optical energy detected by the broadband PDD appears in the narrowband LLS. This is in general agreement with ground-based measurements and with assumptions incorporated into the design of current and planned CCD-imaging sensors. IntroductionThe satellite-based detection of optical emissions from lightning provides an effective way to monitor and study thunder- The goals of this study were to (1) present the basic phenomenology and techniques associated with the joint detection of optical lightning by a space-based photodiode sensor and CCD imager, (2) use the coordinated data set to explore the relationship between PDD optical waveform features and LLS pixel event characteristics, particularly as it pertains to the scattering of light by the intervening clouds, and (3) demonstrate the use of combined waveform/imager data to improve upon current satellite-based lightning data interpretation and discrimination techniques. 17,897

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[1] We have examined lightning flashes in five nighttime, oceanic thunderstorms, which were jointly observed by Tropical Rainfall Measuring Mission (TRMM) and Fast OnOrbit Recording of Transient Events (FORTE). The multiplicity of instruments on board these satellites presents a multiphenomenological snapshot view of oceanic nighttime convection. Data are available for five oceanic storms with a total of 40 flashes. The independent optical imagers on each satellite establish the flash locations. The relative fraction of Lightning Imaging Sensor (LIS)-detected optical pulses that were also observed by the FORTE/photodiode detector varied from 0% for LIS range less than 10 4 J sr À1 m À2 mm À1 to 100% for LIS range greater than 10 6 mJ À1 sr À1 m À2 mm À1 . The FORTE/VHF data sometimes allow estimation of the VHF source emissions heights and identification of individual discharge processes as positive or negative, in-cloud or cloud-to-ground. These observations reinforce the concepts that the VHF pulses are produced by a breakdown process, while the optical pulses are the result of current flow.