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 present three-dimensional simulations of photon transport through clouds, specifically designed to address the characteristics and detection of optical lightning waveforms collected by satellites. The model uses a Monte Carlo approach, in which discrete photons are advanced by a standard time step through a distribution of scattering water droplets, whose size and number density distributions are variable. The model is different from previous work, in that it considers both finite and infinite cloud geometries and simulates sources of emission with arbitrary spatiotemporal properties. The model outputs are designed to be directly comparable to data obtained by the FORTE satellite photodiode detector, which records optical waveforms for lightning events with 15 resolution. The model treats the light propagation through clouds having a variety of shapes, sizes, and optical depths and constructs the delayed/dispersed/attenuated light curve as seen from arbitrary locations outside of the cloud. We compare the simplest case results to previous models and to data from the FORTE satellite and consider certain special cases such as the signal received from impulses occurring below the cloud. We find that the shape of the cloud and the position of the event within the cloud, rather than the motion or extent of the event itself, are the greatest determinants of the resultant distribution of photons in the sky. We also find that the position of the event within the cloud can be as large a determinant in the apparent attenuation of the signal as the cloud optical depth. We find that the class of FORTE optical waveforms with durations >• 1 ms cannot be accounted for by photon scattering alone, but rather, the intrinsic source duration must itself be quite long, which is not the case for return strokes. IntroductionThe FORTE satellite, built by Los Alamos and Sandia Na- spatial resolution optical data set, the optical data analysis is somewhat hindered by the lack of appropriate models of photon transport through cloud. Before we can fully exploit the FORTE optical database, we must better quantify the scattering and absorption of light by clouds. In particular, we must understand how to recover properties of the cloud and discharge from observables such as the measured pulse width and delay. We also seek to understand the detectability of optical transients as a function of event type and viewing angle. This paper presents Monte Carlo simulations of photon transport in clouds designed to output data products similar to that of the FORTE PDD and future satellite-based optical waveform detectors, thereby facilitating comparisons to real transient event data.Section 2 details the results of earlier work in this area, while section 3 describes this new model. In section 4 we examine some of the simplest cases to test the model's validity, and in section 6 we compare the simulations to real PDD data. Previous StudiesRadiative transfer in clouds has been studied extensively, but typically with applications to the stea...

Abstract. We present long optical and radio frequency (RF) time series of lightning events observed with the FORTE satellite in January 2000. Each record contains multiple RF and optical impulses. We use the RF signatures to identify the general type of discharge for each impulse according to the discrimination techniques described by Suszcynsky et al. (2000) and reviewed herein. We see a large number of paired, impulsive events in the RF which allow us to study the heights within clouds of several events. We also see that the rate of RF/optical coincidence depends on the type of discharge: nearly 100% of VHF signals from first negative return strokes have an associated optical signal, whereas a mere 50% of impulsive intracloud events appear to have an optical counterpart. While the RF signals from ground strokes clearly coincide with simple optical signals in almost all cases, the intracloud lightning often shows nearly continuous, complicated RF and optical emissions which do not cleanly correlate with one another. The RF and optical pulses do not show a well-defined relationship of intensities, for any lightning type. The observed delay between the RF and optical pulses we interpret as mainly an effect of the scattering experienced by the light as it traverses the cloud. For intracloud lightning, we find no evidence of an intrinsic delay at the source between the onset of the RF and optical signals. Impulsive in-cloud RF events are seen to occur on average every 0.9 ms during a flash. studied return strokes and found the light signal to begin at or just after the E field peak, and they also found that the optical peak is always greater for initial return strokes than for subsequent strokes. Ganesh et at. [1984] similarly studied return strokes and saw a correlation (with significant scatter) between E field intensity and light intensity. Goodman et at. Introduction Los Alamos and Sandia National

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.

[1] Narrow Bipolar Events (NBEs) are a recently discovered distinct class of intracloud lightning discharges whose associated processes produce the most powerful very high frequency (VHF) radiation observed from lightning. NBEs are thus the prime candidate for proposed satellite-based VHF global lightning mapping and storm tracking missions. In this study, we offer a detailed evaluation of the Great Plains Los Alamos Sferic Array (LASA). We then statistically compare NBE rates to non-NBE lightning rates measured by both the LASA and the National Lightning Detection Network (NLDN) and to NEXRAD radar-inferred metrics of convective strength for thunderstorms in the Great Plains from May to July 2005. We find strong correlations between total lightning rate and convective strength, especially in terms of the height of 30 dBZ radar echo. However, we find much weaker correlations between NBE rate and non-NBE lightning rate and between NBE rate and radar-inferred convective strength. Though NBEs occur in the same storms as other lightning, they cluster more closely in both space and time and may be indicative of specific types of storms and/or specific stages in convective development. Indeed, we find that NBEs are more prevalent in, and perhaps indicative of, the strongest convection. However, even the strongest convection (as inferred by radar) does not always produce NBEs. We compare these results to past studies of NBEs which were based in Florida. We also briefly discuss the implications of these results for satellite-based VHF lightning detection.