T. E. Light

Simulations of lightning optical waveforms as seen through clouds by satellites

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 μs 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.

Coordinated observations of optical lightning from space using the FORTE photodiode detector and CCD imager

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.

Coincident radio frequency and optical emissions from lightning, observed with the FORTE satellite

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.