Introduction

Background

Satellite-based observations of radio frequency (RF) and optical emissions from lightning have been occurring for over 30 years [Vorpahl et al., 1970; Sparrow and Ney, 1971; Turman, 1977, 1978; Vonnegut et al., 1983; Christian and Latham, 1998; McKerras et al., 1998]. Optical emissions from terrestrial lightning were first detected from space by NASA's Orbiting Solar Observatory (OSO) 2 satellite in the mid-1960s. Since that time, additional optical observations of lightning have been made by the OSO, VELA, Defense Meteorological Satellite Program (DMSP) series of satellites. The VELA and DMSP observations provided a time history of optical pulses produced by lightning, but the details of those pulse waveform data were only superficially reported [e.g., Turman, 1977, 1978; MacKerras et al., 1998]. This paper serves to place the previous satellite-based waveform data into perspective via a closer examination of data obtained from a similar type of optical sensor that we describe in detail.

More recently, NASA has launched and operated optical imaging sensors (Optical Transient Detector and Lightning Imaging Sensor) in low Earth orbit to study terrestrial lightning [e.g., Christian and Latham, 1998, 1998]. These imaging sensors provide the capability to determine the geographic location and a measurement of the integrated optical emissions of detected lightning events [e.g., Christian et al., 1989] and thereby contribute to remote-sensing applications such as providing improved input data for meteorological forecasting models and improving our understanding of the relationship between lightning and thundercloud physics [e.g., Baker et al., 1999].

FORTE satellite

The FORTE satellite was launched on August 29, 1997, into a nearly 825-km circular, 70° inclination orbit. This Department of Energy satellite carries optical and RF sensors and possesses an ability to simultaneously monitor the optical and RF emissions of lightning activity on a global basis. In the first year of operation, over 2 million optical and RF events were detected by the sensors on the FORTE satellite. An initial presentation of the RF data has been given by Jacobson et al. [1999]. A study of transionospheric pulse pairs has been reported by Massey et al. [1998a, 1998b]. Jacobson et al. [2000] has also compared FORTE VHF data with stroke-level data reported by the National Lightning Detection Network (NLDN). Additionally, Suszcynsky et al. [2000] presented a dual phenomenology account of detected lightning by closely examining the behavior of RF and optical lightning emissions, employing NLDN data to develop techniques to infer lightning type from the time-dependent RF spectrum.

This paper will focus on data collected by one of the two optical sensors carried by FORTE, a silicon photodiode detector (PDD). This instrument records the amplitude time history of transient optical events within the field of view. We describe the instrument (section 2), present examples of PDD data and describe down-selection algorithms (section 3), compare the PDD data with NLDN data (section 4), and review the statistical character of terrestrial lightning as observed from space (section 5).

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