An image of an unusual luminous electrical discharge over a thunderstorm 250 kilometers from the observing site has been obtained with a low-light-level television camera. The discharge began at the cloud tops at 14 kilometers and extended into the clear air 20 kilometers higher. The image, which had a duration of less than 30 milliseconds,resembled two jets or fountains and was probably caused by two localizd electric charge concentrations at the cloud tops. Large upward discharges may create a hazard for aircraft and rocket launches and, by penetrating into the ionosphere, may initiate whistler waves and other effects on a magnetospheric scale. Such upward electrical discharges may account for unexplained photometric observations of distant lightning events that showed a low rise rate of the luminous pulse and no electromagnetic sferic pulse of the type that accompanies cloud-to-earth lightning strokes. An unusually high rate of such photometric events was recorded during the night of 22 to 23 September 1989 during a storm associated with hurricane Hugo.

Abstract. The FORTE satellite, launched on August 29, 1997, carries both radiofrequency-receiver and optical (imaging and photometric) payloads for the study of lightning. The radio-frequency (RF) data for the first 7 months of operation are described, both to illustrate the satellite's capabilities and to explain the basic statistical findings so far. FORTE's multichannel RF trigger system represents a significant advance in spacebased monitoring of lightning emissions. We are able to observe even rather weak and diffuse RF emissions from lightning and are no longer limited to the brightest known events, "transionospheric pulse pairs," or TIPPs. We do see TIPPs, and we show that the FORTE observations of TIPPs are consistent only with the second pulse's being due to a ground reflection. We find that TIPPs are basically bimodal in character, one type having a steep roll-off of power from 38 to 130 MHz and the other being essentially flat-spectrum in that range. The steep-spectrum TIPPs cluster together in the manner of most RF emissions from lightning, while the flat-spectrum events tend to maintain a wider spacing (>0.1 s) between recurrent emissions. The FORTE Satellite and RF PayloadWe provide here only a sketch of FORTE's RF capabilities. A longer discussion will be given in a separate engineering paper.FORTE was launched on August 29, 1997, into a 70 ø inclination orbit, nearly circular at 800 km altitude. Data acquisition commenced within days and has continued without serious interruption through the writing of this paper (April 1998). The FORTE satellite contains both a suite of RF receivers and the Optical Lightning System (comprising a charge-coupled device (CCD) imager and a fast broadband photometer). This paper will introduce the performance and capabilities of the RF payload [Enemark and Shipley, 1994] as reflected in RF data gathered 337

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 report initial measurements of ionospheric propagation parameters, particularly the total electron content (TEC), using the recently launched FORTE satellite. FORTE, which orbits the Earth at an altitude of 800 km and an inclination of 70 ø, contains a set of wideband radio receivers whose output is digitally recorded. A specialized triggering circuit identifies transient, broadband radio events, which include radiation from lightning, transionospheric pulse pairs, and man-made sources. Event data are transmitted to the ground station for analysis. In this paper we examine signals transmitted from an electromagnetic pulse generator operated at Los Alamos. The transmitter produces nearly impulsive signals in the VHF range. The received signal is dispersed by the ionosphere, and the received signal can be analyzed to deduce the total electron content along the path. By comparing the slant TEC thus measured with results from a ray-tracing code, we can deduce the vertical TEC to 800 km. Data from eight passes are presented. These types of data (in larger quantities) are of interest to operators of radar altimeters, who need data to corroborate their corrections for the ionospheric TEC. The combination of FORTE TEC data to 800 km and TEC measurements to 20,000 km (the Global Positioning System orbital altitude) can provide useful information for assessing the validity of models of plasmaspheric electron density. Initial estimates of the plasmaspheric density, on two daytime passes, are about 6 TECU. The signal received by FORTE, which is linearly polarized at the transmitter, is split into two magnetoionic modes by the ionosphere. The receiving antenna is also linearly polarized and therefore receives both modes. By measuring the beat frequency between the two modes, we can deduce the product of the geomagnetic field and the cosine of the angle between the field and the propagation vector. The possibility of using the measured slant TEC and the beat frequency to geolocate impulsive signals is discussed. IntroductionOn August 29, 1997, a space vehicle named FORTE was placed into a circular orbit with an altitude of 800 km and an inclination of 70 ø by a Pegasus launch vehicle. FORTE is an acronym for fast onboard recording of transient events, and the satellite includes two major payloads. The primary payload is a set of tunable wideband radio receivers followed by fast digitizers. The secondary payload, comprising an imaging camera and a fast photodiode detector, will not be discussed in this paper. One of the purposes for the radio payload is to provide data use this information will be described below. The received signal can also be analyzed, under some circumstances, to infer the product of the geomagnetic field and the cosine of the angle between the field and the propagation vector. This information can be used to further constrain the locus of possible source locations.In the sections to follow we will discuss the theoretical basis of the measurements and data analysis and then describe the transm...

This paper presents further discussion of and new data on fast subvisual increases in the luminosity of the night sky described in our previous papers. A detailed technical description of the simple telescopic photometers used in the project SKYFLASH and their mode of operation including the detection of polarized Rayleigh‐scattered flashes is provided. Distant lightning storms account for many of the events, and the complex relations between short and long luminous pulses with and without sferics are shown by examples from a new computerized data system, supplemented by two low‐light‐level TV cameras. Of particular interest are the previously observed “long” events having a slow rise and fall, 20‐ms duration, and showing small polarization and no coincident sferic. A group of such events on September 22–23 during the invasion of U.S. coasts by Hurricane Hugo, is discussed in detail. The recently observed “plume” cloud‐top‐to‐stratosphere lightning event is suggested as a possible source type for these flashes. An alternative source may be exploding meteors, recently identified during SKYFLASH observations by low‐light‐level television techniques as the origin of some sky‐wide flash events described herein.