[1] The Los Alamos Sferic Array (LASA) recorded VLF/LF electric-field-change signals from over ten million lightning discharges during the period from 1998 to 2001. Using the differential-times-of-arrival of lightning sferics recorded by three or more stations, the latitudes and longitudes of the source discharges were determined. Under conditions of favorable geometry and ionospheric propagation, sensors obtained ionospherically reflected skywave signals from the lightning discharges in addition to the standard groundwave sferics. In approximately 1% of all waveforms, automated processing identified two 1-hop skywave reflection paths with delays indicative of an intracloud (height greater than 5 km) lightning source origin. For these events it was possible to determine both the height of the source above ground and the virtual reflection height of the ionosphere. Ionosphere heights agreed well with published values of 60 to 95 km with an expected diurnal variation. Source height determinations for 100,000+ intracloud lightning events ranged from 7 to 20 km AGL with negative-polarity events occurring above $15 km and positive-polarity events occurring below $15 km. The negativepolarity events are at a suprisingly high altitude and may be associated with discharges between the upper charge layer of a storm and a screening layer of charge above the storm. Approximately 100 of the intracloud events with LASA height determinations were also recorded by VHF receivers on the FORTE satellite. Independent FORTE source height estimates based on delays between direct and ground-reflected radio emissions showed excellent correlation with the VLF/LF estimates, but with a +1 km bias for the VLF/LF height determinations.

The World Wide Lighting Location Network (WWLLN) locates lightning globally, using sparsely distributed very low frequency (VLF) detection stations. Due to WWLLN's detection at VLF (in this case ϳ10 kHz), the lightning signals from strong strokes can propagate up to ϳ10 4 km to WWLLN sensors and still be suitable for triggering a station. A systematic evaluation of the performance of WWLLN is undertaken, using a higher-frequency (0-500 kHz) detection array [the Los Alamos Sferic Array (LASA)] as a ground truth during an entire thunderstorm season in a geographically confined case study in Florida. It is found that (a) WWLLN stroke-detection efficiency rises sharply to several percent as the estimated lightning current amplitude surpasses ϳ30 kA; (b) WWLLN spatial accuracy is around 15 km, good enough to resolve convective-storm cells within a larger storm complex; (c) WWLLN is able to detect intracloud and cloudto-ground discharges with comparable efficiency, as long as the current is comparable; (d) WWLLN detects lightning-producing storms with high efficiency in every 3-h epoch; thus, WWLLN can be useful for locating deep convection for weather forecasting on 3-h update cycles; and (e) WWLLN detects a stroke count in each storm that is weakly proportional to the stroke count detected by LASA. Thus, to the extent that lightning rate can serve as a statistical proxy for rainfall, WWLLN may eventually provide rainfall-proxy data to be assimilated in 3-h forecast update cycles.

The FORTE (Fast On-orbit Recording of Transient Events) satellite records bursts of electromagnetic waves arising from near the Earth's surface in the radio frequency (RF) range of 30 to 300 MHz with a dual polarization antenna. We investigate the possible RF signature of ultra-high energy cosmic-ray particles in the form of coherent Cherenkov radiation from cascades in ice. We calculate the sensitivity of the FORTE satellite to ultra-high energy neutrino (UHE ν) fluxes at different energies beyond the Greisen-Zatsepin-Kuzmin (GZK) cutoff. Some constraints on supersymmetry model parameters are also estimated due to the limits that FORTE sets on the UHE neutralino flux. The FORTE database consists of over 4 million recorded events to date, including in principle some events associated with UHE ν. We search for candidate FORTE events in the period from September 1997 to December 1999. The candidate production mechanism is via coherent VHF radiation from a UHE ν shower in the Greenland ice sheet. We demonstrate a high efficiency for selection against lightning and anthropogenic backgrounds. A single candidate out of several thousand raw triggers survives all cuts, and we set limits on the corresponding particle fluxes assuming this event represents our background level.

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