LA - 3755 80 ^ LOS ALAMOS SCIENTIFIC LABORATORY or the University of California

ColHaated 50*.field photometers were operated during tße 1965 lightning study to obtain data for an analysis of the production of false alarms in air-fluorescence nuclear-explosion detection systems. Detectors were employed in spectral regions useful for air-fluorescence detection! 591* X [Ife* HI (0,0)] and 8900 A [Ife IP (1,0)], both of which are continum channels for lightning; and at wavelengths useful for litfrtnlng discrimination: kiUO k [continm»], 6563 k [fti], anl 8320 A [HI (2)]. flesults from ~ 870 pulses In 58 fleihes, all of which had visible, discrete channels, are presented in terms of source spectral intensities at 591* k (W sr" A') and as ratios of the spectral intensities in other wavelength regions to that at 5914 k. Distances to the channels, ranging from 5 to 80 tan, were measured using a photographic triangulation system, am appropris te inverse-square-law and itjnoBrberic-transmission corrections were made. ■ Observations «re made on all types of lightning phenomena: leaders, return strokes, and interand intra-cloud processes.; U» most probable spectral intensity at 5914 Jl for all these pu'=£es is lO* H sr" A"} values range fron 5 X 10 to 10 W sr' Ä". Average spt ctral intensities relative to 591* k are: 41*0 JL, 1.2 ± 0.5J 6565 A, 2.1 ± o.8j 8220 k, 4.8 ± 2,8; and 89OO A, 0.8 ± 0.4. In first return strokes, the three longest-wavelength signals decrease relative to the 5914-A signal by a factor of 1.5 to 5. I. 1KTRCDUCTI0N This is the second of two reports on the optical emission characteristics of lightning, investiffted at Los Alamos during the swaner of 1965. Because lightning pulses are capable of producing false abinae in nuclear-explosion-detection systems that rely on optical detection of the burst of air fluorescence exejted äircctly or indirectly by an exploal'jn, a lightning study «as conducttd to find cptlnnw methods to discriinlnate agalnet such false alarms. A nuaber of types of optical detestara were operated with strict timing coordination to Obtain different kinds oT spectral Information. Colllnatsd photometers obvained time-resolved signals which «ere recorded continuously by oscilloscope cameras; results are reported here. A slitless spectrograph «as used t? Investigate the returnstroke spectrum, for «hich results are given by Connor in the first report of this series. All-sl^ photcmsters,. identical to the ones designed for the Los Alamoe Air Fluorescence Detection System (LAAFDB), «ere operated by EG&Q to measure the lightning spectrum as seen by the lAAFEBs preliminary results were reported by Anato. All meaaurements have been reduced by invarse-uquare-la« and atmospheric-transmission correction«! to give source characteristics, usinf source.to-deteotor distances measured with a photographic triangulation system. Bemotely operated, time-coordinated camfraa recorded images of the optical channel of a lightning flash simultaneously from three separated stations.* Parallax msasured from pairs of photographs of each channel, together «1th measurements of known landmarks, was used to calculate the slant disttnceto points on the channels. An average projected baseline of 6^0 m between pairs of stations resulted in estimated errors of ± 5$ at a distance of SO ten and 1 25$ at 80 km.* Krider has reported photoelectric measurements of lightning made with colllmated detectors, with both narrow spectral passbends and broadband. Ite was Interested in the times at which raäiatlon freut the various molecular and atonic species reached its peak, and in the advance rate of the return-stroke streamerss he gave no relative strengths of the detected signals as a function of wavelength. Subsequent reports «ill contain the applications of the results of the optical-emission measurements reported here and elsewhere.' Volune HI will contain a derivation of the propagation chsracterifitics of lightning-produced signtils by which light is scattered into an all-sky detector. An aggregate of collimated-detector relativ« and absolute spectral data is required for comparison with similar all-sky detector results to ciitaln the scattering paracaten». Volume IV will treat discriaination against lightning by nuclear-explosion air-fluorescence detection systems, aid requires the distribution functions of source characteristics presented in this report. H. HBTRUMEOTATIDN AND CALIBRATIOK Five "co^^llBated' photomefcers with parallel, circitlar, 50° fields of view were used during each lightning storm. The term "collimated" is derived from the cellimation of light by lenses before It passes through interference filters. The center of the coBjnon field of view was approximately parallel to tl»t of the slitless spectrograph at all times. In an, the spectrograph and photometers were pointed toward the greatest lightning activityj hence, for ~ 60$ of the recorded flashes, the actual lightning source and the majority of the light scattered fron it by clouds were within the photaneters' field of view.