Publication | Open Access
Absolute Spectral Irradiance Measurements of Lightning from 375 to 880 nm
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1984
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EngineeringAtomic Emission SpectroscopyAbsorption SpectroscopyOptical CharacterizationSpectral IrradianceSpectrochemical AnalysisTerrestrial Gamma-ray FlashesRadiative TransferOptical DiagnosticsStrongest EmissionsInstrumentationPhysicsImaging SpectroscopyRadiative AbsorptionSpectral ImagingRadiation MeasurementStandard DeviationNear-infrared SpectroscopyRadiometryNatural SciencesSpectroscopy
A spectrometer-detector optical multichannel analyzer system capable of absolute spectral irradiance measurements has been used to record the time-integrated emissions (150 or 300 ms) from cloud-to-ground lightning. Two detectors, one operating in the 375–695 nm region and one in the 650–880 nm region, recorded emissions from the same lightning. Ten flashes containing 46 recorded strokes were selected for detailed analysis from 500 flashes recorded in 1981. Thew ten flashes occurred within three degrees of the optical axis of the spectrometer system. The average stroke spectral irradiance from 375 to 650 nm for flashes at approximately 15 km is 3.5 × 10−5 J m−2 per stroke with a standard deviation of 2.0 × 10−5 and a range from 0.7 × 10−5 to 6.8 × 10−5 J m−2 per stroke. The average stroke spectral irradiance from 650 to 880 nm for the same strokes is 1.2 × 10−5 j m−2 per stroke with a standard deviation of 0.7 × 10−5 and a range from 0.5 × 10−5 to 3.2 × 10−5 J m−2 per stroke. The ratio of the visible to infrared spectral irradiance for the previously specified wavelength ranges, 375–650 and 650–880 nm, is 3. The average total spectral irradiance from 375 to 880 nm is 4.7 × 10−5 J m−2 per stroke with a standard deviation of 2.6 × 10−5 and a range of 1.2 × 10−5 to 9.6 × 10−5. A summary table of spectral irradiance values in 50 nm increments is presented. The strongest emissions in 50 nm increments occur in the visible region centered at 505 nm. These emissions are over four times greater than at a similar 50 nm increment centered at 825 nm. The integrated stroke emission energy in 50 nm increments from 430 to 850 nm decreases with increasing wavelength twice as fast as the decrease of solar energy over the same wavelength range. Spectral emission data show that unresolved neutral nitrogen lines (NI) at 744.2 nm are more intense than the H-alpha emission at 656.3 nm. A flash with a continuing current shows strong emissions at 384.0 nm. These are identified as cyanogen (CN) emissions and appear to be a unique spectral signature of cloud-to-ground lightning flashes with continuing currents.