Spectral intensities emitted by lightning discharges

Peak spectral intensities emitted by seven types of lightning phenomena, viz., first and subsequent return strokes, stepped and dart leaders, cloud-to-air flashes, intracloud pulses, and M components of return strokes, are reported in five narrow regions of the visible and near-visible spectrum. At 3914 A the peak continuum radiation varies from 300 to 107 watts ster−1 A−1, averaged over a 23-A bandwidth and integrated over the whole source. For the average lightning stroke, the spectral intensities at the other regions studied, relative to that near 3914 A, are: 0.95 at 4140 A (continuum), 2.1 at 6563 A (Hα plus continuum), 4.8 at 8220 A (NI[2] plus continuum), and 0.7 at 8900 A (continuum). Return strokes are usually the most intense lightning phenomena but can vary in intensity by a factor of 1000 from storm to storm and by a factor of 50 within one flash. The most intense return strokes appear to have the highest radiating temperature. Stepped and dart leaders are a factor of 20 to 100 less intense than their return strokes, but the leaders and return strokes have similar spectra. Pulses originating within clouds vary in intensity from the weakest pulses observed to ∼105 watts ster−1 A−1 near 3914 A and are substantially stronger in the near-infrared channels, relative to the 3914-A channel, than the average stroke. M-component brightenings of return-stroke channels are of lower excitation than typical return strokes but are otherwise similar to them. There is no indication in any of these strokes of the strong N2+ 1N (3914 A) radiation present in some slit spectrograms in the literature.