The excitation temperature of lightning

Abstract

Excitation temperatures have been obtained for individual lightning strokes, using time-resolved slitless spectrums on which relative strengths of N II multiplets have been determined with a microdensitometer. Films were calibrated with a xenon flash tube of known spectral emittance; errors due to intermittency and reciprocity effects have been practically eliminated. Oscillator strengths (i.e., f values) have been newly calculated for sixteen N II lines: these are tabulated separately. Local thermodynamic equilibrium and an optically thin emitting gas are assumed. Calculated temperatures represent data on five different strokes, with two or three positions densitometered along three of the channels—for a total of nine values. These range from 24,200° to 28,400°K; each is a weighted average based on five values obtained from multiplets at λ3995, 4041, 4433, 4630, and 5679 angstroms. Corresponding standard deviations range from 400° to 1000°K. It appears therefore that temperatures vary significantly not only from stroke to stroke, but also for different positions on the same stroke. To compare intensities at widely separated parts of the spectrum, atmospheric attenuation as a function of wavelength and air path must be included in the corrections. In the course of this investigation, it was found that both the air path (i.e., apparent distance) and the temperature of a stroke can be determined from the spectrum alone, using relative intensities measured for only three N II multiplets: at λ3995, 4041, and 5679 angstroms.