The Auroral Spectrum

The first-negative bands of nitrogen, which comprise most of the nitrogen radiation in the auroral spectrum, have been excited under conditions which suggest those in the aurora very closely. Active nitrogen is produced in uncondensed discharges in concentrations sufficient to give a strong visible glow. A very strong flash is observed at the beginning of the afterglow, and this flash indicates a high concentration of active material in the exciting discharge itself. Under the best conditions for the production of the afterglow the spectrum of the exciting discharge consists almost entirely of the first-negative bands. The most important characteristic of these bands, as excited under the present conditions, is the absence of lines due to ${\mathrm{N}}^{+}$. The usual excitation of the ${\mathrm{N}}_2^{+}$ bands in discharges at low pressures produces these lines, whereas they are almost entirely absent in the auroral spectrum. The excitation of ${\mathrm{N}}_2^{+}$ bands in the present experiments is thought to be due to the large concentration of metastable nitrogen molecules in the $A\left({}_{}{}^3{}_{}{}^{}\Sigma \right)$ state, and this experiment is presented as a proof of their presence in both the aurora and in nitrogen afterglows.