Excitation of N₂ Band Systems in Aurora

The near-ultraviolet region of the auroral spectrum has been observed by a photoelectric spectrometer for the purpose of deriving accurate relative intensities of the N 2 band systems. To increase the signal-to-noise ratio with the faint auroras measured, long "exposures" were accumulated in a memory unit. From the band intensities in the Vegard–Kaplan system, the relative populations of the first three vibrational levels of the metastable [Formula: see text] state could be found; the ratios appear to vary significantly. These bands give a rotational temperature of 800 ± 200 °K; they must therefore be emitted from a height of about 220 km. The vibrational populations are most reasonably explained if nearly all the excitation of the A state is by cascading from higher levels. A detailed discussion is given of these processes, supplemented by measurements of the second positive bands. Cascading is found to supply an appreciable fraction of the excitation of the B 3 Π g state; the C 3 Π u state seems to be excited purely by electron impact. Previous evidence that the latter is not always true is examined and rejected.