Electron impact excitation of auroral spectral features in the visible and ultraviolet are computed by solving the complete electron transport equation. Excitation rates are given for several bands of N2 (A 3&Sgr;, B 3&Pgr;, W 3Δ, a 1&Pgr;, C 3&Pgr;) and of N2 +, for bands of O2 (a 1Δ, b 1&Sgr;) and of O2 +, and for several states of O (1D, 1S, 5S, 3S) and of O+. The theoretical results are tested by comparing the predicted emission rate ratios N2 2PG(0,0)/N2 + 1NG(0,1) to ratios derived from photometer measurements of I(3371 ¿) and I(4278 ¿) that were acquired over many hours of observations from a high-flying aircraft. The observations spanned a wide range of auroral types that were ordered by their electron spectral hardness. The results show that the ratio I(3371 ¿)/I(4278 ¿) is a better indicator of the characteristic energy of the electron spectrum than the so-called ''red to blue'' ratio, I(6300 ¿/I(4178 ¿), which has been used over the years. Results of observations of the I(3371 ¿)/I(4278 ¿) ratio acquired by rocket-borne photometers, by satellite borne photometers and by a spectrometer show poor agreement with the airborne experimental results and with the model predictions. Significant differences between the model results reported here and previously published predictions of this spectroscopic ratio are also noted. A relationship between the energy flux and the characteristic energy of electron precipitation, first reported by Eather and Mended (1972), is found to hold over a wide range of fluxes. ¿ American Geophysical Union 1989