Top side N2+ content and scale height from Isis 2 auroral scanning photometer data at 3914 ¿ for three evening twilights are found to be about 3 times as great in winter as in summer over the Pacific sector. The observed intensity distributions are compared with a photochemical model that underestimates the summer intensities by about a factor of 2 and that offers no possibility of explaining the winter enhancement. Estimates of enhancement by conjugate photoelectrons, neutral winds, and Penning ionization by metastable helium indicate that none of these are sufficient to explain the seasonal variation.