The effects of auroral ionization on the gas phase chemistry of meteoric sodium in the upper atmosphere are considered via computer simulation and with a case study. We find that under some circumstances, auroral ionization can significantly affect the chemical state of sodium. In particular, at auroral altitudes altitudes, where recently deposited material or sudden atom layers are typically found, neutral Na can be ionized via charge transfer reactions with aurorally enhanced ionization products. The time constants for these reactions are of the order of 15 min. Measurements collected at the Sondrestrom Facility, in Greenland, are analyzed, and a case study is presented for which this gas phase chemistry is sufficient to explain the results. In addition to supporting the predicted influence of auroral ionization on neutral sodium, this case study is also consistent with a chemical mechanism for the formation of a sporadic atom layer. ¿ 2000 American Geophysical Union