Abstract Observations of the occurrence of the H α line in auroral spectra obtained at Saskatoon and at Churchill are reported. These observations are discussed with particular reference to the auroral theory of Martyn. The diurnal dependence of the H α emission does not seem to be in agreement with this theory. The dependence of H α intensity on latitude and magnetic disturbance index suggests that the zone of maximum proton precipitation moves southwards as the magnetic disturbance becomes more intense. This observation is consistent with Martyn's theory, and also with Störmer's theory and other theories derived from Stökmer's analysis.