Optical spectra of a mid-latitude aurora on September 21--22 indicated that the observed emissions were caused by energetic ions ⟨O+ or He+⟩. In an earlier paper it was shown that the spectral features could be fairly well explained by a flux of precipitating energetic O+ with characteristics similar to O+ fluxes that have been observed in large magnetic storms. A comprehensive model is used to trace the energy degradation of the incoming O+ ions, the production of energetic neutrals, and the subsequent scattering of these neutrals in altitude and energy. In this study we have computed the atmospheric impact ⟨in terms of heating, ionization, and excitation of certain emissions⟩ if the incoming flux is He+ instead of O+. The results show that an energetic He+ flux is very efficient in producing both ionization and a backscattered flux out of the atmosphere. Of the total energy influx, approximately 50% goes into ionization ⟨compared with ~2% in the case of O+⟩ and 40% is backscattered to the magnetosphere or escapes ⟨compared with 16% for O+⟩. By contrast the He+ flux is less efficent at heating the atmosphere ⟨10% compared with 82%⟩. The He+ flux is very efficient in producing N2+ 1 N ⟨first negative⟩ emission, but essentially does not excite the N2 2 P ⟨second positive⟩ system. From these results it is concluded that the aurora observed on September 22, 1982, is well explained by an influx of energetic O+ together with a 6% admixture of He+.