Ionic and neutral concentrations of Mg and Fe near 92 km

Lidar observations of atmospheric sodium reveal a diurnally steady layer with a maximum concentration near 92 km of 3?5 × 10³cm^?3. These data imply that both Fe and Mg should have peak concentrations near 10⁵ cm^?3 on the basis of their relative abundance to sodium in chondritic meteorites. We review the available metallic ion data below 100 km and find typical maxima of 10² cm^?3 for Na and 10³ cm^?3 for both [Fe⁺] and [Mg⁺] for undisturbed daytime conditions. We argue that enhancements of up to 10⁴ cm^?3 for Fe⁺ and Mg⁺ during aurora support the viewpoint that [Fe] and [Mg] both approach 10⁵ cm^?3 near 92 km. We briefly discuss the lack of spectroscopic evidence for such concentrations. In our review of the metallic ion data we find evidence for about a factor of ten diurnal variation in both Fe⁺ and Mg⁺ and probably Na⁺. Minimum concentrations of 10² cm^?3 are attained just before dawn for [Fe⁺] and [Mg⁺].