We present a model of neutral and ionized magnesium in the equatorial ionosphere at altitudes of 50--350 km. The model was developed in an attempt to understand recent spectroscopic measurements made from the space shuttle. The model incorporates ablation of cosmic dust particles, ion and neutral chemistry, diffusion, and vertical drift due to the equatorial electric field. We find that in the absence of electric field drift, there is little or no variation in Mg and Mg+ density profiles above 120 km. With the electric field included, the high-altitude behavior of the Mg+ ion changes dramatically. At dawn, almost all the ion density is confined to the 90- to 125-km altitude range, while at dusk there is significant ion density as high as 350 km. A radiance model is developed to allow for direct comparison with measurements. The model incorporates shielding of incident solar UV by ozone and variable local time of observation and tangential height of instrument line of sight. Comparisons between model and measurement are encouraging. ¿ American Geophysical Union 1996