Metallic ions in the equatorial ionosphere

Four positive ion composition measurements of the equatorial E region at Thumba, India, are presented. During the day, the major ions between 90 and 125 km are NO+ and O2+. The relative concentrations are similar to those observed at midlatitudes but exhibit unusual structural behavior with altitude. A metallic ion layer centered at 92 km is found to contain Mg+, Fe+, Ca+, K+, Al+, Na+, and possibly Si+ ions. The layer is explained in terms of a similarly shaped altitude distribution of neutral atoms that are photoionized and charge exchanged with NO+ and O2+. Three-body reactions form molecular metallic ions that are rapidly lost by dissociative ion-electron recombination. Unfortunately reactions that create molecular ions from Na+ and K+ using CO2 as a third body to form cluster ions are too slow to account for the observed alkali metal ion distribution. Nighttime observations show downward drifting of the metallic ion layer caused by equatorial dynamo effects. The increased ion-electron recombination, induced by the downward drifting electrons, produces an observed depletion of NO+ and O2+ within the metallic ion region. The inclusion of drift effects is found to be necessary in producing a proper description of metal ion behavior at the magnetic equator. Similar considerations are probably appropriate at other latitudes.