Seasonal and diurnal variation of electron and iron concentrations at the meteor heights above Arecibo

Abstract

We report observations of seasonal and local time variation of the averaged electron and iron concentrations, as well as simultaneous measurements of the two species, above the Arecibo Observatory (18.35°N, 66.75°N), Puerto Rico. The average Fe profile between 21:00 and 24:00 LT has a single peak at about 85 km with the exception of the summer when an additional peak exists at about 95 km. The higher Fe peak in the summer is correlated with higher electron concentrations in this season. The three nights of simultaneous measurements of electron and iron concentrations show that narrow layers of Fe and electrons are well correlated. Comparison of the climatological and simultaneous Fe and electron data suggests that recombination of Fe + plays an important role in determining the Fe profile in the upper part of the Fe layer. Above 93 km, the Fe concentration appears to increase after sunset if the electron concentration exceeds about 4000 electrons cm −3. The average rate of Fe production is about 0.1 atom cm −3 s −1 for all seasons at 100 km in the early evening hours. A chemical model reveals that the concentration of Fe + must be 50–80% of the total ionization over Arecibo for typical equinox conditions to explain the observed rate of Fe production. These high relative Fe + concentrations are consistent with in situ observations that Fe + is usually the dominant ion in sporadic E layers in the nighttime lower E region. This suggests that the source of Fe + is provided by sporadic E layers descending over Arecibo after sunset. The Fe density between 80 and 85 km decreases during the night, for all seasons. This is attributed to the formation of stable molecular Fe species, such as FeOH, due to the increase in O 3 and decrease in atomic O and H during the night at these altitudes.