Dependence of mesospheric Na and Fe distributions on electron density at Arecibo

We present case studies of the mesospheric alkali and non-alkali metals, Na and Fe, along with electron concentrations [N_e] obtained from measurements made at Arecibo on nights of 17-18 and 18-19 March 2004. The background mesospheric conditions as recorded by an airglow all-sky imager displayed ripple- and band-type structures on these nights. Both of the metals display detailed structures within their neutral sporadic layer but are more pronounced in Na than for Fe. A sporadic-E (E_s) with electron concentrations [N_e] exceeding 3000 electrons cm^-3 is accompanied by a strong Na enhancement and a weak sporadic Fe (Fe_s) layer around 95-km altitude. The concentration of Fe⁺ and Na⁺ is estimated to be close to 600 and 30 ions.cm^-3, respectively, within the sporadic-E layer. In order to investigate ion-neutral coupling, a correlative analysis was performed in two altitude regions. Similar features are seen between neutrals and electrons in the 96-100-km altitude range, while within the altitude range of 80-90 km, an opposite behavior is seen. A comparative study between neutral layers below 90 km often referred to as the main or permanent layer and sporadic activity above 90 km reveals different characteristics for alkali and non-alkali metal. Fe concentrations in the main layer are higher than in Fe_s resulting in a density ratio of less than 1 determined from two layers of 3 km thickness centered at 97 and 87 km. For the case of Na, the ratio exceeds 1 during E_s activity on both the nights. The case studies discussed in this work facilitate our understanding of different factors that can influence the sporadic activity in alkali and non-alkali. In a region dominated by ion-molecule chemistry, temperature fluctuations that can be induced by wave activity will have more impact on Na than for Fe within their layers depending on altitude.