Structure and seasonal variability of the nighttime mesospheric Fe layer at midlatitudes

The Fe layer abundance and root-mean-square (RMS) width have strong annual variations, with minima in summer. The abundance varied from 3.5 × 10⁹ to 25 × 10⁹ cm^-2 with a mean of 10.6 × 10⁹ cm^-2, and the RMS width varied from 2.3 to 5.2 km with a mean of 3.4 km. The centroid height of the Fe layer has a strong semiannual variation, with minima at the solstices. The centroid varies from 86.0 to 90.3 km and has a mean of 88.1 km. Sporadic Fe (Fe_s) layers were present about 27% of the total observation time. The Fe measurements are compared with the extensive Na layer observations obtained during the past decade at Urbana and with common volume observations made simultaneously on 24 nights with a Na temperature lidar. The mean Fe column abundance is approximately twice the mean Na column abundance. The Fe layer centroid height is also on average nearly 4 km lower and the RMS width is approximately 24% narrower than the corresponding Na layer parameters. -from Authors