Evidence for substantial seasonal variations in the structure of the mesospheric Fe layer

Lidar measurements of mesospheric Fe were conducted at Urbana, IL (40°N, 88°W) during the fall of 1989 and the spring through fall of 1991. The Fe layer abundance, centroid height and rms width varied substantially during the observation period with minimum values for all three parameters in summer. The abundance varied from 3.3×10⁹ ‐ 25×10⁹ cm⁻², the centroid height from 86.7 – 90.4 km and the rms width from 2.1 – 3.9 km. Sporadic Fe (Fe_s) layers were also frequently observed. The Fe measurements are compared with the extensive Na layer observations obtained during the past decade at Urbana. The mean Fe column abundance is more than twice the average Na column abundance. The Fe layer is also on average approximately 3.6 km lower and 32 % narrower than the Na layer. The seasonal behavior of the Fe layer is discussed in light of our preliminary temperature measurements of the mesopause region as well as a simple chemical model.