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

Abstract

Lidar measurements of mesospheric Fe were conducted for 325 h during 75 nights at Urbana, Ill. (40°N, 88°W), in fall 1989 and from spring 1991 through summer 1992. 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 5.3 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 (Fes) 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. A chemical model of the mesospheric Fe layer is described and compared to various experimental results. The reaction of Fe with O3 to form FeO on the bottom side of the layer and the subsequent reaction of FeO with CO2 to form FeCO3 appear to be the dominant chemical sinks for Fe. The temperature dependency of the latter reaction may explain the annual variation in the column abundance. The lidar observations and the chemical model calculations suggest that the expected cooling of the mesopause region by approximately 10 K due to the doubling of CO2 and other greenhouse gases during the next century may reduce the mean Fe abundance by as much as 45% and the mean Na abundance by 55%.

Average properties and small-scale variations of the mesospheric Na and Fe layers as observed simultaneously by two closely colocated lidars at 30° N

“…In addition, the Na layer always shows a few kilometers more extension on its top side than the Fe layer. These features can be seen also from a comparison of simultaneously observed Na and Fe layers at high, mid, and low latitudes respectively (Kane and Gardner, 1993;Lynch et al, 2005;Shibata et al, 2006), which may be universal. They provide a connection between the two different metallic species in the mesopause region.…”

“…This is consistent with the fact that Fe is the main component of the meteoric source, whereas the alkali metals are the minor ones (Heide and Wlotzka, 1995). Among the alkali metals, only the Na layer has a typical abundance value approaching that of the Fe layer (Kane and Gardner, 1993;Gardner et al, 2005;Yi et al, 2009). Hence a detailed quantitative comparison between the Na and Fe layers at a given site is feasible.…”

“…Hence a detailed quantitative comparison between the Na and Fe layers at a given site is feasible. Kane and Gardner (1993) compared the Na and Fe layer data obtained simultaneously by two lidars at a midlatitude site (40 • N). They noticed that even though the individual density profiles appeared quite different, the temporal variations of Na and Fe layer parameters (abundances, centroid heights, and RMS widths) were highly correlated in most cases.…”

Sporadic Ca and Ca<sup>+</sup> layers at mid-latitudes: Simultaneous observations and implications for their formation

“…These authors found an average of 6.2% occurrence of strong Na s , although cited as much higher than observed at other mid-latitude locations. Kane and Gardner (1993) report on a total Fe s occurrence rate of 27% for the site of Urbana, Illinois (40 • N, 88 • W). In spite of this, the Ca s rate at Kühlungsborn, Germany was found to be 43% in annual mean.…”

Impact of a major meteor storm on Earth's ionosphere: A modeling study

“…The peak Fe density, occurring around 85 km, is some 1.5 x 10 4 cm -3 at maximum [Kane and Gardner, 1993] compared to a total density at this altitude of some 10 TM cm -3 [Hedin, 1991]. The total density of ionized metal, on the other hand, is observed to be perhaps only an order of magnitude or so less dense than the total ionosphere [e.g., Kopp, 1997] in the region where the metal layers predominate, around 95 km.…”