Impact of horizontal transport, temperature, and PMC uptake on mesospheric Fe at high latitudes

Abstract

AbstractWe analyze year‐round Fe lidar observations made in Antarctica at Rothera (67.7°S), South Pole (90°S), and McMurdo (77.8°S). During midsummer, when the mesopause region is continuously sunlit, the Fe density between 84 and 88 km is independent of temperature, because photolysis of FeOH is so fast that virtually all of the FeOH is converted to Fe via this path, rather than via the temperature‐dependent FeOH + H reaction. The extremely low summertime densities at Rothera and South Pole are caused primarily by meridional transport northward out of the polar cap and, to a lesser extent, by uptake of Fe species on polar mesospheric cloud (PMC) ice particles. In midwinter, both meridional transport and temperature dominate Fe variations. The temperature sensitivity of Fe during winter is 2.2%/K at Rothera and 3.0%/K at South Pole. The annual mean Fe abundance at McMurdo is more than 50% larger than that observed at any other lidar site in both the Northern and Southern Hemispheres. McMurdo is located at the magnetic poleward edge of the auroral oval, just north of the deep polar cap. We hypothesize that southward transport of Fe+ out of the auroral oval in winter and northward transport of Fe+ out of the deep polar cap and auroral zone in summer, to McMurdo where it is neutralized, could be the source of the enhanced Fe. Theoretical calculations show that Fe+ densities of ~13,000 cm−3 in midwinter and ~1600 cm−3 in midsummer, between 84 and 88 km, are required to account for the high Fe densities at McMurdo.