Seasonal variations of the mesospheric Fe layer at Rothera, Antarctica (67.5°S, 68.0°W)

Lidar observations of Fe densities between 75 and 105 km above Rothera, Antarctica, are used to characterize the seasonal variations of the mesospheric Fe layer near the Antarctic Circle and the differences are compared to the South Pole. The maximum Fe abundance occurs in late autumn (early May) at Rothera, rather than in midwinter. A secondary Fe enhancement occurs 6 months later in late spring (October-November) prior to the formation of polar mesospheric cloud (PMC) layers in summer. The midsummer Fe layer is 3 km lower at Rothera because Fe depletion by PMC layers near the mesopause is not as extensive or as complete as at the South Pole. These observations are modeled satisfactorily using a mesospheric one-dimensional Fe chemistry model driven by a general circulation model and including a detailed micrometeoroid flux and ablation model. Our study shows that the autumnal maximum in the Fe abundance is caused primarily by the seasonal temperature maximum in the mesopause region, reinforced by the seasonal peak in the meteor input function (MIF). The Fe abundance at Rothera declines throughout the winter in response to the decrease in the MIF and the slowly falling temperatures. The modeled Fe injection rate is ∼5 times smaller while the eddy diffusivity values between 80 and 90 km are 4.1 times smaller than the corresponding values used in the South Pole model. This comparison demonstrates the sensitivity of the metal atom densities to the balance between injection by meteoric ablation and removal by downward transport.