Roles played by electric field, vertical wind and aurora in the source, formation and evolution of thermospheric Fe/Fe+ layers at high latitudes

Thursday, 18 December 2014
Zhibin Yu, Univ. of Colorado at Boulder, Boulder, CO, United States and Xinzhao Chu, University of Colorado at Boulder, Boulder, CO, United States
The least-understood region of 100-200 km is crucial to the thermosphere, ionosphere and space weather. The roles of atmospheric gravity waves in transporting energy and momentum and causing atmospheric and ionospheric disturbances have been recognized by theoretical studies and observations. The thermospheric neutral Fe layers discovered by lidar observations at McMurdo, Antarctica, exhibit well-defined gravity wave signatures in the altitude range of 100-200 km. Another Fe lidar at Davis also observed Fe layers up to at least 150 km with a diurnal period of reoccurrence. These thermospheric metal layers provide an excellent tracer for measuring neutral temperatures and winds in the thermosphere as well as in studying wave dynamics. Our theory argues that the observed Fe layers are a result of coupling of electrodynamical, neutral dynamical and chemical processes.

A time-dependent, 1-D, high-latitude Fe/Fe+ model has been developed to simulate the observed Fe layers based on the first principles of physics and chemistry. In this paper, we will provide quantitative analyses of the source, formation and transport of thermospheric Fe atoms and confirm that they are produced by neutralization of converged Fe+. The model shows that gravity-wave-induced wind shears converge Fe+ layers and the wave-induced vertical wind transports Fe layers to form the observed layer shapes. Furthermore, electric field causes upward flow transporting Fe+ ions from the main deposition region into the thermosphere. At the same time, electric field can help converge but can also destroy wind-shear-converged Fe+ layer, depending on the relative phase. In this paper, the competitions between wind-shear and electric field driving forces will be investigated to study the neutral-ion (Fe/Fe+) coupling. Our observational data also show that gravity-wave-driven neutral Fe layers are modulated by longer period waves or tides. These events will also be examined by our numerical model.