Formation mechanisms of neutral Fe layers in the thermosphere at Antarctica studied with a thermosphere-ionosphere Fe/Fe~+ (TIFe) model

摘要

With a thermosphere-ionosphere Fe/Fe~+ (TIFe) model developed from first principles at the University of Colorado, we present the first quantitative investigation of formation mechanisms of thermospheric Fe layers observed by lidar in Antarctica. These recently discovered neutral metal layers in the thermosphere between 100 and 200 km provide unique tracers for studies of fundamental processes in the space-atmosphere interaction region. The TIFe model formulates and expands the TIFe theory originally proposed by Chu et al. that the thermospheric Fe layers are produced through the neutralization of converged Fe~+ layers. Through testing mechanisms and reproducing the 28 May 2011 event at McMurdo, we conceive the lifecycle of meteoric metals via deposition, transport, chemistry, and wave dynamics for thermospheric Fe layers with gravity wave signatures. While the meteor injection of iron species is negligible above 120 km, the polar electric field transports metallic ions Fe~+ upward from their main deposition region into the E-F regions, providing the major source of Fe~+ (and accordingly Fe) in the thermosphere. Atmospheric wave-induced vertical shears of vertical and horizontal winds converge Fe~+ to form dense Fe~+ layers. Direct electron-Fe~+ recombination is the major channel to neutralize Fe~+ layers to form Fe above ~120 km. Fe layer shapes are determined by multiple factors of neutral winds, electric field, and aurora activity. Gravity-wave-induced vertical wind plays a key role in forming gravity-wave-shaped Fe layers. Aurora particle precipitation enhances Fe~+ neutralization by increasing electron density while accelerating Fe loss via charge transfer with enhanced NO~+ and O_2~+ densities. Plain Language Summary The discoveries of neutral metal layers reaching near 200 km in the thermosphere have significant scientific merit because such discoveries challenge the current understandings of upper atmospheric composition, chemistry, dynamics, electrodynamics, and energetics. Furthermore, these neutral metal layers provide tracers for direct measurements of the neutral properties in the least understood but crucially important space-atmosphere interaction region in the altitude range of 100-200 km. With a thermosphere-ionosphere Fe/Fe~+ (TIFe) model developed from first principles at the University of Colorado, we present the first quantitative investigation of formation mechanisms of thermospheric Fe layers observed by lidar in Antarctica. The TIFe model formulates the TIFe theory depicting the lifecycle of meteoric metals via deposition, transport, chemistry, and wave dynamics. Antarctic lidar observations were successfully reproduced with the TIFe model simulations. These TIFe layers provide a unique natural laboratory for understanding the fundamental processes critical to advancing the research of space-atmosphere interactions.