Chemical analysis
; F region
; Helium
; Ionosphere
; Atmospheric constituents
; Atomic oxygen
; Frame of reference
; Horizontal transport
; Ionospheric winter anomaly
; Seasonal bulge
; Transport
; Vertical transports
; Atmospheric movements
英文摘要:
A mechanistic description of neutral species transport is presented, reconciling two long-standing yet ostensibly incompatible interpretations of the “wind-induced diffusion” of thermospheric constituents. Regarding the creation and sustainment of anomalous winter distributions of light species (e.g., atomic oxygen or helium), the theory of Reber and Hays (1973) cites vertical advection as the main cause while Mayr et al. (1978) cite horizontal transport—both in the presence of a diffusively separated atmosphere. We demonstrate that these theories are not at odds with each other. Instead, they describe two coherent aspects of a single process: convergent (divergent) horizontal motion increases downwelling (upwelling), thereby transporting light constituents both horizontally and vertically simultaneously. Such a connection between horizontal and vertical transport is straightforward and well established for the mass-averaged circulation. Yet this connection has remained obscure for a single minor species entrained within the flow, in part due to the appearance of the composition equation when expressed in a Eulerian frame of reference. Concrete evidence is presented in terms of model equations and diagnostics, establishing a direct link between the horizontal and vertical transport of an atmospheric constituent. Consequently, we are able to show that winter accumulations of light species arise from an interhemispheric migration operating on seasonal timescales, rather than from their redistribution within the vertical column. This new understanding has implications for the timescales at play in both the F region ionosphere and lower exosphere, two regions profoundly impacted by light neutral species. Published 2016. This article is a US Government work and is in the public domain in the United States of America.