Global transport and localized layering of metallic ions in the upper atmosphere

Abstract

A numerical model has been developed which is capable of simulating all phases of the life cycle of metallic ions, and results are described and interpreted herein for the typical case of Fe + ions. This cycle begins with the initial deposition of metallics through meteor ablation and sputtering, followed by conversion of neutral Fe atoms to ions through photoionization and charge exchange with ambient ions. Global transport arising from daytime electric ®elds and poleward/ downward diusion along geomagnetic ®eld lines, localized transport and layer formation through descending convergent nulls in the thermospheric wind ®eld, and ®nally annihilation by chemical neutralization and compound formation are treated. The model thus sheds new light on the interdependencies of the physical and chemical processes aecting atmospheric metallics. Model output analysis con®rms the dominant role of both global and local transport to the ion's life cycle, showing that upward forcing from the equatorial electric ®eld is critical to global movement, and that diurnal and semidiurnal tidal winds are responsible for the formation of dense ion layers in the 90±250 km height region. It is demonstrated that the assumed combination of sources, chemical sinks, and transport mechanisms actually produces F-region densities and E-region layer densities similar to those observed. The model also shows that zonal and meridional winds and electric ®elds each play distinct roles in local transport, whereas the ion distribution is relatively insensitive to reasonable variations in meteoric deposition and chemical reaction rates.

The intermediate layers and associated tidal motions observed by a digisonde in the equatorial anomaly region

“…On the other hand, layer density does not increase during the afternoon layer descents. The phenomenon of the layer descent and concurrent density decrease resembles the simulation result of Carter and Forbes (1999), who found the evening layer to be less intense than the morning one. Carter and Forbes (1999) further reported that the diffusing zonal wind component in the afternoon would reduce the density (Carter and Forbes, 1999).…”

“…However, if the layer motion is considered, the increase/decrease of densities is more meaningful. During the single-peak months, the layer descent and associated density increase demonstrate that the descending convergent null could capture a large number of molecular and metallic ions (Osterman et al, 1994(Osterman et al, , 1995Carter and Forbes, 1999;Earle et al, 2000). Moreover, the morning descents in the doublepeak months may also share the same cause.…”

“…Regarding the plasma density of the layer, Osterman et al (1994Osterman et al ( , 1995 and Carter and Forbes (1999) demonstrated that molecular metallic ions dominate the compositions of the intermediate layers. In this work, the observed median values for the foEs of the layers (H and C type) are nearly 4-6 MHz (Fig.…”

Meteoric material—an important component of planetary atmospheres

“…A simple balance between these loss processes and ion production, including the effects of diffusion, would produce a single ion layer in the ablation region. Complexities are introduced by ionosphere dynamics whose importance is established for Earth; converging ion drifts due to combining neutral wind drag or electric fields with a magnetic field produce very narrow ion layers (see review by Kelley, [1989], and the modeling of Carter and Forbes, [1999]). One can assume with confidence that the terrestrial dynamic processes alsoapply, butto different degrees, to theother planets withmagnetic fields.…”

Global transport and localized layering of metallic ions in the upper atmospherer

“…We focus our attention here on Fe and Fe + since these are the most abundant of the upper atmosphere metallic atoms and ions, but the results are easily extrapolated to Mg/Mg + , Ca/Ca + , etc. In the model, Fe and Fe + are assumed to be deposited by meteor ablation in the 100±130 km height range (an extensive discussion including many historical references may be found in Carter, 1995 The following section details the numerical model, the Fe + chemistry, the dynamical and electrodynamic inputs to the model, and other input parameters. Section 3 describes the results of several numerical simulations, illustrating localized layering, the eects of large-scale transport by winds and electric ®elds, and the responses to geomagnetic storm and meteor shower perturbations.…”