Observation And Simulation Of Mesospheric Metal Layers

This dissertation aims mainly at researching the physical mechanisms behind the behavior and variation characteristics of mesopause metal layers. It particularly focuses on the potential influence of meteor ablation on metal layers, which is the source of mesopause metal materials. This dissertation also attempts to explain some unsolved and controversial questions about metal layers. The main contents are listed as followed:1. The seasonal/annual characteristics of the high-altitude sporadic metal atom layers are presented on the basis of extensive Na and Fe lidar measurements at 30°N during the past several years. It is found that the extremely high sporadic Na (Nas) and Fe (Fes) layers above 105 km occurred mostly during summer. They had long durations (a few hours) and broad layer widths (much larger than 2 km). Their absolute peak densities could be comparable to or even larger than those of the corresponding main layers on a few nights. By using all the raw data profiles including sporadic layers, we have constructed the contour plots of Na and Fe densities versus month and altitude at 30°N. The Na and Fe layers both exhibit evidence for summer topside extension, which is consistent with the earlier observations for K and Ca at different latitudes. The summer topside extension of mean metal atom layers might represent a universal phenomenon that is alike for different atom species, different geographic locations and different measurement years. The extremely high sporadic metal atom layers above 105 km occurring during summer give rise to the phenomenon.2. Using the resonance fluorescence lidars, we have carried out observations of metal layers during 2001 (Na) and 2004 (Na and Fe) Leonid meteor showers at Wuhan (30.5°N,114.4°E), China. The strong outburst of Leonid shower on 18-19 November 2001 resulted in our capture of a strong Na atom meteor trail. However, there was unlikely to exist any corresponding enhancement of Na column density by the ablation of meteor shower. On the same night three years later, during the calm 2004 Leonid shower, we captured no meteor trail but saw complex sporadic Na and Fe layer events around 95 km after the radiant of Leonid shower rose above the horizon. By comparison, the column densities above 92 km of both Na and Fe atoms showed prominent enhancements after midnight. In addition, the Na and Fe column densities on this night were much bigger than those on the night one day earlier. Among a total of four night observations during Leonid shower, the sporadic layers were observed on three nights. They appeared at similar time and altitude as well as moved with similar tendencies. Current cometary theory supports that Leonid meteor shower could contain both meteoroids that would produce visual meteors and micrometeoroids. Based on this knowledge, we consider that the Earth in 2004 may encounter a large swarm of small-sized micrometeoroids which belong to the Leonid meteor shower. The mass flux of these micrometeoroids might be much higher than that of the visual meteor shower.3. We establish the single meteoroid ablation model, which has taken into account the charging and sputtering processes besides some traditional factors. With this model, we can calculate the ablation profiles of both ablated metal atoms and ions. Also, we have modeled the mass and temperature evolutions of an ablated meteor trail. These models provide a basis for researching the relations between meteoric ablation and metal layers. Based on reliable parameters of sporadic meteoroid sources, we have modeled the temporal variation of mean metal atoms input in the mesosphere and lower thermosphere (MLT) region. Surprisingly, the annual mean diurnal variation of modeled atoms input shows similar to that of observed atoms density under certain condition. As none of known factors are found to be the reason for the mean diurnal variation of atoms density, we have made the following conjecture. Some short-lifetime (no longer than several hours) atoms dominate the mean diurnal variation of atoms density and the other traditional long-lifetime (several days) ones provide a nearly constant background density. The two together compose our metal layers. Obviously, these predicted short-lifetime atoms need a fast sink mechanism, which acts mainly on the newly ablated atoms. The mechanism is not clear but may have relation to high temperature caused by ablation.