The role of electric fields in the occurence, structure, and drift of thin metallic ion layers in the high-latitude ionosphere

Abstract

Metallic ions deposited in the upper atmosphere through the process of meteoroid ablation can, on occasion, be forced into dense layers at altitudes of 90-120 km with a thickness of <2 km. These layers result from a combination of appropriately directed neutral winds and electric fields. The objective of this thesis is to gain new insights into several poorly understood aspects of these atmospheric structures. An experimental program has been developed to determine layer morphology and temporal occurrence in relation to ionospheric electric fields. These metallic ion structures have been found to be often spatially limited, and highly variable in both their location and time of occurrence. Simultaneous electric field measurements confirmed the dominant role of these fields in the formation of thin layers at high latitudes. A time-dependent numerical model was used to simulate data, in an attempt to understand why layers tend to be observed at lower altitudes than theoretically predicted. It was found that adopting a reduced value for the ion-neutral collision frequency brings observations and theory into agreement. Empirical determinations of the collision frequency indicated values that are about a factor of ten smaller than predicted by the induced-dipole model now used in other ionospheric studies. Observations indicate a greater rate of layer occurrence during the summer months. An explanation for this seasonal effect is proposed that invokes an annual variation in large-scale electric fields, suggested by an empirical model of the high-latitude convection pattern. The large-scale circulation of metallic ions has been investigated. This examination suggests that the structure of the convection pattern controls the redistribution of metallic ions, which in turn defines where and when layers may occur. The results of this analysis explain the limited times of layer occurrence, as well as the absence of layers even when appropriate formation conditions exist. Finally, a theoretical analysis indicated that layers can drift horizontally at speeds exceeding 100 m/s. Observations confirmed this result. This suggests that advection may be important when interpreting observations of evolving metal layers.