• Radiowave Scattering Structure In The Disturbed Auroral Ionosphere: Some Measured Properties

      Fremouw, Edward Joseph; Philip, K.; Parthasarathy, R.; Tryon, J.; Owren, L. (1966)
    • The role of electric fields in the occurence, structure, and drift of thin metallic ion layers in the high-latitude ionosphere

      Bedey, David Franklin (1996)
      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.
    • Time-dependent electron transport and optical emissions in the aurora

      Peticolas, Laura Marie; Lummerzheim, Dirk (2000)
      This thesis presents the first time-dependent transport model of auroral electrons. The evolution of the spherical electron intensity in phase space is studied for a variety of incident electron intensities. It is shown that the secondary electrons with energies <10 eV and at altitudes >150 km can take over 300 ms to reach steady state in phase space. Since there are bright optical emissions in this region, such a time dependence in the auroral electrons is important. The emissions of N2(2PG) 3371 A and <math> <f> <rm>N<sup>+</sup><inf>2</inf></rm></f> </math> (1NG) 4278 A are studied for time-varying electron pulses to show for the first time that this ratio will change until the secondary electrons reach steady state in the ionosphere. The way in which the 3371A/4278A ratio changes with time-varying precipitation depends on the precipitating electron spectra. The changes in the emission ratio can be used to learn more about the auroral acceleration region and the role of the ionosphere in auroral emissions. Field-aligned bursts (FABs), often observed in electron spectra of instruments flying over flickering aurora, are modeled with the time-dependent transport model. How the ionosphere modifies these electrons is shown. The 3371 and 4278 A emissions of flickering FABs are modeled to study the optical effects of modulated electron intensities in time. A study of 4278 A emissions for electron source regions from 630 to 4,000 km are studied along with frequency variations from 5 to 100 Hz. This study shows that the percent variation of the maximum to the minimum column brightness is less for higher frequencies and more distant source regions. It is shown that with an accurate time-dependent transport calculation and 4278 A emission observations of flickering aurora it should be possible to deduce the source altitude of the modulated electrons creating the optical flickering.