Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

Legacy Department


Committee Member

Dr. Miguel Larsen, Committee Chair

Committee Member

Dr. Gerald Lehmacher

Committee Member

Dr. Jens Oberheide

Committee Member

Dr. Chad Sosolik


During the hours near sunset, Earth’s equatorial ionosphere undergoes signif-icant changes as it transitions from day to night. This period is a dynamic time that preconditions the ionosphere for a number of plasma instabilities that may develop over the course of the night, including equatorial spread F. The neutral dynamics of the sunset ionosphere are of critical importance to the generation of the currents and electric fields which drive spread F and other instabilities; however, the behavior of the neutrals is experimentally understood through single-altitude measurements or measurements that provide weighted altitude means of the winds as a function of time. Vertically resolved neutral wind measurements in the F region near and after sunset are extremely rare. In this work, are presented several sounding rocket chemical tracer measurements, which are vertically resolved and show large westward winds and wind shears in the F region near sunset. Winds and shears of this magni-tude near sunset are unexpected based on current wind models, which show eastward neutral flow with very small gradients above 200 km altitude. The observed chemical tracer neutral wind profiles are applied to an existing spread F model in order to in-vestigate the effects of such large westward winds and shears during the transitional period near sunset on the subsequent development of spread F plumes. The unusual wind profiles observed in the experiments are shown to have a suppressing effect on instability development.



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