Date of Award

Spring 2024

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Ningyu Liu

Second Advisor

Joseph Dwyer

Third Advisor

Kai Germaschewski

Abstract

Streamers are self-consistent ionization waves that are generated within an electric field. Since they were first proposed in the theory of spark breakdown developed in the 1930s by Raether, Loeb, and Meek [Raether, 1939; Loeb and Meek, 1940], they have demanded the attention of scientists across many different fields. They are crucial to the Earth environment because of their natural occurrence in the form of sprites, at near-ionosphere altitudes above a thunderstorm, and their proposed roles in the formation and propagation of lightning. % that can occur above a thunderstorm, or However, they also play a role in the human environment, from their use in removing NOx byproducts from the imperfect combustion of a diesel engine [vanVeldhuizen, 2000] to their use in plasma medicine. Understanding their dynamics is of paramount importance in order to both reveal the electrical coupling of the atmospheric regions over which they occur and enhance their useful application. Although much effort has been put towards understanding their processes, many questions still remain. By investigating streamer discharges through the use of numerical modeling and high-speed image analysis, more insight may be gleaned. In order to explore the role of streamer discharge in lightning initiation, we use a 3D axisymmetric fluid model to analyze streamer initiation from a hydrometeor with dimensions similar to that found in a thunderstorm. Hydrometeors are ice or water particles which form in a variety of shapes and sizes. Our results show that a negative streamer is able to initiate from the tip of cone-shaped hydrometeor within ambient electric field conditions weaker than that necessary for electrical breakdown. A positive streamer is able to form from the base of the cone and is found to be less dependent on the hydrometeor geometry. The findings reveal the importance of the structure of the hydrometeor for negative streamer initiation and that if a negative streamer forms alone, the geometry is likely not an isolated cone shape. Our next investigation involves the analysis of the electromagnetic frequency emission spectrum from a positive and negative streamer colliding. This type of collision is believed to occur within the streamer zone of a leader, a component of lightning propagation, and in sprites. By performing multiple simulations in which the colliding streamers are initiated from the cone-shaped hydrometeors we are able to test the effects of ambient field conditions and length of propagation on the emission spectrum. We find that an increased ambient field magnitude shifts the spectrum to higher frequencies and that streamer propagation length increases the amplitude of the signal. Sprites are formed within the quasi-static electric field generated by a cloud-to-ground lightning discharge at 40-90 km altitudes above a thunderstorm and consist, primarily, of systems of streamers. Recent analysis of high-speed sprite image observation revealed a potential method that uses the rate of optical intensity decay of a streamer interacting with the D-region ionosphere to estimate the electron number density of the local region above a thunderstorm. We simulate an upward negative sprite streamer interacting with the ionosphere in order to interpret these results. Upon interacting with the ionosphere, our simulated streamer shows similar behavior to that seen in the observations. Furthermore, the optical intensity decay leads to a small factor lower estimation of the electron density, which promotes the validity of the proposed method. Our results also provide more insight into the interactive dynamics between a streamer and a conductive body. Through analysis of high speed sprite images of a carrot sprite event, we probe streamer system dynamics. We first develop a detection/tracking algorithm to analyze individual streamers within the images. Then we apply the algorithm to two branching events, where we find that the total optical intensity is unchanged from parent streamer to daughter streamers. We then investigate the collision of a single streamer and a separate streamer channel. Our findings indicate that it may have an effect on the streamers whose channel it connected. We do observe that upon connecting the streamer rapidly grows as it propagates down the channel. Finally, we present a statistical distribution of the branching angle, calculated by the tracking algorithm, from 41 separate events. Our findings are consistent with previous sprite observations, however, differ from the findings in laboratory experiments.

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