PHOTOIONIZATION AND PHOTODETACHMENT EFFECTS OF LIGHTNING DISCHARGES IN THE D REGION OF THE EARTH’S IONOSPHERE

Open Access
- Author:
- Janalizadeh, Reza
- Graduate Program:
- Electrical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 07, 2021
- Committee Members:
- Eugene Clothiaux, Outside Unit & Field Member
Julio Urbina, Major Field Member
Victor Pasko, Chair & Dissertation Advisor
Tim Kane, Major Field Member
Kultegin Aydin, Program Head/Chair - Keywords:
- sprite
streamers
nonthermal gas discharges
extreme ultraviolet
photoionization
photodetachment
meteoroid
ionosphere
transient luminous events
plasma inhomogeneities
molecular nitrogen
molecular oxygen - Abstract:
- Sprites are large-scale mesospheric gas discharges produced by intense cloud-to-ground (CG) lightning discharges in the underlying thunderstorms. These luminous discharges often exhibit a brief descending high-altitude diffuse glow in the shape of a pancake with diameters up to 80 km near 75 km altitude, referred to as a sprite halo, and develop into fine-structured filaments with diameters up to several hundred meters in the altitude range of 40 to 90 km, commonly referred to as sprite streamers. Since the first video documentation, sprites have attracted extensive research interest in the last two decades, primarily due to their potential as natural resources for the study of streamer physics, their potential impact on the chemistry of the upper atmosphere, and their ability to perturb the subionospheric radio signals. However, up to date, the inception mechanism of sprite streamers, among other scientific questions remains an outstanding issue to be resolved. In particular, although the streamer nature of sprites has been explained theoretically and confirmed experimentally, the origin of plasma inhomogeneities from which sprite streamers are initiated in the lower ionosphere has not yet been identified. We emphasize that plasma inhomogeneities in the lower ionosphere are crucial for the initiation of sprite streamers, and the ultimate goal of this research is to find the answer to the question: ``What is the origin of plasma inhomogeneities, which are located at sprite altitudes and are necessary for initiation of sprite streamers"? In this dissertation we investigate sprite streamer initiation for three different scenarios. In the first case we consider sprite streamer initiation due to the photoionization and electron impact ionization of neutral metallic species, which are deposited in the lower ionosphere through meteoric ablation, under the application of the lightning-induced electric field and energetic emissions of the sprite halo. In order to investigate this hypothesis we calculate the electron impact ionization rate of these species by compiling the electron impact ionization cross sections of meteoric species and inputting them to BOLSIG+ software to obtain the ionization rate. To calculate the photoionization rate of these species due to emissions from the sprite halo we compile the necessary photoionization cross sections and follow the same approach taken for the modeling of photoionization of molecular oxygen, O2, due to nonthermal gas discharges in air. The ionization potential of meteoric species determines the energy of halo emissions which result in photoionization. The ionization potential of the species considered is much less than that of O2, and therefore, not only extreme ultraviolet (EUV) photons (capable of photoionizing O2) but also Lyman-Birge-Hopfield (LBH) emissions of molecular nitrogen, N2, which are observed in the sprite halo spectrum, contribute to photoionization of metallic species considered. In the second case we investigate sprite streamer initiation as a result of photodetachment of negative ions in recently-formed meteor trails due to the optical and ultraviolet (UV) emissions of the sprite halo. The electron affinity of the ions considered is much less than the ionization potential of meteoric species. Therefore, sprite halo emissions with less energy (compared to UV photons) are also energetic enough to cause photodetachment of the considered negative ions. Thus, in addition to UV photons, optical emissions of the sprite halo are considered. The approach to the photodetachment problem is similar to the photoionization problem mentioned above. Photodetachment cross sections of the main negative ions of the lower ionosphere are compiled from the literature and photodetachment rates due to UV and optical emissions of the sprite halo are calculated. In the third (and final) case we consider sprite streamer initiation from enhanced levels of electron density observed in meteor trails shortly after their formation under the application of the lightning-induced electric field. We start with studying the relaxation of meteor trail ionization in the background ionosphere. The level of electron density in the trail at specific moments of time is compared against the level required for initiation of sprite streamers. While at lower altitudes, e.g., 70 km, the trail ionization dissipates in a fraction of a second, it is demonstrated that at higher altitudes, e.g., 85 km, sufficient levels of electron density persist in the trail for as long as a few minutes. In this case, initiation of sprite streamers from relatively dense meteor trails is demonstrated. In all scenarios inception of sprite streamers is studied via a two-step halo-streamer technique, which is partially comprised of a previously introduced plasma fluid model for numerical modeling of sprite halos and sprite streamers. To address the science question formulated above, we combine this technique with newly developed theoretical modeling, which stems from each of the scenarios mentioned.