Numerical modeling of the inception, morphology and radio signals of sprites produced by lightning discharges with positive and negative polarity

Open Access

Author:

Qin, Jianqi

Graduate Program:

Electrical Engineering

Degree:

Doctor of Philosophy

Document Type:

Dissertation

Date of Defense:

February 20, 2013

Committee Members:

• Victor P Pasko, Dissertation Advisor
• Victor P Pasko, Committee Chair
• John David Mathews, Committee Member
• Julio Urbina, Committee Member
• Deborah A Levin, Committee Member
• Sebastien Celestin, Special Member

Keywords:

• Lightning
• Sprites
• Halo
• Streamers
• Inception
• Morphology
• Radio signals
• Modeling

Abstract:

Sprites are large-scale mesospheric gas discharges produced by intense cloud-to-ground 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 great impact on the chemistry in the upper atmosphere, and their ability to perturb the subionospheric radio signals. However, up to date, the inception mechanism of sprite streamers, the origin of different sprite morphology, the lightning polarity asymmetry in producing sprites, and the characteristics of the electromagnetic radiation from sprites remain outstanding issues to be resolved. The purpose of this dissertation is to investigate the above-mentioned outstanding issues through numerical modeling of sprite halos and sprite streamers using a previously introduced plasma fluid model and some newly developed models and simulation techniques. The first outstanding issue that is addressed by this dissertation is the inception mechanism of sprite streamers. Although the streamer nature of sprites has been explained theoretically and confirmed experimentally, how these filamentary plasmas are initiated in the lower ionosphere has not yet been well understood, as many spatial and temporal features of sprites in their initial stage cannot be explained by the modeling studies in the existing literature. To better understand the initiation of sprite streamers, a plasma fluid model, an improved avalanche-to-streamer transition criterion, and a 'two-step' technique are used to numerically simulate the initiation of sprite streamers from electron density inhomogeneities during the development of a sprite halo. The reported modeling results suggest an original mechanism for the inception of sprite streamers. This mechanism specifically explains the spatial and temporal offsets between sprite streamers and the preceding sprite halo, the 10s to 100s millisecond long delay of some sprites with respect to the onset of their causal lightning discharge, and the initiation of sprite streamers by lightning discharges associated with charge moment changes as small as ~200 C km. The inception of sprite streamers is demonstrated to depend strongly on the charge moment change of the causative lightning discharge and the ambient conditions in the lower ionosphere (i.e., the ambient electron density profile and electron density inhomogeneities), which together determine the size and location of the streamer initiation region. We specifically emphasize the crucial role of electron density inhomogeneities in the lower ionosphere for the initiation of sprite streamers, especially in the case of small charge moment changes. In the last two decades, video observations have revealed the complexity of sprite morphology, showing that sprites can be composed of many distinct, spatially separated filamentary streamers with a complex mixture of sizes, orientations and shapes. Generally, sprites are classified into carrot sprites, that exhibit both upward and downward propagating streamers, and column sprites, that contain predominantly vertical downward streamers. The origin of these different sprite morphologies is the second outstanding issue that is addressed by this dissertation. In order to find out the physical conditions that lead to the production of carrot sprites and column sprites, we study the dependence of sprite morphology on lightning characteristics and upper atmospheric ambient conditions. It is demonstrated that the morphology of sprites is mainly determined by the characteristics of their causative lightning discharges, namely the polarity, the total charge moment change, the impulsiveness of the return stroke, and the strength of the continuing current. A detailed relation between the above-mentioned lightning characteristics and the sprite morphology is established. We also quantify the impact of lower ionospheric ambient conditions (i.e., the ambient electron and ion density) on the threshold charge moment changes required to produce column sprites and carrot sprites. Another well-known puzzle is that sprites are almost exclusively produced by positive cloud-to-ground lightning discharges (+CGs), and that 'negative sprites' (produced by -CGs) are extremely rare, with only nine events documented in the existing literature compared to thousands of their positive counterparts. To understand the lightning polarity asymmetry in producing sprites, we investigate possible differences between the requirements for the initiation of positive and negative sprites. A parametric study shows that positive sprites require an ambient electric field with smaller magnitude and shorter persistence when compared to those required for the initiation of negative sprites. Consequently, the initiation of positive sprites requires a smaller threshold charge moment change than that of negative sprites (320 C km and 500 C km, respectively, under typical nighttime conditions). This difference between the initiation thresholds represents one of the key factors accounting for the polarity asymmetry of sprites. Other factors, including the charge moment contrast of +CGs and -CGs, and the differences originating from physical properties of streamers and observability of positive and negative sprites are also discussed. Furthermore, radio atmospherics observed in association with sprites demonstrate that significant electrical current is flowing in the body of sprites, which produces detectable electromagnetic radiation typically observed in the Extremely Low Frequency (ELF) band. In the previous literature, it was generally believed that sprites were unable to produce radio signals with higher frequencies in the Very Low Frequency (VLF) to Low Frequency (LF) range. Up to date, no theoretical work exists that is dedicated to studies of the possible VLF and LF radio signals from sprites. This dissertation work presents the first theoretical estimates of the electromagnetic radiation produced by individual sprite streamers using simulation results from plasma fluid modeling. It is demonstrated that the spectral content of the sprite radiation is highly dependent on the air density and the ambient electric field, i.e., sprite streamers propagating at lower altitudes with higher air density or propagating in a higher electric field produce a higher frequency radiation. Most interestingly, we quantitatively demonstrate that sprite streamers at low altitudes (~40 km) can produce VLF and LF radiation, that needs to be confirmed by future radio observations. Finally, this dissertation work presents research efforts that are devoted to a direct comparison of morphological features predicted by the streamer model with those appearing in video observations in order to derive the mechanism of column and carrot sprites from optical and radio observations, a direct comparison of optical emissions produced by the modeled halo events and high-speed video observations in order to develop a useful remote sensing technique to study the poorly known ambient conditions in the lower ionosphere, and an investigation of the impact of successive lightning strokes that are separated by tens of milliseconds on the initiation and propagation of sprite streamers.