Simulations of the dynamics of positive and negative sprite halos
Open Access
- Author:
- Xi, Hao
- Graduate Program:
- Electrical Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- March 16, 2015
- Committee Members:
- Victor P Pasko, Thesis Advisor/Co-Advisor
Jianqi Qin, Thesis Advisor/Co-Advisor
Julio Urbina, Thesis Advisor/Co-Advisor - Keywords:
- Sprites
streamers
halos
TLEs
Plasma
Fluid
Numerical Modeling
Numerical Instability
Electromagnetics
Atmospheric Science - Abstract:
- Sprite halos exhibit a brief descending diffuse glow in the shape of a pancake with diameters up to $\sim$80 km and near $\sim$75 km altitude. Sprite halos are diffuse optical phenomenon observed at high altitudes in Earth's atmosphere that is produced by intense cloud-to-ground lightning discharges in the underlying thunderstorms. Often times, sprite halos may also develop into sprite streamers, highly-structured filaments with diameters up to hundreds of meters in the altitude range of $\sim$40 km to $\sim$90 km. Extensive video observations and modeling studies have been directed towards the topic of sprite halos and sprite streamers over the past two decades. A recent study has suggested that the pre-existing plasma irregularities in the D-region of ionosphere play a vital role in sprite streamer initiation, which, in an agreement with up-to-date experimental conclusions, might challenge former sprite streamer initiation theory that sprite streamers are initiated as a result of the sharpening and collapse of the screening ionization wave associated with sprite halos. This thesis demonstrates that what was considered as a primary sprite streamer initiation in former modeling studies would actually be a manifestation of a numerical instability. In order to accomplish the purpose of this thesis, we construct a two-dimensional plasma fluid model, capable of numerically modeling the developmental stages of sprite halo events. The model is designed with the flexibilities of altering essential parameters and thus allowing a better comparison of model results with observations. Next, a parametric study is conducted for model validation purposes. Comparisons and analyses are made for different model cases of charge moment changes of various magnitudes, as well as lightning waveform with different parameters of impulsiveness. The charge moment change is an important quantitive parameter in the present study that is defined as the amount of charge transferred by cloud-to-ground lightning times the altitude from which it was removed. Consistency of our modeling results with previous modeling studies and video observations is demonstrated. A literature review on polarity asymmetry of sprites and relative discussions are also included. Our simulation results indicate that the numerical instabilities, identified as regions with electron densities at least 10 times higher than that of surrounding areas, show great dependencies on the spatial resolution of a numerical scheme. These instabilities always emerge at a relatively fixed location for different charge moment changes and morphologically relate to the polarity of lightning current moment. Finally, a quantitive criterion is proposed to identify and detect the appearance of such instabilities by monitoring the increase of the spatial gradient of electrons within the simulation domain. We introduce a minimum ``e-folding'' parameter, $m_0$, which measures the steepest spatial gradient between adjacent grids within the simulation domain. We conclude that when the minimum ``e-folding'' parameter drops to half of the spatial resolution, due to spatial discretizations failing to correctly resolve such a steep spatial gradient, numerical instabilities develop. In practical calculation, such instabilities become apparent when $m_0$ drops to $\sim$0.4 times smaller than the spatial resolution. After that, the results obtained should be considered as containing incorrect numerical artifacts. In the end, a summary of the work in this thesis as well as suggestions on possible future research topics are presented.