Influence Of Lightning Multiplicity And Ionospheric Inhomogeneity On The Initiation Process Of Sprites

Sprites are one of the prevalent types of transient luminous events(TLEs)occurring in the altitude range of 50～90 km,and they are typically triggered by intense positive cloud-to-ground(+CG)discharges in thunderstorms.Research on sprites has important academic value and potential application value,which will help to reveal the energy transmission in the atmosphere,global electrical circuit and the safe operation of aircraft in the adjacent space.In this dissertation,we have established three-dimensional lightning electromagnetic coupling numerical model.The effects of tropospheric lightning multiplicity and atmospheric inhomogeneity of the ionospheric D-layer on sprites are studied,and some possible causes of horizontal displacement between sprites and parent lightning discharges are simulated and verified.The main conclusions are summarized as follows:(1)Two three-dimensional lightning electric field coupling models are established,including a three-dimensional quasi-electrostatic(3-D QES)field model and a three-dimensional electromagnetic pulse(3-D EMP)field model.The 3-D QES model is established based on the finite difference method(FDM),and the 3-D EMP model is established based on the finite difference time domain method(FDTD)in three-dimensional cartesian coordinate system.The nonlinear effect of ionosphere and simulation calculation of optical emissions are considered in both models.The simulation accuracy of the 3-D models is verified by comparing with the existing literature results or 2-D axisymmetric model results.The parameter setting of three-dimensional model is more flexible than 2-D axisymmetric model,which provides a reliable model tool for further studying the influence of the tropospheric lightning multiplicity and inhomogeneities of ionospheric atmosphere on sprites.(2)The effect of tropospheric lightning multiplicity on the initiation and optical emissions of sprites are studied.Below 60 km,the lightning-generated electric fields with the same charge moment change(CMC)are strongly influenced by different discharge models,but above 60 km,the electric fields and light radiation intensity in the ionospheric D-layer are not influenced by the discharge models.When the horizontal radius of the charge region is 10 km,the electric fields and optical emissions generated by the lightning in the ionospheric D-layer are the largest.The electric fields and optical emissions produced by the lightning of asymmetric ellipsoidal charge region in the ionospheric D-layer are also asymmetric,and the horizontal distribution and intensities of optical emissions are affected by the observation orientation.For the inclined discharge model,the positions of optical emissions in the ionospheric D-layer depend on the position of the tropospheric charge region.Considering two adjacent charge regions separated by 40 km,and two flashs have the same intensity and the same polarity,the electric fields in the ionospheric D-layer have the same direction generated by two +CGs,and the total electric field amplitude increases due to superposition.The corresponding optical emissions center is located in the middle of the two charge regions,and the displacement from the center of the charge region is 20 km.When the two ground flashes have the same intensity but opposite polarity,the directions of the electric fields in the ionospheric D-layer are opposite generated by the +CG and the adjacent –CG,the total electric field amplitude decreases and splits into two regions with opposite directions,and the corresponding optical emissions are also divided into two regions.The center positions of the optical emissions are offset from the respective charge regions by 13 km.(3)The influence of atmospheric gravity waves on the initiation and optical emissions of sprites are investigated.Less air density at atmospheric gravity waves trough leads to lower breakdown threshold,making it easier to produce air brokdown,so sprite halos and subsequent sprite streamers are more likely initiate at atmospheric gravity waves trough.The spatial distribution of atmospheric gravity waves will lead to the distortion of sprite halos optical emissions: the intensities of sprite halos optical emissions along the trough of atmospheric gravity waves is larger than that along the peak position.Sprite halos optical intensity and morphological characteristics will also be affected by the observation orientation;the greater atmospheric gravity waves disturbance,the greater impact on sprite halos optical emissions morphology,when the disturbance amplitude of atmospheric gravity waves exceeds30%,the relative change of sprite halos optical radiation amplitude will exceed 50%.The larger the tropospheric lightning intensity,the less influence of atmospheric gravity waves on the optical emissions.(4)The possible causes of horizontal displacement between sprites and parent lightning are simulated and verified.The 3-D QES model and the 3-D EMP model are used to simulate and verify the existing possible hypotheses in the literature.In addition,new possible causes are proposed and verified based on the previous simulation results in this paper.The possible causes that can cause the horizontal displacement between sprites and parent lightning mainly include: horizontal discharge channels of lightning within the thunderstorm cloud,inclined discharge model of thunderstorm cloud,electron density inhomogeneities distribution in the ionospheric D-layer,and the influence of two ground flashes in adjacent charge regions.The simulation results in this paper will provide the theoretical basis for explaining the phenomenon of sprites horizontal displacement.