Statistical Analysis And Reasons For Oceanic And Continental Distribution Of Red Sprites

Red sprite is a kind of TLEs with the most complex structure and the longest luminescence time affected by the parent lightning.A scientific understanding of the morphological distribution characteristics and causes of the red sprites is essential for studying the exchange of matter and energy between the troposphere and the middle and upper space and the future prediction of the electromagnetic space environment.Thus,more and more scientific researchers began to pay attention to the red sprites.The morphological characteristics of red sprites are closely related to the discharge behaviour of the parent lightning.So,to statistically analyze the distribution and morphological differences of the red sprites and the causes,combining the long-term optical data and sferics of red sprites and lightning is necessary.Much ground-based comprehensive observations about the red sprites and its parent lightning have been studied.However,due to the limitations of ground-based observations,the few cases over the ocean cannot show the complete distribution of red sprites over the land and sea,let alone the reasons.Based on the long-term ISUAL optical data for TLEs and the ground-based sferics data for lightning,supplemented by ground-based optical data,radar data,cloud top brightness temperature data,ERA5 reanalysis data,and other meteorological data,this paper explores the distribution differences of the polarity,structure,and luminescence duration of red sprites over the ocean,land,and sea-land junctions in the middle and low latitudes.Combined with the analysis of the development of thunderstorms and the discharge characteristics of intense cloud-to-ground strokes(CG),the distribution of red sprites and the reasons for their different morphological manifestations are discussed.The main conclusions are summarized as follows:(1)The occurrence and continuous luminescence of red sprites are the result of the joint action of the driving electric field triggered by ground flashes and the atmospheric background conditions at the height of occurrence.Halo is mainly produced by the initial part(<1 ms)of the continuing current during the causative+CG stroke,At the same time,the streamers are more dependent on the effect of the subsequent continuous current of the parent lightning on the middle and upper atmosphere.E-field simulations show that the triggering of the halo is mainly the contribution of the peak current of lightning return stroke;the E-field generated by the instantaneous charge transfer may be more important than the E-field generated by the charge reconfiguration.Compared with the halo,the electrostatic field generated by charge reconfiguration under the long-term continuous current of lightning is more important to the streamers than the induction field generated by instantaneous charge transfer.(2)Regarding sea and land distribution,the pure halos and the negative red sprites accompanied by halos are more likely to appear in the border area between the ocean and the land.In contrast,the positive red sprites accompanied by streamers are evenly distributed over the sea and land with no apparent geographical tendency.The distribution density of negative red sprites over the ocean is more significant than that over land.The junction of sea and land where more than one-third of red sprites recorded is a high-incidence area of red sprites.In terms of luminescence duration,there is little difference between sea and land for the luminescence duration of pure halo events.However,the glowing time of sprite halos over the ocean is longer than that in the land area,particularly for the +sprite halo,the median difference of duration between sea and land reaches 1.5 ms,while for-sprite halo is only 0.65 ms.The glowing duration of sprite without halo feature is significantly longer than that of pure halo and sprite halo.Its incidence near the coast is about only half that of land and sea.(3)Before return stroke,the breakdown time scale of intense-CG is higher than that of+CG,no matter over the ocean or land,since the electromagnetic pulse triggered by the breakdown when negative leader extends is richer than that of the positive leader.However,it is opposite after the return stroke.Especially over the ocean,the breakdown time scale of intense+CG is about 10 ms longer than that of-CG.The fact that the breakdown time of+CG is higher than that of-CG under the same underlying surface suggests that the collection time of positive charges inside the cloud after the return stroke is longer,which may result in a more transferred charge and a longer transfer time.This may provide a possible explanation for the longer glow duration of positive red sprites with streamer feature than that of negative red sprites.(4)The intense flashes over the middle and low latitude continents and the nearby seas are concentrated near the coastline.The lightning distribution in this latitude strongly correlates with CAPE value and precipitation distribution.The coastline area with high CAPE value area and high precipitation leads it to become the area where the intense flashes are concentrated.Atmospheric conditions between lakes and land,as well as oceans and land,are conducive to the development of thunderstorms and further promote the triggering of lightning.This attraction of thunderstorms by lake shorelines can be strong enough for thunderstorms moving along the large circulation field to change their former direction.The thunderstorm moving from the ocean to the continent slows down and expands significantly after reaching the coastline with absorbing the small thunderstorm cells generated nearby.More importantly,the number of CGs,incredibly intense CGs,begins to increase.The TLEs pay more attention to high peak current intensively outbreak when thunderstorms reach the coastline,such as elves,pure halo,sprite halo.After the thunderstorm continued to expand its coverage area at the coastline by absorbing newly generated single cells,the TLE paying more attention to long-lasting current mainly appeared,for example,red sprite without halo feature.Through observation and case analysis,we briefly explained the possible reasons for the high incidence of halo feature sprites on the coastline.Based on the long-term observation data of red sprite and lightning,this thesis revealed the sea-land distribution and morphological differences of the red sprites and their causes through the combination of statistics and case analysis.A scientific understanding of the distribution and causes of the red sprites is of great significance to ensure the safe exploration of the near-Earth space environment,and also provides a theoretical basis for the future environmental prediction of the space electromagnetic environment.