Observation And Study On Oceanic And Continental Distribution Of Sprite Polarity Difference

Red sprite,as one of the most common phenomena of lightning-induced transient luminous events(TLEs)in the middle and upper atmosphere,is of very importance to the study of material and energy exchange between troposphere and mesosphere/low ionosphere,which has attracted extensive attention of scientific researchers in recent years.Although domestic and foreign scholars have carried out a large number of observation experiments on sprites,these experiments are mainly carried out in the form of ground-based observation.Therefore,there are still many scientific issues for the study of sprites in oceanic areas that have not been clearly understood.In this paper,the sprites and the electromagnetic waveform of parent lightning in the middle and high latitudes of China and near equatorial low latitude ocean areas are studied and analyzed by using the measured optical observation data and magnetic field detection data obtained by the foundation observation station independently set up by our group.Combined with lightning location data,ISUAL(Imager of Sprites and Upper Atmospheric Lightning)observation data,cloud top brightness temperature data,radar data and other data,this paper also analyzed the temporal and spatial relationship of parent lightning and parent thunderstorm between land and ocean,as well as the reasons for the distribution difference of the sprite polarity between ocean and land.The main conclusions are summarized as follows:1.To increase the possibility of observing TLEs in the middle and high latitudes of China,we set the observed and experimental area in Northeast and North China to further explore the occurrence of sprites and the development characteristics of parent thunderstorms in middle and high latitudes.We carried out the ground-based observation experiment in Northeast China in the summer of 2017.This experiment is the first sprite ground-based observation experiment in Northeast China.A total of 26 positive sprites were recorded in this experiment,of which about 2/3 of the sprites were generated over a large-scale mesoscale convection system(horizontal scale about 500 km),and the other 1/3 were induced by three small-scale mesoscale convection systems(horizontal scale about 100km).The parent lightning occurs near convective areas of mesoscale convection systems,and there is no obvious large-scale stratiform cloud area behind convective areas.Since a single observation station can not well describe the spatial characteristics of the sprite morphology,we carry out triangulation observation in North China to further analyze the formation mechanism of sprites and the characteristics of parent thunderstorms in middle and high latitudes of China.In this experiment,20 spirits were observed on a mesoscale convection system on August 8,2017,of which 6 sprites were simultaneously observed by Binzhou,Shandong and Xinzhou,Shanxi.All sprites are generated by positive cloud-to-ground lightning.The parent lightning is located in the stratiform area of the mesoscale convection system,which is consistent with the spatial relationship between sprites and parent thunderstorms in the Midwest of the United States.This experiment is the first triangulation observation on sprites in Chinese mainland,and the observation results are obtained to achieve the purpose of the observation experiment.The ground-based observation in the middle and high latitudes of China provides a new understanding on the generation mechanism of sprites and the characteristics of parent thunderstorm on the land.2.At present,the observation and research of sprites mainly focus on continental thunderstorms,but there are few reports on the characteristics of oceanic sprites and parent thunderstorms.To obtain more observation data of oceanic sprites,we carried out ground-based observation experiments in Puerto Rico in 2017 and Malacca in 2019,respectively,which further supplemented the observation and research data of oceanic sprites.In the observation experiment in Malacca in 2019,a total of 7 positive sprites were recorded over three oceanic thunderstorms.The parent thunderstorms were small-scale mesoscale convection systems without obvious stratiform regions or supercell.The parent lightning is mainly located near the convection area of thunderstorm,which may be due to the extensive positive charge layer distributed in the convection area or the adjacent cloud anvil area.In addition,sprites often occur after the lightning frequency decreases briefly,which may be a signal of the convective activity weakening of oceanic thunderstorms in this area.This experiment is the first ground-based observation experiment of sprites in Southeast Asia,which provides a further understanding of the production conditions and mechanisms of oceanic sprites in different regions of the world.In the ground-based observation experiment in Puerto Rico in 2017,6 negative sprites and 18 Gigantic jets were observed over the hurricane Harvey(2017).The parent lightning is located near the convective core,and the impulse charge moment change is greater than-500 C km.In the generation stage of sprites,the cloud-to-ground lightning rate decreases and the structure of tropical thunderstorm tends to be stable.The charge structure differences of parent thunderstorms may be the main reason for the difference of polarity distribution between oceanic and continental sprites.In addition,the frequent occurrence of Gigantic jets also indicates that there is a wide range of main negative charge layers in the thunderstorm.This experiment further supplements the observed and experimental data of oceanic sprites.3.Previous studies have shown that the intracloud discharge process is related to the charge distribution in the cloud.When the charge amount of the upper positive charge layer is small,the negative leader will escape from the cloud top and form jet-like discharge;When the charge amount of the lower positive charge is smaller,the negative charge will escape from the cloud bottom and form hybrid IC-NCG lightning;When the charge amount of the upper positive charge layer is small and the centers of positive and negative charges are offset,the negative leader will escape from the side of the cloud,forming bolt-from-the-blue lightning.In the ground-based observation experiment in Puerto Rico in 2017,Gigantic jets and negative sprites were also observed in hurricane Harvey(2017).Therefore,it is reasonable to speculate that the negative sprites in Harvey(2017)is generated by hybrid IC-NCG lightning or bolt-from-the-blue.Because the negative sprites recorded in the ground-based observation is limited,this paper also analyzed the ISUAL observation data to obtain more data,and it was found that there was sprite current characteristics in the magnetic field waveform of a bright negative sprite.By comparing the optical signal and the magnetic field waveform signal,it was determined that the second pulse signal in the magnetic field waveform was the slow electromagnetic signal generated by the charge flow in the negative sprite.By analyzing the characteristics of parent thunderstorm and the intensity of parent lightning,it is found that the generation of sprite current tends to be caused by the long-time discharge process in the cloud,which further indicates that the complex charge structure in the cloud affects the generation of negative sprites and the development of streamer structures.This finding is the first observation of sprite current characteristics in negative sprites,which provides a further understanding of the generation conditions and mechanisms of negative sprites.