Statistical Study Of Plasmaspheric Plumes And Corresponding Simulated Evolution Under Different Levels Of Geomagnetic Storms

The plasmasphere is the coldest and densest inner region of the magnetosphere.The cold plasma particles in the plasmasphere are generally from the ionosphere.Under the role of the magnetic field reconnection at the magnetopause on the dayside,the enhanced sunward convection erodes the outer layer of the plasmasphere,which causes the outer boundary of the plasmasphere to move inward on the night side and outward on the day side.Furthermore,under the action of both convection and corotating electric field,these dense plasmas of sunward motion forming a dense plasmaspheric plume.The structure of the plasmasphere plays a vital role in the physical process of magnetosphere.In order to deeply understand the structure and formation process of this plasmaspheric plume,we have done the following researches:In the first part,Using the plasma density data measured by EMFISIS(Electric and Magnetic Field Instrument Suite and Integrated Science)on Van Allen Probes satellite in a total of 72 months from January 2013 to December 2018,the plasmaspheric plume of the inner magnetosphere was found according to the judgement.Firstly,the distribution of plasmaspheric plumes in MLT-L coordinates in the inner magnetosphere is statistically analyzed,and the characteristics of the spatial temporal distribution and occurrence rate of plasmaspheric plumes are analyzed,respectively.The results show that the plasmaspheric plumes mainly occur in the region with higher L shells,and the occurrence rate of plasmaspheric plumes is much higher in the afternoon side.Secondly,we analyzed the relationship between the occurrence rate of plasmaspheric plumes and phase of geomagnetic storm.We found that plasmaspheric plumes mainly occurred during the recovery phase of the storms,and less occurred in the primary and main phases of the storms.In addition,we found that the occurrence rate incidence of plasmaspheric plume in the plasmasphere is higher at the level of moderate geomagnetic activity and strong geomagnetic activity.This finding is not completely consistent with the observation of the outer magnetosphere by Lee et al[1],which analyze the distribution of plasmaspheric plume in the outer magnetosphere based on the data of the Cluster satellite.Lee et al suggest that the plasmaspheric plume has a highest incidence during moderate storms.In the second part,to explain the difference in the incidence of plasmaspheric plume associated with geomagnetic activity levels observed by Van Allen Probes(primary observation of plasmaspheric plume in the inner magnetosphere L≤6.2)and Cluster(primary observation of plasmaspheric plume in the outer magnetosphere L≥8),We conducted a group of experimental particle simulations to compare the evolution of plasmaspheric plume during different geomagnetic storms.In this regard,we analyzed two geomagnetic storm events,a small geomagnetic storm(in March 2013,with a minimum DST index of-37nt)and a strong geomagnetic storm(in May 2013,with a minimum DST index of-72nt).We analyzed the evolution of plasmasphere and plasmaspheric plume during the two storms by group particle simulation.The simulation results show that the plasmasphere plume lasts longer at L=8 than at L=6 during the weak geomagnetic storms.In other words,the plasmaspheric plume lasts longer at L=8 than at L=6 during the weak geomagnetic storms.Plasmaspheric plume is more easily observed at L=8.At the level of strong geomagnetic activity,plasmaspheric plume lasted longer at L=6 than at L=8.In other words,plasmaspheric plume was more easily observed at L=6 during a strong magnetic storm.Through this study,the spatial and temporal distribution and incidence of plasmaspheric plume and the relationship between plasmaspheric plume and geomagnetic activity level and geomagnetic storm phase are further clarified.From the perspective of simulation,the reason for the difference between the statistical results of Van Allen Probes satellite and that of Lee et al.using Cluster satellite is explained.The formation process of plasmaspheric plume is described intuitively by simulation evolution image[1].