Propagation properties of foreshock cavitons: Cluster observations  

作　　者：WANG MengMeng YAO ShuTao SHI QuanQi ZHANG Hui TIAN AnMin DEGELING Alexander William ZHANG Shuai GUO RuiLong SUN WeiJie LIU Ji BAI ShiChen SHEN XiaoChen ZHU XiaoQiong FU SuiYan PU ZuYin 

机构地区：[1]School of Space Science and Physics,Shandong University,Weihai 264209,China [2]National Space Science Center,Chinese Academy of Sciences,Beijing 100190,China [3]Geophysical Institute,University of Alaska Fairbanks,Fairbanks,Alaska 99775,USA [4]Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China [5]Department of Climate and Space Sciences and Engineering,University of Michigan,Ann Arbor 48113,USA [6]Institute of Space Physics and Applied Technology,Peking University,Beijing 100871,China

出　　处：《Science China(Technological Sciences)》2020年第1期173-182,共10页中国科学（技术科学英文版）

基　　金：supported by the National Natural Science Foundation of China(Grant Nos.41574157,41628402&41774153);partially supported by National Science Foundation(Grant No.AGS-1352669);the International Space Science Institute-Beijing for supporting the international team “Dayside Transient Phenomena and Their Impact on the Magnetosphere-Ionosphere”;supported by the specialized research fund for State Key Laboratories

摘　　要：Foreshock cavitons are transient phenomena observed in the terrestrial foreshock region.They are characterized by a simultaneous depression of magnetic field magnitude and plasma density,which are bounded with enhancements of these two parameters and surrounded by ultralow frequency(ULF)waves.Previous studies focused on the interplanetary magnetic field(IMF)conditions,solar wind(SW)conditions,and the growth of the foreshock waves related to the generation of foreshock cavitons.Previously,a multipoint spacecraft analysis method using Cluster data was applied to analyze only two foreshock cavitons,and this method did not consider uncertainties.In this study,multipoint spacecraft analysis methods,including the timing method,the minimum directional derivative(MDD)method,and the spatiotemporal difference(STD)method are applied to determine the velocity in both spacecraft and solar wind frames.The propagation properties show good agreement with previous results from simulations and observations that most cavitons move sunward in the solar wind frame,with the velocities larger than the Alfvén speed.The propagation properties of foreshock cavitons support the formation mechanism of cavitons in previous simulations,which suggested that cavitons are formed due to the nonlinear evolution of compressive ULF waves.We find that there is clear decreasing trend between the size of cavitons and their velocity in the solar wind frame.In addition,the timing method considering errors has been applied to study the evolution properties by comparing the velocities with errors of the leading and trailing edges,and we identify three stable cavitons and one contracting caviton,which has not been studied before.Most cavitons should remain stable when they travel toward the Earth’s bow shock.The relationship between the size of foreshock cavitons and their distance from the bow shock is also discussed.Foreshock cavitons are transient phenomena observed in the terrestrial foreshock region. They are characterized by a simultaneous depression of magnetic field magnitude and plasma density, which are bounded with enhancements of these two parameters and surrounded by ultralow frequency(ULF) waves. Previous studies focused on the interplanetary magnetic field(IMF) conditions, solar wind(SW) conditions, and the growth of the foreshock waves related to the generation of foreshock cavitons. Previously, a multipoint spacecraft analysis method using Cluster data was applied to analyze only two foreshock cavitons, and this method did not consider uncertainties. In this study, multipoint spacecraft analysis methods, including the timing method, the minimum directional derivative(MDD) method, and the spatiotemporal difference(STD) method are applied to determine the velocity in both spacecraft and solar wind frames. The propagation properties show good agreement with previous results from simulations and observations that most cavitons move sunward in the solar wind frame, with the velocities larger than the Alfvén speed. The propagation properties of foreshock cavitons support the formation mechanism of cavitons in previous simulations, which suggested that cavitons are formed due to the nonlinear evolution of compressive ULF waves. We find that there is clear decreasing trend between the size of cavitons and their velocity in the solar wind frame. In addition, the timing method considering errors has been applied to study the evolution properties by comparing the velocities with errors of the leading and trailing edges, and we identify three stable cavitons and one contracting caviton, which has not been studied before.Most cavitons should remain stable when they travel toward the Earth’s bow shock. The relationship between the size of foreshock cavitons and their distance from the bow shock is also discussed.

关 键 词：foreshock transient phenomena cavitons nonlinear evolution of ULF waves propagation and evolution properties of structures multipoint spacecraft methods 

分 类 号：P315.72[天文地球—地震学]