Observational Studies On Cross-scale Transient Phenomena In The Terrestrial Foreshock

The space near the terrestrial bow shock serves as a natural laboratory for studying collisionless shocks and the foreshock is filled with various MHD scale transient phenomena,including hot flow anomalies(HFAs)and foreshock bubbles(FBs),of which the boundaries can form fast magnetosonic shocks,i.e.,the foreshock transient shocks(FT shocks).Similar to the terrestrial bow shock,the secondary FT shocks radiate precursor waves around lower hybrid frequency or ion inertial scale,reflect solar wind ions near ion gyro-radius scale,and have their own foreshock.Moreover,FTs with large variation of dynamic pressure can drive the motion of the magnetopause,which then excite ultra-low frequency waves in the magnetosphere and disturb the ionosphere.Sometimes multiple FTs can merge into a larger FT,resulting in the complex signal of dynamic pressure being unpredictable and complex geo-effects.The studies presented in the dissertation are based on high-resolution data provided by Magnetospheric Multiscale(MMS)mission and focus on the properties and generation mechanism of precursor waves upstream of FT shocks,the process of reflected ions at an FT shock,the presence of multiple transient structures.The goal is to understand the cross-scale physics processes in the terrestrial foreshock.Firstly,for the statistical investigation of precursor waves upstream of FT shocks,the multi-spacecraft analysis method and the singular value decomposition method based on a single satellite were used to analyze data from the MMS mission.The statistical study examined the properties of precursor waves in both spacecraft and plasma rest frames.The precursors’propagation characteristics relative to FT shocks were investigated.Additionally,the study analyzed the properties of precursor waves in relation to the parameters of FT shocks.In the plasma rest frame,precursor waves are found to be right-handed polarized,with frequencies close to the lower hybrid frequency.Based on these observations and without assuming any theoretical models,they are directly determined that they are whistler waves.Concerning FT shocks,the majority of precursor waves propagate quasi-parallel to the FT shock wave normal.The phase velocities of precursor waves point either upstream or downstream in a roughly equal number of events,and the group velocities of all events point upstream of FT shocks.The appearance and properties of precursor waves do not exhibit a clear correlation with the Alfvenic Mach number and shock normal angle of FT shocks.These pieces of evidence suggest that the dispersion radiation process of FT shocks is the most likely mechanism for exciting whistler precursor waves.The statistical study is of significant importance for understanding the generation and evolution of collisionless shocks.Secondly,a case study was conducted on the phenomenon of ion reflection by an FT shock near the ion gyro-radius scale.The study primarily investigates the propagation characteristics of an FT shock event and the velocity distribution of the reflected ions.It analyzes the relationship between reflected ions and solar wind ions in phase space and discusses the connection between reflected ions and whistler precursor waves.The case study reveals that solar wind ions are specularly reflected at the FT shock,and the reflected ions and solar wind ions have not yet mixed in phase space.The whistler precursors are radiated from the FT shock rather than driven by the reflected ions.This case study suggests that specular reflection is a possible reflection mechanism at FT shocks,enhancing our understanding of FTs and FT shocks.Thirdly,we attempt to conduct a case study on phenomena associated with multiple transient structures spanning from the ion gyro-radius scale to the MHD scale.First,the study identifies the types of transient structures.In the first event,a spontaneous hot flow anomaly(SHFA),an FB,and subsequently a SHFA were observed by satellites.In the second event,a SHFA,an HFA,and subsequently a SHFA were successively observed by satellites.Second,in conjunction with joint observations from other satellites in the upstream solar wind,it confirms the existence of interplanetary discontinuities that drive the formation of the FB and the HFA.Additionally,the ion velocity distribution within and around each transient structure is analyzed.The analysis reveals that both of these events occurred in the ultra-low frequency(ULF)foreshock region.The presence of the interplanetary discontinuity carried by the solar wind in the ULF foreshock region is a key factor leading to the occurrence of the presence of multiple different types of transient structures.Furthermore,the ion velocity distributions within transient structures of different scales or evolutionary stages are significantly different.Complex substructures within the HFA may be related to local ULF waves.The dissertation primarily focuses on FTs and their secondary effects.It reanalyzes several cross-scale phenomena ranging from ion inertial scales to MHD scales.It delves into the dynamic processes of secondary FT shocks,including the excitation of whistler waves and the reflection of ions.It also provides an initial exploration of various types of transient structures occurring concurrently.In particular,the research on precursor waves and reflected ions associated with FT shocks contributes to our understanding of the generation,evolution,and impacts of collisionless shocks.The exploration of different types of transient structures occurring simultaneously enriches our knowledge of FTs and dissipation processes in the foreshock.