Study Of Kinetic Processes In The Earth's Magnetotail Using Satellite Observation And Numerical Simulation

The Earth's Magnetotail is a crucial ingredient of our magnetosphere. Many explosive phenomena happened in the magnetotail could greatly influence our living on the earth, such as satellites in the near earth region may be damaged by energetic particles produced during the magnetosphere storm, the dramatic change of the properties of ionosphere may affect the wireless communication. In addition, magnetotail is an excellent natural laboratory for us to study the plasma physics.Magnetic reconnection is a universal process in the astrophysics, which could transfer the magnetic energy to plasma kinetic and thermal energy in a short time period, and also change the large scale topology of magnetic field. There are rich wave activities in the reconnection region. Which wave dominates the reconnection layer? Whether these waves are just the byproduct of reconnection or they could trigger or mediate the reconnection process? These are open questions for space and experimental plasma scientist. Magnetic null point is a crucial region of reconnection, where magnetic field lines break and reconnect. Revealing the detailed structures, waves and particle dynamics around null points could be significantly important to understand the reconnection in three dimensional regime. Energetic particle injection and dipolarization are two important ingredients of substorm. Studying these phenomena could tell us how energy releases during the substorm and how the substorm be triggered.In this thesis, by combining satellite observation and numerical simulation, we primarily studied multi-scale kinetic processes in the magnetotail, in particular the wave-particle interaction and acceleration of ions and electrons. Following are our main results:1, We studied different wave characteristics at different regions inside one reconnection diffusion region observed by Cluster spacecraft.We identified both electrostatic and electromagnetic modes of lower hybrid drift (LHD) wave in the reconnection region around a thin current sheet. During the crossing of the separatrix with the reversal of plasma flow and Hall magnetic fields, strong electrostatic LHD mode was observed. Strong electromagnetic fluctuations were observed in the center of the current sheet in the diffusion region. The dispersion properties of the electromagnetic wave were studied by using the interferometer method and are consistent with the properties of LHD wave. This is the first observation evidence of electromagnetic mode of LHD wave inside reconnection diffusion region. We estimated the anomalous resistivity provided by the electromagnetic mode of LHD wave, and found that it could not balance the measured electric field in the reconnection region.We also studied low frequency wave characteristics at the earthward region of the X-line, which was a highβregion with small guide field. We obtained wave vectors in low frequency range using the k-filtering method and found that waves in the diffusion region are highly oblique propagating mode. We compared the measured dispersion relation with the theoretical dispersion relation and confirm the existence of Alfven-Whistler waves in the reconnection region.In addition, we identified a density depletion layer inside the diffusion region and examined the wave-particle interaction associated with the layer. Strong anti-parallel electric current was observed through the layer. Electrostatic wave enhancements between the ion cyclotron frequency and the lower hybrid frequency were observed. The polarization analysis shows the wave is mainly linearly and quasi perpendicularly polarized, which is consistent with the lower hybrid wave. Moreover, oblique propagating whistler wave were observed at the same time. The electron distribution shows there were strong parallel beams. The possible mechanism of wave-particle interaction and the role of density cavities in electron acceleration are discussed.2, We show the Cluster in situ observation of magnetic null structures in the diffusion region, as well as the electron dynamics and associated waves. Possible spiral null pair and null clusters formed by three nulls have been identified in the diffusion region. There is a close relation among the null points, the bipolar signature of the Z-component of magnetic field and enhancement of the flux of energetic electrons up to 100 keV. The four satellites were located in different topological domains of the 3D reconnection structure, with one being located just 19 km apart from the separator line of magnetic null structures in electron scale. On crossing of the separator line, a very thin current sheet with half-width of 4-6 electron initial scale lengths and a peak of anti-parallel current density were found, and high energetic electron enhancement was observed with the hardest energy spectrum (-3.4). Electrostatic solitary waves, whistler-mode waves and lower hybrid waves were identified near the separator line, indicating that electron dynamics and wave-particle interactions play an important role in collisionless reconnection. It is found that the angle between the fans of the nulls is quite close to the theoretically estimated maximum value of the group-velocity cone angle for whistler wave regime of reconnection.3, We performed a series two dimensional Particle-In-Cell (PIC) simulations to study the possible generation mechanism of modulated electron plasma waves often observed in the magnetosphere. It is shown that weak beam instability could generate the modulated Langmuir wave, and when the ambient magnetic field is strong, there is no modulation on the perpendicular electric field. The observed perpendicular polarized modulated waves could be generated by weak loss cone instability. When the weak beam has loss cone distribution, the modulated waves show quick transition between parallel and perpendicular polarization, which explains the observed waveform around the reconnection layer. The quick change of polarization might be the result of trapping phase difference of different modulated waves.4, We studied one substorm injection event by THEMIS and LANL observation, as well as large scale kinetic simulation. We followed millions of particles in the magnetic and electric field obtained from a global MHD simulation to model the energetic ion injection. It is found that our simulation could capture the main feature of ion injection observed by both THEMIS and LANL spacecraft, including the timing and dispersion properties of energetic flux increase. It is found that there were primarily two energization regions for particles to gain energy during this substorm. One is around the near-earth X-line (Ⅹ～-20 RE), where particles were mostly accelerated in non-adiabatic motion under strong inductive electric field. The other were several stretched or localized regions between X=-18 RE and X=-7 RE, where particles were also accelerated in non-adiabatic motion but under potential electric field. Our results imply the importance of reconnection and non-adiabatic motion in the energization of ions during substorm. This is a significant supplement and revision to the previous models which either believes particles only gain energy in the dipolarization region, or adiabatic motion dominates the energization process during substorm.5, We investigated the micro-physics associated with several dipolarization fronts observed by THEMIS observation.On Feb 15,2008, multiple dipolarization fronts were observed by THEMIS spacecraft in the near Earth magnetotail during a substorm. The dipolarization fronts were located at the leading edge of earthward propagating plasma bubbles. Major energetic electron flux enhancements were observed at the dipolarization fronts, which were also associated with large wave fluctuations extending from below the lower hybrid frequency to above the electron cyclotron frequency. Intense electric field wave packets, primarily contributed by the Hall electric field and LHD wave, were observed right at the front, which was a thin current layer with size of the order of the ion inertial length. The LHD wave was believed to be generated by a diamagnetic current in the presence of density and temperature gradients. Electrostatic electron cyclotron waves were detected slightly after the front. The electrostatic electron cyclotron waves were probably generated by the positive slope of the electron perpendicular velocity distribution. Both of these waves are suggested to be able to heat electrons. The observation of these waves at the dipolarization front could be important for the understanding of electron energization during substorm injection, as well as the mechanism of current disruption.On Feb 27,2009, four THEMIS spacecraft, located between X=-20 RE and X=-10 RE, captured one earthward propagating dipolarization front. The dipolarization front was also located at the leading edge of plasma bubble and a kinetic structure with width on the order of ion inertial length. The ion and electron distribution varies significantly between outer (P1/P2) and inner (P3/P4) probes. Two outer spacecraft (P1/P2) detected whistler waves around the front; however, two inner spacecraft (P3/P4) did not. On Mar 15,2009, Five THEMIS spacecraft observed one dipolarization front one by one. Similar as the event of Feb 27,2009, two outer spacecraft (P1/P2) detected whistler waves around the front, while the other three spacecraft did not. Above evidence implies that dipolarization front has different characteristics in different regions during its propagation.