Browsing by Author "Liu, Xu"
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Item Instability in a Relativistic Magnetized Plasma(American Institute of Physics Inc.) Liu, Xu; Chen, Lunjin; 0000-0002-7211-0546 (Liu, X); 0000-0003-2489-3571 (Chen, L); Liu, Xu; Chen, LunjinWe present a general relativistic linear growth rate formula of electromagnetic waves for any wave normal angle and a general distribution function in a uniform magnetized plasma with a dominant cold plasma component and a tenuous hot plasma component. Such a general linear growth rate formula can be applied to different plasma environments, such as the Jovian Magnetosphere and laboratory plasma. The relativistic resonant condition for different wave modes is discussed and summarized. Then, the formula is applied to a parametric study for local instability of Earth's plasmaspheric hiss. We study the effects of the electron temperature, electron temperature anisotropy, types of distribution functions, plasma density, background magnetic field, and wave normal angle on the relativistic linear growth rate of the whistler mode. We find that (1) the energetic electrons with larger energy resonate with the lower frequency wave. The relativistic effect becomes significant for the electron with energy >100 keV. (2) The anisotropy only increases the growth rate and expands the growth wave band. (3) The high density and low background magnetic field tend to decrease the wave frequency and increase the growth rate. (4) The field-aligned growth rate is larger than the oblique growth rate, and the lower frequency whistler waves are easier to propagate obliquely. © 2019 Author(s).Item Modulation of Locally Generated Equatorial Noise by ULF Wave(Blackwell Publishing Ltd, 2019-04-23) Zhu, Hui; Chen, Lunjin; Liu, Xu; Shprits, Y. Y.; 0000-0003-3556-8096 (Zhu, H); 0000-0003-2489-3571 (Chen, L); 0000-0002-7211-0546 (Liu, X); Zhu, Hui; Chen, Lunjin; Liu, XuIn this paper we report a rare and fortunate event of fast magnetosonic (MS, also called equatorial noise) waves modulated by compressional ultralow frequency (ULF) waves measured by Van Allen Probes. The characteristics of MS waves, ULF waves, proton distribution, and their potential correlations are analyzed. The results show that ULF waves can modulate the energetic ring proton distribution and in turn modulate the MS generation. Furthermore, the variation of MS intensities is attributed to not only ULF wave activities but also the variation of background parameters, for example, number density. The results confirm the opinion that MS waves are generated by proton ring distribution and propose a new modulation phenomenon. ©2019. American Geophysical Union. All Rights Reserved.Item One-Dimensional Full Wave Simulation of Equatorial Magnetosonic Wave Propagation in an Inhomogeneous Magnetosphere(Amer Geophysical Union) Liu, Xu; Chen, Lunjin; Yang, Lixia; Xia, Zhiyang; Malaspina, David M.; Malaspina, David M.; 0000-0002-7211-0546 (Liu, X); 0000-0003-2489-3571 (Chen, L); 0000-0001-8922-6484 (Xia, Z); Liu, Xu; Chen, Lunjin; Yang, Lixia; Xia, ZhiyangThe effect of the plasmapause on equatorially radially propagating fast magnetosonic (MS) waves in the Earth's dipole magnetic field is studied by using finite difference time domain method. We run 1-D simulation for three different density profiles: (1) no plasmapause, (2) with a plasmapause, and (3) with a plasmapause accompanied with fine-scale density irregularity. We find that (1) without plasmapause the radially inward propagating MS wave can reach ionosphere and continuously propagate to lower altitude if no damping mechanism is considered. The wave properties follow the cold plasma dispersion relation locally along its trajectory. (2) For simulation with a plasmapause with a scale length of 0.006 R_E compared to wavelength, only a small fraction of the MS wave power is reflected by the plasmapause. WKB approximation is generally valid for such plasmapause. (3) The multiple fine-scale density irregularities near the outer edge of plasmapause can effectively block the MS wave propagation, resulting in a terminating boundary for MS waves near the plasmapause.Item Propagation and Excitation of Equatorial Electrostatic and Electromagnetic Emissions In the Magnetosphere(2020-04-14) Liu, Xu; Chen, LunjinThe plasma density is an important parameter for affecting the distribution of magnetospheric waves. This dissertation addresses the effects of plasma density on the propagation of magnetosonic (MS) waves and generation and distribution of electron cyclotron harmonic (ECH) waves. These two waves are important for the dynamics of energetic electrons in the Earth’s magnetosphere. The plasma density effect on the radial propagation of MS waves is evaluated by the FiniteDifference Time-Domain (FDTD) method. We find that the radially propagating MS waves can propagate down to ionospheric altitude with a smoothly varying plasma density profile if no damping mechanism is considered. The fine-scale density structures near the outer edge of plasmapause can effectively block MS wave propagation, and therefore, such a structured plasmapause can serve as a terminating boundary of MS waves, which was often shown by Van Allen Probes observation. We extend the linear growth rate formula of the electromagnetic modes in previously assumed non-relativistic regimes to the relativistic regime. We also derive a general electrostatic wave linear growth rate solver for a realistic and arbitrary plasma distribution function and apply this to examine the instability of ECH waves. The ECH wave growth rate increases with loss-cone size, parallel temperature of hot electrons, cold electron temperature and electron density, and decreases with hot electron perpendicular temperature. Such a linear instability solver can be readily applied to simulation and observation. We, for the first time, simulate the global ECH wave evolution during a geomagnetic storm using a Ring current-Atmosphere interactions Model with Self-Consistent Magnetic field (RAM-SCB) combined with our linear growth rate solver. Our simulation shows that the ECH wave instability becomes more intensive with stronger geomagnetic activity or during the main phase of geomagnetic storms. ECH wave instability is much stronger at nightside and dawnside, compared to that at dayside and duskside. The unstable region of ECH waves extends to larger MLT and lower L shell regions as geomagnetic activity increase or geomagnetic storms evolve to the main phase, and the inner boundary of ECH wave instability is traced well by plasmapause location. Finally, we investigate the relation between ECH waves and the plasmapause using Van Allen Probes observation. Two categories of ECH waves are shown by their different behaviors near the plasmasphere boundary layer (PBL, i.e., plasmapause). Category I ECH waves are terminated at the outer boundary of PBL because of the rapid suppression of ECH wave instability due to a dramatic increase of cold plasma density and a dramatic decrease of hot electron flux across the PBL. Category II ECH waves, limited at nightside and dawnside, can be excited across the PBL. Lower harmonic bands can be excited further inside the plasmasphere. Such Category II ECH waves occur when the hot electron flux penetrates across the PBL/plasmasphere and cold plasma density increases gradually. Statistically, the wave power of Category II ECH waves is much more intense than that of Category I, and Category II ECH waves are accompanied by more injected energetic electrons overlapping with the cold dense electrons in the PBL.Item Spectral Properties and Associated Plasma Energization by Magnetosonic Waves in the Earth's Magnetosphere: Particle-In-Cell Simulations(Amer Geophysical Union, 2017-05-24) Sun, Jicheng; Gao, Xinliang; Lu, Quanming; Chen, Lunjin; Liu, Xu; Wang, Xueyi; Tao, Xin; Wang, Shui; 0000-0002-5059-5394 (Sun, J); 0000-0003-2489-3571 (Chen, L); Sun, Jicheng; Chen, Lunjin; Liu, XuIn this paper, we perform a 1-D particle-in-cell (PIC) simulation model consisting of three species, cold electrons, cold ions, and energetic ion ring, to investigate spectral structures of magnetosonic waves excited by ring distribution protons in the Earth's magnetosphere, and dynamics of charged particles during the excitation of magnetosonic waves. As the wave normal angle decreases, the spectral range of excited magnetosonic waves becomes broader with upper frequency limit extending beyond the lower hybrid resonant frequency, and the discrete spectra tends to merge into a continuous one. This dependence on wave normal angle is consistent with the linear theory. The effects of magnetosonic waves on the background cold plasma populations also vary with wave normal angle. For exactly perpendicular magnetosonic waves (parallel wave number k(parallel to) = 0), there is no energization in the parallel direction for both background cold protons and electrons due to the negligible fluctuating electric field component in the parallel direction. In contrast, the perpendicular energization of background plasmas is rather significant, where cold protons follow unmagnetized motion while cold electrons follow drift motion due to wave electric fields. For magnetosonic waves with a finite k(parallel to), there exists a nonnegligible parallel fluctuating electric field, leading to a significant and rapid energization in the parallel direction for cold electrons. These cold electrons can also be efficiently energized in the perpendicular direction due to the interaction with the magnetosonic wave fields in the perpendicular direction. However, cold protons can be only heated in the perpendicular direction, which is likely caused by the higher-order resonances with magnetosonic waves. The potential impacts of magnetosonic waves on the energization of the background cold plasmas in the Earth's inner magnetosphere are also discussed in this paper.