Browsing by Author "Xia, Zhiyang"
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Item Eigenmode Analysis of Compressional Poloidal Modes in a Self-Consistent Magnetic Field(Amer Geophysical Union) Xia, Zhiyang; Chen, Lunjin; Zheng, Liheng; Chan, Anthony A.; 0000-0003-2489-3571 (Chen, L); /0000-0001-9068-4431 (Zheng, L); Xia, Zhiyang; Chen, Lunjin; Zheng, LihengIn this study, we simulate a self-consistent magnetic field that satisfies force balance with a model ring current that is radially localized, axisymmetric, and has anisotropic plasma pressure. We find that the magnetic field dip forms near the high plasma pressure region with plasma β >~ 0.6, and the formed magnetic dip becomes deeper for larger plasma β and also slightly deeper for larger anisotropy. We perform linear analysis on a ppol of self-consistent equilibria for second harmonic compressional poloidal modes of sufficiently high azimuthal wave number. We investigate the effect of anisotropic pressure on the eigenfrequency of the poloidal modes and the characteristics of the compressional magnetic field component. We find that the eigenfrequency is reduced at the outer edge of the thermal pressure peak and increased at the inner edge. The compressional magnetic field component occurs primarily within 10 degrees of the equator on both the inner and outer edges, with stronger compressional magnetic field component on the outer edge. Larger β and smaller anisotropy can increase the change of eigenfrequency and the strength of the compressional magnetic field component. The critical condition on plasma β and pressure anisotropy of an Alfven ballooning instability is also identified.Item Generation of Magnetosonic Waves Over a Continuous Spectrum(Blackwell Publishing Ltd, 2016-01-08) Chen, Lunjin; Sun, J.; Lu, Q.; Gao, X.; Xia, Zhiyang; Zhima, Zeren; 0000-0003-2489-3571 (Chen, L); Chen, Lunjin; Xia, Zhiyang; Zhima, ZerenMagnetosonic waves, also known as equatorial noise emission, were found to have discrete frequency structures, which is consistent with instability caused by proton ring distribution. Nonetheless, nondiscrete structure, i.e., a broadband spectrum over a continuous frequency range, has been reported. We investigate the question whether proton ring distribution can generate nondiscrete spectra for perpendicularly propagating magnetosonic waves. We propose discrete and nondiscrete characteristics of the local instability for explaining the observation of discrete, continuous, and mixed spectra. The criterion for transition from discrete and continuous instability is given, γ > ~ Ω(h)/2, where γ is wave growth rate and Ω(h) is proton cyclotron frequency. The condition is verified by particle-in-cell simulation using more realistic electron-to-proton mass ratio and speed of light than in previous studies. Such criterion of generating a continuous spectrum can be tested against simultaneous in situ measurement of wave and particle. We also find that the modes at low Ω(h) harmonics, including the fundamental Ω(h'), can be still excited through nonlinear wave-wave coupling, even when they are neutral modes (γ = 0) according to the linear kinetic theory. Comparison with magnetosonic waves in cold plasma limit and electromagnetic ion Bernstein mode is also discussed.Item Modulation of Very Low Frequency Whistler Waves by Ultra Low Frequency Waves(2019-08) Xia, Zhiyang; 0000-0001-8922-6484 (Xia, Z); Chen, LunjinThis dissertation focuses on the study of the modulation of very low frequency (VLF) whistler mode waves in the Earth’s magnetosphere by ultra low frequency (ULF) waves. First, I provide an in-situ observation of chorus wave modulated by ULF waves deep in the inner magnetosphere. The observed ULF wave can modulate the distribution of both protons and electrons and amplify the intensity of chorus waves. Then I build a two-dimensional selfconsistent magnetic field (SCB) model to analyze the eigenmode of ULF field line resonance (FLR) with the effect of the anisotropic ring current pressure included. The results show that the eigenfrequency is reduced at the negative radial pressure gradient while increases at the positive pressure gradient. The compressional component of FLR magnetic field perturbation can be found in both the positive and negative gradient regions of the pressure and enhanced by larger plasma β and smaller anisotropy. Using about 2 years’ observations of three THEMIS satellites and over 5.5 years of observations of two Van Allen Probes satellites, I perform a statistical study of the chorus wave modulation events. The results indicate that in most of the modulation events, the intensity of chorus wave correlates to the background magnetic field negatively and plasma density positively. The intensity of chorus wave strongly depends on the amplitude of the background magnetic field perturbation but weakly depends on the amplitude of plasma density perturbation. Besides the work on VLF whistler mode waves modulated by ULF waves, I also perform two other relevant studies. The first one is using the two-dimensional self-consistent magnetic field (SCB) model to study the effects of localized thermal pressure on the magnetic field configuration and the formation of magnetic dip structure. The modeling results demonstrate that the magnetic perturbation increases with increasing plasma β and decreasing width of pressure distribution. The formation of magnetic dip requires a critical β value that increases with increasing width of pressure distribution and decreasing L shell. The other study is using the observations of DEMETER satellite to investigate propagation characteristics of low altitude ionospheric hiss. The ionospheric hiss can propagate from the high latitude regions to the equator within a waveguide near the region of cutoff frequency and plasma density peak, which results in the narrow frequency banded spectrum of ionospheric hiss waves with the central frequency around the local proton cyclotron frequency. The power of ionospheric hiss is stronger on the dayside than the nightside, under higher geomagnetic activity, in local summer and confined near the region where the local proton cyclotron frequency is equal to the wave frequency.Item Observational Evidence of the Drift-Mirror Plasma Instability in Earth's Inner Magnetosphere(American Institute of Physics Inc.) Soto-Chavez, A. R.; Lanzerotti, L. J.; Manweiler, J. W.; Gerrard, A.; Cohen, R.; Xia, Zhiyang; Chen, Lunjin; Kim, H.; 0000-0003-2489-3571 (Chen, L); Xia, Zhiyang; Chen, LunjinWe report on evidence for the generation of an ultra-low frequency plasma wave by the drift-mirror plasma instability in the dynamic plasma environment of Earth's inner magnetosphere. The plasma measurements are obtained from the Radiation Belt Storm Probes Ion Composition Experiment onboard NASA's Van Allen Probes Satellites. We show that the measured wave-particle interactions are driven by the drift-mirror instability. Theoretical analysis of the data demonstrates that the drift-mirror mode plasma instability condition is well satisfied. We also demonstrate, for the first time, that the measured wave growth rate agrees well with the predicted linear theory growth rate. Hence, the in-situ space plasma observations and theoretical analysis demonstrate that local generation of ultra-low frequency and high amplitude plasma waves can occur in the high beta plasma conditions of Earth's inner magnetosphere. © 2019 Author(s).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 Relativistic Electron's Butterfly Pitch Angle Distribution Modulated by Localized Background Magnetic Field Perturbation Driven by Hot Ring Current Ions(Amer Geophysical Union, 2017-05-21) Xiong, Ying; Chen, Lunjin; Xie, Lun; Fu, Suiyan; Xia, Zhiyang; Pu, Zuyin; 0000-0003-2489-3571 (Chen, L); Chen, Lunjin; Xia, ZhiyangDayside modulated relativistic electron's butterfly pitch angle distributions (PADs) from ~ 200 keV to 2.6 MeV were observed by Van Allen Probe B at L = 5.3 on 15 November 2013. They were associated with localized magnetic dip driven by hot ring current ion (60-100 keV proton and 60-200 keV helium and oxygen) injections. We reproduce the electron's butterfly PADs at satellite's location using test particle simulation. The simulation results illustrate that a negative radial flux gradient contributes primarily to the formation of the modulated electron's butterfly PADs through inward transport due to the inductive electric field, while deceleration due to the inductive electric field and pitch angle change also makes in part contribution. We suggest that localized magnetic field perturbation, which is a frequent phenomenon in the magnetosphere during magnetic disturbances, is of great importance for creating electron's butterfly PADs in the Earth's radiation belts.