Browsing by Author "Chen, Lunjin"

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    Eigenmode analysis of compressional poloidal modes in a self‐consistent magnetic field
    (Wiley, 2017) Xia, Zhiyang; Chen, Lunjin; Zheng, Liheng; Chan, Anthony A.
    In 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 βand 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° 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 Alfvén ballooning instability is also identified.
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    Modulation of chorus intensity by ULF waves deep in the inner magnetosphere
    (Wiley, 2016) Xia, Zhiyang; Chen, Lunjin; Dai, Lei; Claudepierre, Seth G.; Chan, Anthony A.; Soto-Chavez, A.R.; Reeves, G.D.
    Previous studies have shown that chorus wave intensity can be modulated by Pc4-Pc5 compressional ULF waves. In this study, we present Van Allen Probes observation of ULF wave modulating chorus wave intensity, which occurred deep in the magnetosphere. The ULF wave shows fundamental poloidal mode signature and mirror mode compressional nature. The observed ULF wave can modulate not only the chorus wave intensity but also the distribution of both protons and electrons. Linear growth rate analysis shows consistence with observed chorus intensity variation at low frequency (f <∼ 0.3fce), but cannot account for the observed higher-frequency chorus waves, including the upper band chorus waves. This suggests the chorus waves at higher-frequency ranges require nonlinear mechanisms. In addition, we use combined observations of Radiation Belt Storm Probes (RBSP) A and B to verify that the ULF wave event is spatially local and does not last long.
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    UBER v1.0: a universal kinetic equation solver for radiation belts
    (European Geosciences Union, 2021) Zheng, Liheng; Chen, Lunjin; Chan, Anthony A.; Wang, Peng; Xia, Zhiyang; Liu, Xu
    Recent proceedings in radiation belt studies have proposed new requirements for numerical methods to solve the kinetic equations involved. In this article, we present a numerical solver that can solve the general form of the radiation belt Fokker–Planck equation and Boltzmann equation in arbitrarily provided coordinate systems and with user-specified boundary geometry, boundary conditions, and equation terms. The solver is based upon the mathematical theory of stochastic differential equations, whose computational accuracy and efficiency are greatly enhanced by specially designed adaptive algorithms and a variance reduction technique. The versatility and robustness of the solver are exhibited in four example problems. The solver applies to a wide spectrum of radiation belt modeling problems, including the ones featuring non-diffusive particle transport such as that arising from nonlinear wave–particle interactions.