Chen, Lunjin

Permanent URI for this collectionhttps://hdl.handle.net/10735.1/4965

Lunjin Chen is an Assistant Professor of Physics. His interest is in the interaction between electromagnetic waves and charged particles in the Earth's magnetosphere. Dr. Chen's research areas include:

Magnetosphere physics

Interaction of electromagnetic waves and energetic charge particles in geospace plasma

Modeling of radiation belt dynamics

Instability and propagation of plasma waves

Applications of plasma waves

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Two-Dimensional gcPIC Simulation of Rising-Tone Chorus Waves in a Dipole Magnetic Field
(Blackwell Publishing Ltd, 2019-06-18) Lu, Q.; Ke, Y.; Wang, X.; Liu, K.; Gao, X.; Chen, Lunjin; Wang, S.; 0000-0003-2489-3571 (Chen, L); Chen, Lunjin
Rising-tone chorus waves have already been successfully produced in a mirror magnetic field with the use of one- and two-dimensional particle-in-cell (PIC) simulations. However, in reality, the background magnetic field in the inner Earth's magnetosphere is a dipole magnetic field, unlike symmetric mirror fields. In this paper, with the two-dimensional (2-D) general curvilinear PIC (gcPIC) code, we investigate the generation of rising-tone chorus waves in the dipole magnetic field configuration. The plasma consists of three components: immobile ions, cold background, and hot electrons. In order to save computational resource, the topology of the magnetic field is roughly equal to that at L = 0.6 R_{E}, although the plasma parameters corresponding to those at L = 6 R_{E} (R_{E} is the Earth's radius) are used. Whistler mode waves are first excited around the magnetic equator by the hot electrons with a temperature anisotropy. The excited whistler mode waves propagate almost parallel and antiparallel to the background magnetic field in their source region, which is limited at ∣λ ∣ ≤ 3° (where λ is the magnetic latitude). When the waves leave from the source region and propagate toward high latitudes, both their amplitude and wave normal angle become larger. However, the group velocity of the waves is directed toward high latitudes almost along the magnetic field. During such a process, the waves have a frequency chirping, as shown by a rising tone in the frequency-time spectrogram. To our best knowledge, it is for the first time that rising-tone chorus are generated in a dipole magnetic field with a PIC simulation. ©2019. American Geophysical Union. All Rights Reserved.
Modeling Energetic Electron Nonlinear Wave-Particle Interactions with Electromagnetic Ion Cyclotron Waves
(Blackwell Publishing Ltd, 2019-04-15) Zheng, Liheng; Chen, Lunjin; Zhu, Hui; 0000-0003-2489-3571 (Chen, L); 0000-0001-9068-4431 (Zheng, L); 0000-0003-3556-8096 (Zhu, H); Zheng, Liheng; Chen, Lunjin; Zhu, Hui
Electromagnetic ion cyclotron (EMIC) waves in duskside plasmasphere and plasmaspheric plume scatter megaelectron volt electrons into the loss cone and are considered a major loss mechanism for the outer radiation belt. Wave-particle interaction between energetic electrons and EMIC waves has been studied extensively by the quasi-linear diffusion theory. However, EMIC waves are typically strong enough to trigger nonlinear wave-particle interaction effects and transport electrons in very different ways from quasi-linear diffusion. New mathematical method is therefore in demand to study the evolution of energetic electron distribution in response to nonlinear wave-particle interaction. In this work, we present a Markov chain description of the wave-particle interaction process, in which the electron distribution is represented by a state vector and is evolved by the Markov matrix. The Markov matrix is a matrix form of the electron response Green's function and could be determined from test particle simulations. Our modeling results suggest that electron loss rate is not significantly affected by phase bunching and phase trapping, but for strong EMIC waves, electron distribution is more saturated near loss cone than quasi-linear theory prediction, and negative electron phase space density slope develops inside loss cone. ©2019. American Geophysical Union. All Rights Reserved.
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, Xu
In 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.
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, Lunjin
We 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).
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, Lunjin
We 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).
Two-Dimensional Particle-In-Cell Simulation of Magnetosonic Wave Excitation in a Dipole Magnetic Field
(Blackwell Publishing Ltd) Chen, Lunjin; Sun, Jicheng; Lu, Q.; Wang, X.; Gao, X.; Wang, D.; Wang, S.; 0000-0002-5059-5394 (Sun, J); Chen, Lunjin; Sun, Jicheng
The excitation of magnetosonic waves in the meridian plane of a rescaled dipole magnetic field is investigated, for the first time, using a general curvilinear particle-in-cell simulation. Our simulation demonstrates that the magnetosonic waves are excited near the equatorial plane by tenuous ring distribution protons. The waves propagate nearly perpendicularly to the background magnetic field along both radially inward and outward directions. Different speeds of inward and outward propagation result in the asymmetrical distribution about the source region. The waves are accompanied by energization of both cool protons and electrons near the wave source region. The cool protons are heated perpendicularly, while the cool electrons can be heated in the parallel direction and also experience enhanced perpendicular drift at the presence of intense wave power. The implications of simulation results to the observations of magnetosonic waves and related particle heating in the inner magnetosphere are also discussed. ©2018 American Geophysical Union. All Rights Reserved.
Statistical Properties of Plasmaspheric Hiss from Van Allen Probes Observations
(Amer Geophysical Union) Hartley, D. P.; Kletzing, C. A.; Santolik, O.; Chen, Lunjin; Horne, R. B.; 0000-0003-2489-3571 (Chen, L); Chen, Lunjin
Van Allen Probes observations are used to statistically investigate plasmaspheric hiss wave properties. This analysis shows that the wave normal direction of plasmaspheric hiss is predominantly field aligned at larger L shells, with a bimodal distribution, consisting of a near- field aligned and a highly oblique component, becoming apparent at lower L shells. Investigation of this oblique population reveals that it is most prevalent at L < 3, frequencies with ⨍/⨍_{ce} 0.01 (or ⨍ > 700 Hz), low geomagnetic activity levels, and between 1900 and 0900 magnetic local time. This structure is similar to that reported for oblique chorus waves in the equatorial region, perhaps suggesting a causal link between the two wave modes. Ray tracing results from HOTRAY confirm that it is feasible for these oblique chorus waves to be a source of the observed oblique plasmaspheric hiss population. The decrease in oblique plasmaspheric hiss occurrence rates during more elevated geomagnetic activity levels may be attributed to the increase in Landau resonant electrons causing oblique chorus waves to be more substantially damped outside of the plasmasphere. In turn, this restricts the amount of wave power that can access the plasmasphere and evolve into oblique plasmaspheric hiss. These results confirm that, despite the difference in location of this bimodal distribution compared to previous studies, a direct link between oblique equatorial chorus outside of the plasmasphere and oblique hiss at low L shells is plausible. As such, these results are in keeping with the existing theory of chorus as the source of plasmaspheric hiss.
Generation of Lower Harmonic Magnetosonic Waves through Nonlinear Wave-Wave Interactions
(Blackwell Publishing Ltd) Gao, X.; Sun, J.; Lu, Q.; Chen, Lunjin; Wang, S.; Chen, Lunjin
Although magnetosonic waves in the Earth's magnetosphere have been well understood by the linear theory, low harmonic magnetosonic waves, which often lack of free energy, can be unusually present. By employing a 1-D particle-in-cell simulation model, we have investigated the generation of those unusual lower harmonic magnetosonic waves in a plasma containing a proton ring distribution. In our simulation, the higher harmonic magnetosonic waves (from~9Ω_h to ~12Ω_h) are firstly excited due to the unstable proton ring, which can be well explained by the linear theory. Several lower harmonic magnetosonic waves (below 5Ω_h), which well separates away from the higher harmonics, soon appear in the system. Those lower harmonics, which do not have any positive linear growth rates, can be generated by a nonlinear mechanism. The bicoherence analysis demonstrates that there is a strong phase coupling among the unusual lower harmonic magnetosonic waves and the magnetosonic waves generated due to the proton ring, supporting the idea that the lower harmonic waves could be driven by the wave-wave couplings of the generated magnetosonic waves. This wave-wave coupling generation mechanism is further confirmed by another two simulations, where two or three pump magnetosonic waves are initially injected. The lower-frequency waves, that is, the fundamental wave and its second harmonic, are also successfully reproduced due to the nonlinear coupling of pump magnetosonic waves. Our simulations not only propose a potential generation mechanism of unusual lower harmonic magnetosonic waves in the Earth's magnetosphere, but also give some new insights on the evolution of magnetosonic spectra.
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, Liheng
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 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.
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, Zhiyang
The 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.
On the Diffusion Rates of Electron Bounce Resonant Scattering by Magnetosonic Waves
(Amer Geophysical Union) Maldonado, Armando A.; Chen, Lunjin; 0000-0001-5286-1898 (Maldonado, AA); 0000-0003-2489-3571 (Chen, L); Maldonado, Armando A.; Chen, Lunjin
Magnetosonic waves have been demonstrated as effective for bounce resonant scattering. Electron scattering rates due to bounce resonance interaction with magnetosonic waves are derived in a general form, where the effects of the finite Larmor radius, of violation in the first adiabatic invariant, and of latitudinal wave power distribution are considered. Such bounce resonance diffusion coefficients are important, but missing, from radiation belt modeling. Additionally, we provide a parametric study on the electron energy and equatorial pitch angle, magnetosonic wave, and background parameters to identify the factors that determine effective bounce resonant scattering.
Fast Magnetosonic Waves Observed by Van Allen Probes: Testing Local Wave Excitation Mechanism
(Amer Geophysical Union) Min, Kyungguk; Liu, Kaijun; Wang, Xueyi; Chen, Lunjin; Denton, Richard E.; 0000-0003-2489-3571 (Chen, L); Chen, Lunjin
Linear Vlasov theory and particle-in-cell (PIC) simulations for electromagnetic fluctuations in a homogeneous, magnetized, and collisionless plasma are used to investigate a fast magnetosonic wave event observed by the Van Allen Probes. The fluctuating magnetic field observed exhibits a series of spectral peaks at harmonics of the proton cyclotron frequency Ω_p and has a dominant compressional component, which can be classified as fast magnetosonic waves. Furthermore, the simultaneously observed proton phase space density exhibits positive slopes in the perpendicular velocity space, ∂f_p/∂v_⊥ > 0, which can be a source for these waves. Linear theory analyses and PIC simulations use plasma and field parameters measured in situ except that the modeled proton distribution is modified to have larger ∂f_p/∂v_⊥ under the assumption that the observed distribution corresponds to a marginally stable state when the distribution has already been scattered by the excited waves. The results show that the positive slope is the source of the proton cyclotron harmonic waves at propagation quasi-perpendicular to the background magnetic field, and as a result of interactions with the excited waves the evolving proton distribution progresses approximately toward the observed distribution.
Coherently Modulated Whistler Mode Waves Simultaneously Observed over Unexpectedly Large Spatial Scales
(Amer Geophysical Union) Li, Jinxing; Bortnik, Jacob; Li, Wen; Thorne, Richard M.; Ma, Qianli; Chu, Xiangning; Chen, Lunjin; Kletzing, Craig A.; Kurth, William S.; Hospodarsky, George B.; Wygant, John; Breneman, Aaron; Thaller, Scott; Chen, Lunjin
Utilizing simultaneous twin Van Allen Probes observations of whistler mode waves at variable separations, we are able to distinguish the temporal variations from spatial variations, determine the coherence spatial scale, and suggest the possible mechanism of wave modulation. The two probes observed coherently modulated whistler mode waves simultaneously at an unexpectedly large distance up to ~4.3 R_E over 3 h during a relatively quiet period. The modulation of 150-500 Hz plasmaspheric hiss was correlated with whistler mode waves measured outside the plasmasphere across 3 h in magnetic local time and 3 L shells, revealing that the modulation was temporal in nature. We suggest that the coherent modulation of whistler mode waves was associated with the coherent ULF waves measured over a large scale, which modulate the plasmaspheric density and result in the modulation of hiss waves via local amplification. In a later period, the 500-1500 Hz periodic rising-tone whistler mode waves were strongly correlated when the two probes traversed large spatial regions and even across the plasmapause. These periodic rising-tone emissions recurred with roughly the same period as the ULF wave, but there was no one-to-one correspondence, and a cross-correlation analysis suggests that they possibly originated from large L shells although the actual cause needs further investigation.
On the Origin of Ionospheric Hiss: A Conjugate Observation
(Amer Geophysical Union, 2018-11-05) Zhima, Zeren; Chen, Lunjin; Xiong, Ying; Cao, Jinbin; Fu, Huishan; Chen, Lunjin
We present a conjugate observation on whistler mode electromagnetic hiss from the low Earth orbit satellite Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) and the high-altitude elliptical orbit spacecraft Time History of Events and Macroscale Interactions during Substorms (THEMIS). The conjugate observation was performed at 14:51:10 to 15:12:00 UT on 15 June 2010, when DEMETER was flying across the L shell region from ~1.39 to 2.80 at an altitude of ~660 km; meanwhile, THEMIS probes were passing through the L shell region from ~1.64 to 1.91 at altitudes from ~1.6 to 2.0 R_E. The conjugated observations demonstrate similar time-frequency structures between the ionospheric hiss (~350 to 800 Hz) captured by DEMETER and the plasmaspheric hiss (~350 to 900 Hz) recorded by THEMIS probes, including similar peak frequencies (~500 to 600 Hz), similar lower cutoff frequencies (~350 to 400 Hz), and upper cutoff frequencies (~730 to 800 Hz). The wave vector analyses show that the ionospheric hiss propagates obliquely downward to the Earth and slightly equatorward with right-handed polarization, suggesting that its source comes from higher altitudes. Ray tracing simulations with the constraint of observations verify that the connection between ionospheric and plasmaspheric hiss is physically possible through wave propagation. This study provides direct observational evidence to support the mechanism that high-altitude plasmaspheric hiss is responsible for the generation of low-altitude ionospheric hiss.
The Characteristic Response of Whistler Mode Waves to Interplanetary Shocks
(Amer Geophysical Union, 2018-11-05) Yue, Chao; Chen, Lunjin; Bortnik, Jacob; Ma, Qianli; Thorne, Richard M.; Angelopoulos, Vassilis; Li, Jinxing; An, Xin; Zhou, Chen; Kletzing, Craig; Reeves, Geoffrey D.; Spence, Harlan E.; 0000-0003-2489-3571 (Chen, L); Chen, Lunjin
Magnetospheric whistler mode waves play a key role in regulating the dynamics of the electron radiation belts. Recent satellite observations indicate a significant influence of interplanetary (IP) shocks on whistler mode wave power in the inner magnetosphere. In this study, we statistically investigate the response of whistler mode chorus and plasmaspheric hiss to IP shocks based on Van Allen Probes and THEMIS satellite observations. Immediately after the IP shock arrival, chorus wave power is usually intensified, often at postmidnight to prenoon sector, while plasmaspheric hiss wave power predominantly decreases near the dayside but intensifies near the nightside. We conclude that chorus wave intensification outside the plasmasphere is probably associated with the suprathermal electron flux enhancement caused by the IP shock. Through a simple ray tracing modeling assuming the scenario that plasmaspheric hiss is originated from chorus, we find that the solar wind dynamic pressure increase changes the magnetic field configuration to favor ray penetration in the nightside and promote ray refraction away from the dayside, potentially explaining the magnetic local time-dependent responses of plasmaspheric hiss waves following IP shock arrivals.
The Radiation Belt Electron Scattering by Magnetosonic Wave: Dependence on Key Parameters
(Amer Geophysical Union, 2018-10-22) Lei, Mingda; Xie, Lun; Li, Jinxing; Pu, Zuyin; Fu, Suiyan; Ni, Binbin; Hua, Man; Chen, Lunjin; Li, Wen; Chen, Lunjin
Magnetosonic (MS) waves have been found capable of creating radiation belt electron butterfly distributions in the inner magnetosphere. To investigate the physical nature of the interactions between radiation belt electrons and MS waves, and to explore a preferential condition for MS waves to scatter electrons efficiently, we performed a comprehensive parametric study of MS wave-electron interactions using test particle simulations. The diffusion coefficients simulated by varying the MS wave frequency show that the scattering effect of MS waves is frequency insensitive at low harmonics (f < 20 f(cp)), which has great implications on modeling the electron scattering caused by MS waves with harmonic structures. The electron scattering caused by MS waves is very sensitive to wave normal angles, and MS waves with off 90 degrees wave normal angles scatter electrons more efficiently. By simulating the diffusion coefficients and the electron phase space density evolution at different L shells under different plasma environment circumstances, we find that MS waves can readily produce electron butterfly distributions in the inner part of the plasmasphere where the ratio of electron plasma-to-gyrofequency (f(pe)/f(ce)) is large, while they may essentially form a two-peak distribution outside the plasmapause and in the inner radiation belt where f(pe)/f(ce) is small.
Propagation Characteristics of Plasmaspheric Hiss: Van Allen Probe Observations and Global Empirical Models
(Amer Geophysical Union, 2017-03-13) Yu, J.; Li, L. Y.; Cao, J. B.; Chen, Lunjin; Wang, J.; Yang, J.; 0000-0003-2489-3571 (Chen, L); Chen, Lunjin
Based on the Van Allen Probe A observations from 1 October 2012 to 31 December 2014, we develop two empirical models to respectively describe the hiss wave normal angle (WNA) and amplitude variations in the Earth's plasmasphere for different substorm activities. The long-term observations indicate that the plasmaspheric hiss amplitudes on the dayside increase when substorm activity is enhanced (AE index increases), and the dayside hiss amplitudes are greater than the nightside. However, the propagation angles (WNAs) of hiss waves in most regions do not depend strongly on substorm activity, except for the intense substorm-induced increase in WNAs in the nightside low L-region. The propagation angles of plasmaspheric hiss increase with increasing magnetic latitude or decreasing radial distance (L-value). The global hiss WNAs (the power-weighted averages in each grid) and amplitudes (medians) can be well reproduced by our empirical models.
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, Zhiyang
Dayside 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.
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, Xu
In 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.
Observations of Discrete Harmonics Emerging from Equatorial Noise
(Nature Publishing Group, 2015-07-14) Balikhin, Michael A.; Shprits, Yuri Y.; Walker, Simon N.; Chen, Lunjin; Cornilleau-Wehrlin, Nicole; Dandouras, Iannis; Santolik, Ondrej; Carr, Christopher; Yearby, Keith H.; Weiss, Benjamin; 0000-0003-2489-3571 (Chen, L)
A number of modes of oscillations of particles and fields can exist in space plasmas. Since the early 1970s, space missions have observed noise-like plasma waves near the geomagnetic equator known as 'equatorial noise'. Several theories were suggested, but clear observational evidence supported by realistic modelling has not been provided. Here we report on observations by the Cluster mission that clearly show the highly structured and periodic pattern of these waves. Very narrow-banded emissions at frequencies corresponding to exact multiples of the proton gyrofrequency (frequency of gyration around the field line) from the 17th up to the 30th harmonic are observed, indicating that these waves are generated by the proton distributions. Simultaneously with these coherent periodic structures in waves, the Cluster spacecraft observes 'ring' distributions of protons in velocity space that provide the free energy for the waves. Calculated wave growth based on ion distributions shows a very similar pattern to the observations.

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