Xinliang Gao

New evidence for generation mechanisms of discrete and hiss‐like whistler mode waves

Linear theory suggests that whistler mode wave growth rates are proportional to the ratio of hot electron (~1 to 30 keV) density to total electron density (Nh/Nt), whereas nonlinear wave theory suggests that an optimum linear growth rate is required to generate rising tone chorus from hiss-like emissions. Using the Time History of Events and Macroscale Interactions during Substorms waveform data collected by three probes over the past ~5 years, we investigate the correlation between Nh/Nt and wave amplitude/wave occurrence rate for rising tone, falling tone, and hiss-like emissions separately. Statistical results show that the rising and falling tones preferentially occur in the region with a limited Nh/Nt range, whereas both the occurrence rate and wave amplitudes of hiss-like emissions become larger for higher values of Nh/Nt. Our statistical results not only provide an important clue on the generation mechanism of hiss-like emissions, but also provide supporting experimental evidence for the nonlinear theory of generating rising tone chorus.

Transmission of large‐amplitude ULF waves through a quasi‐parallel shock at Venus

There exist large-amplitude ultralow frequency (ULF) waves in the upstream region of a quasi-parallel shock, which are excited due to the reflected ions by the shock. These waves are then brought back to the shock by the solar wind, and at last they coalesce and merge with the shock. In this paper, with the magnetic field measurements from Venus Express, for the first time we observe the transmission of large-amplitude ULF waves from the upstream region to the downstream under quasi-parallel shock conditions. These waves exist in both the upstream and downstream regions of the Venusian bow shock, which have the similar characteristics: their peak frequencies are 0.04–0.05 Hz in the spacecraft frame, their propagation angles do not change greatly, they have left-hand polarization with respect to the mean magnetic field in the spacecraft frame, and they also have a large compressibility. We conclude that they are magnetosonic waves. The generation mechanism of such waves at the Venusian bow shock is also discussed in the paper.

Generation of magnetosonic waves over a continuous spectrum

Magnetosonic 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 >∼ Ωh/2, where γ is wave growth rate and Ωh 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 Ωh harmonics, including the fundamental Ωh, 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.

The effect of different solar wind parameters upon significant relativistic electron flux dropouts in the magnetosphere

Superposed epoch analyses were performed on 193 significant relativistic electron flux dropout events, in order to study the roles of different solar wind parameters in driving the depletion of relativistic electrons, using ~16 years of data from the POES and GOES missions, and the OMNIWEB solar wind database. We find that the solar wind dynamic pressure and interplanetary magnetic field (IMF) Bz play key roles in causing the relativistic electron flux dropouts, but also that either large solar wind dynamic pressure or strong southward IMF Bz by itself is capable of producing the significant depletion of relativistic electrons. The relativistic electron flux dropouts occur not only when the magnetopause is compressed closer to the Earth but also when the magnetopause is located very far (> ~10 RE). Importantly, our results show that in addition to the large solar wind dynamic pressure, which pushes the magnetopause inward strongly and causes the electrons to escape from the magnetosphere, relativistic electrons can also be scattered into the loss cone and precipitate into the Earths atmosphere during periods of strong southward IMF Bz, which preferentially provides a source of free energy for electromagnetic ion cyclotron (EMIC) wave excitation. This is supported by the fact that the strongest electron precipitation into the atmosphere is found in the dusk sector, where EMIC waves are typically observed in the high-density plasmasphere or plume and cause efficient electron precipitation down to ~1 MeV.

Generation of multiband chorus by lower band cascade in the Earth's magnetosphere

Chorus waves are intense electromagnetic whistler mode emissions in the magnetosphere, typically falling into two distinct frequency bands: a lower band (0.1–0.5fce) and an upper band (0.5–0.8fce) with a power gap at about 0.5fce. In this letter, with the Time History of Events and Macroscale Interactions during Substorms satellite, we observed two special chorus events, which are called as multiband chorus because upper band chorus is located at harmonics of lower band chorus. We propose a new potential generation mechanism for multiband chorus, which is called as lower band cascade. In this scenario, a density mode with a frequency equal to that of lower band chorus is generated by the ponderomotive effect (inhomogeneity of the electric amplitude) along the wave vector, and then upper band chorus with the frequency twice that of lower band chorus is generated through wave-wave couplings between lower band chorus and the density mode. The mechanism provides a new insight into the evolution of whistler mode chorus in the Earths magnetosphere.

Plasma density behavior with new graphite limiters in the Hefei Tokamak-7

A new set of actively cooled toroidal double-ring graphite limiters has been developed in the Hefei Tokamak-7 (HT-7) [X. Gao et al., Phys. Plasmas 7, 2933 (2000)] for long pulse operation. The extension of operational region and density behavior with graphite (C) limiters have been studied in this paper. Extended high-density region at the high plasma current low-qa was obtained. The density profile with the C limiter was studied to compare with the previous molybdenum (Mo) limiter. The critical density of multifaceted asymmetric radiation from the edge (MARFE) onset is observed in the region of Zeff1∕2fGW=0.9∼1.2, where fGW=n¯e∕nGW. (Here n¯e is the maximum line average electron density and nGW is the Greenwald density.) Under the same injected power, the critical density of MARFE onset with the new C limiter is much higher than the previous Mo limiter.

Particle acceleration and generation of diffuse superthermal ions at a quasi‐parallel collisionless shock: Hybrid simulations

[1] A large scale one-dimensional hybrid simulation is performed to investigate the generation mechanism of diffuse superthermal ions at a high Mach number quasi-parallel collisionless shock. The shock exhibits a cyclic behavior and reforms periodically. The generation of the diffuse ions is associated with the reformation of the shock. At the beginning of the reformation cycle, a part of ions are reflected by the shock due to the existence of the cross shock potential. At the same time, an upstream wave is brought back by the upstream plasma and interacts with the shock. The upstream wave begins to steepen as it approaches the shock, and then a new shock front is formed. The reflected ions are trapped between the new and old shock fronts. They are accelerated every time they are reflected by the new shock front until the reformation cycle of the shock is finished and the particles escape from the shock. In this way, diffuse superthermal ions are generated in the quasi-parallel shock, which may be further accelerated to higher energy due to shock diffusive acceleration.

Generation of Multiband Chorus in the Earth's Magnetosphere: 1‐D PIC Simulation

Multiband chorus waves, where the frequency of upper band chorus is about twice that of lower band chorus, have recently been reported based on THEMIS observations. The generation ofmultiband chorus waves is attributed to the mechanism of lower band cascade, where upper band chorus is excited via the nonlinear coupling process between lower band chorus and the associated density mode with the frequency equal to that of lower band chorus. In this letter, with a one-dimensional (1-D) particle-in-cell (PIC) simulation model, we have successfully reproduced multiband chorus waves. During the simulation, the significant density fluctuation is driven by the fluctuating electric field along the wave vector of the pump wave (lower band chorus), which canbedirectly observed in this self-consistent plasma system. Then, the secondharmonic of the pump whistler-mode wave (upper band chorus) is generated. After quantitatively analyzing resonant conditions amongwave numbers, we can confirm that the generation is caused due to the coupling between the pump wave and the density fluctuation along its wave vector. The third harmonic can also be excited through lowerbandcascade if thepumpwhistler-modewavehas a sufficiently largeamplitude.Our simulation results not only provide a theoretical support to themechanism of lower band cascade to generatemultiband chorus but also propose a new pattern of evolution for whistler-mode waves in the Earth’s magnetosphere.

A parametric study for the generation of ion Bernstein modes from a discrete spectrum to a continuous one in the inner magnetosphere. II. Particle-in-cell simulations

In this paper, we perform one-dimensional particle-in-cell simulations to investigate the properties of perpendicular magnetosonic waves in a plasma system consisting of three components: cool electrons, cool protons, and tenuous ring distribution protons, where the waves are excited by the tenuous proton ring distribution. Consistent with the linear theory, the spectra of excited magnetosonic waves can change from discrete to continuous due to the overlapping of adjacent unstable wave modes. The increase of the proton to electron mass ratio, the ratio of the light speed to the Alfven speed, or the concentration of protons with a ring distribution tends to result in a continuous spectrum of magnetosonic waves, while the increase of the ring velocity of the tenuous proton ring distribution leads to a broader one, but with a discrete structure. Moreover, the energization of both cool electrons and protons and the scattering of ring distribution protons due to the excited magnetosonic waves are also observed...

Ion dynamics at supercritical quasi-parallel shocks: Hybrid simulations

By separating the incident ions into directly transmitted, downstream thermalized, and diffuse ions, we perform one-dimensional (1D) hybrid simulations to investigate ion dynamics at a supercritical quasi-parallel shock. In the simulations, the angle between the upstream magnetic field and shock nominal direction is θBn=30°, and the Alfven Mach number is MA∼5.5. The shock exhibits a periodic reformation process. The ion reflection occurs at the beginning of the reformation cycle. Part of the reflected ions is trapped between the old and new shock fronts for an extended time period. These particles eventually form superthermal diffuse ions after they escape to the upstream of the new shock front at the end of the reformation cycle. The other reflected ions may return to the shock immediately or be trapped between the old and new shock fronts for a short time period. When the amplitude of the new shock front exceeds that of the old shock front and the reformation cycle is finished, these ions become thermal...