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The wave normal angle of excited whistler waves was previously considered to be controlled by the parallel plasma beta (β∥h) of anisotropic hot electrons, while the effects of thermal electrons were usually neglected. By combining both the linear theoretical and 2‐D particle‐in‐cell (PIC) simulation models, we have investigated the effects of thermal electrons on the whistler anisotropy instability. In the high‐beta (β∥h ≥ 0.025) regime, the wave normal angle of the dominant whistler mode with the largest growth rate is always 0°, which is not affected by thermal electrons, while, its wave frequency and linear growth rate decrease with the density and temperature of thermal electrons. These results are also confirmed by PIC simulations. In the low‐beta (β∥h ≤ 0.025) regime, with the increase of the density and temperature of thermal electrons, the wave normal angle of the dominant whistler mode turns to zero from a large value. This change could be due to the stronger damping caused by thermal electrons for oblique whistler mode, since oblique wave usually has a smaller cyclotron resonant velocity than parallel wave. PIC simulations also show a consistent result, but reproduce a broad magnetic spectrum, even in the case including sufficient thermal electrons. Furthermore, thermal electrons with large parallel velocities are resonantly accelerated in the perpendicular direction, while parts of hot electrons are trapped and accelerated in the parallel direction. Our study suggests that the wave normal angle of whistler mode in the Earth's magnetosphere could be determined by both anisotropic and thermal electrons. Key Points: Both the linear theory and 2‐D PIC simulations show the WNA and spectrum of excited whistlers is dependent on both hot and thermal electronsIn high‐beta regime, the WNA of the excited dominant whistler is always 0°, which is not affected by thermal electronsIn low‐beta regime, the WNA of the excited dominant whistler turns to 0° from a large value if sufficient thermal electrons exist [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

In order to detect the cerebral microbleed (CMB) voxels within brain, we used susceptibility weighted imaging to scan the subjects. Then, we used undersampling to solve the accuracy paradox caused from the imbalanced data between CMB voxels and non-CMB voxels. we developed a seven-layer deep neural network (DNN), which includes one input layer, four sparse autoencoder layers, one softmax layer, and one output layer. Our simulation showed this method achieved a sensitivity of 95.13%, a specificity of 93.33%, and an accuracy of 94.23%. The result is better than three state-of-the-art approaches. [ABSTRACT FROM AUTHOR]

Aim Emotion recognition based on facial expression is an important field in affective computing. Current emotion recognition systems may suffer from two shortcomings: translation in facial image may deteriorate the recognition performance, and the classifier is not robust. Method To solve above two problems, our team proposed a novel intelligent emotion recognition system. Our method used stationary wavelet entropy to extract features, and employed a single hidden layer feedforward neural network as the classifier. To prevent the training of the classifier fall into local optimum points, we introduced the Jaya algorithm. Results The simulation results over a 20-subject 700-image dataset showed our algorithm reached an overall accuracy of 96.80 ± 0.14%. Conclusion This proposed approach performs better than five state-of-the-art approaches in terms of overall accuracy. Besides, the db4 wavelet performs the best among other whole db wavelet family. The 4-level wavelet decomposition is superior to other levels. In the future, we shall test other advanced features and training algorithms. [ABSTRACT FROM AUTHOR]

An overall and comparative ecological risk assessment of heavy metals (including Cd, Cr, Cu, Pb, Zn, Hg and As) in surface sediments from China's eight major aquatic bodies was conducted to better understand their potential risks on a national scale. By applying the joint approach of Hakanson risk index ( RI) and Monte Carlo simulation, ecological risk in this work is expressed as probability distribution of RI values instead of single point calculations to reflect the uncertainties in risk assessment process. The results show that the highest ecological risks posed by heavy metals existed in Xiangjiang River and Dianchi Lake. Although only a slim margin of high risk (651.88/600 = 1.08 and 700.61/600 = 1.17) was identified based on average RI values, the probabilities of high risk level derived from Monte Carlo simulation reached as high as 56.7 and 52.9 % in these two aquatic bodies, respectively. And the probability of low risk level was less than 1.6 %. Furthermore, the risk was mainly contributed by Hg and Cd, discharged through local intensive mining and industrial activities. The findings indicate that rigid control and effective management measures to prevent heavy metal pollution are urgently needed in China, especially for the high-risk aquatic bodies. This study shows that the joint approach can be used to identify the high risk water bodies and the major metal pollutants. It may avoid overestimating or underestimating the ecological risk and provide more decision-making support for risk alleviation in the polluted aquatic bodies. [ABSTRACT FROM AUTHOR]

Alfven waves with a finite amplitude are found to be unstable to a parametric decay in low beta plasmas. In this paper, the parametric decay of a circularly polarized Alfven wave in a proton-electron-alpha plasma system is investigated with one-dimensional (1-D) hybrid simulations. In cases without alpha particles, with the increase of the wave number of the pump Alfven wave, the growth rate of the decay instability increases and the saturation amplitude of the density fluctuations slightly decrease. However, when alpha particles with a sufficiently large bulk velocity along the ambient magnetic field are included, at a definite range of the wave numbers of the pump wave, both the growth rate and the saturation amplitude of the parametric decay become much smaller and the parametric decay is heavily suppressed. At these wave numbers, the resonant condition between the alpha particles and the daughter Alfven waves is satisfied, therefore, their resonant interactions might play an important role in the suppression of the parametric decay instability. [ABSTRACT FROM AUTHOR]

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 thermalized ions in the downstream. No noticeable heating can be found in the directly transmitted ions. The relevance of our simulations to the satellite observations is also discussed in the paper. [ABSTRACT FROM AUTHOR]

The ion motions in obliquely propagating Alfven waves with sufficiently large amplitudes have already been studied by Chen et al. [Phys. Plasmas 8, 4713 (2001)], and it was found that the ion motions are stochastic when the wave frequency is at a fraction of the ion gyro-frequency. In this paper, with test particle simulations, we investigate the ion motions in obliquely propagating magnetosonic waves and find that the ion motions also become stochastic when the amplitude of the magnetosonic waves is sufficiently large due to the resonance at sub-cyclotron frequencies. Similar to the Alfven wave, the increase of the propagating angle, wave frequency, and the number of the wave modes can lower the stochastic threshold of the ion motions. However, because the magnetosonic waves become more and more compressive with the increase of the propagating angle, the decrease of the stochastic threshold with the increase of the propagating angle is more obvious in the magnetosonic waves than that in the Alfven waves. [ABSTRACT FROM AUTHOR]

Previous studies have shown that obliquely propagating Alfven waves may be excited in the electromagnetic alpha/proton instability. In this paper, two-dimensional hybrid simulations are performed to investigate the nonlinear evolution of the electromagnetic alpha/proton instability. We further find that the obliquely propagating Alfven waves excited by the alpha/proton instability have nearly linear polarization. The background proton component, as well as the alpha component, can be resonantly heated by the oblique Alfven waves. The implications of our results to the unsolved solar coronal hearing problem are also discussed in this paper. [ABSTRACT FROM AUTHOR]

It is well known that the parallel cuts of the parallel and perpendicular electric field in electron phase-space holes (electron holes) have bipolar and unipolar structures, respectively. Recently, electron holes in the Earth's plasma sheet have been observed by THEMIS satellites to have detectable fluctuating magnetic field with regular structures. Du et al. () investigated the evolution of a one-dimensional (1D) electron hole with two-dimensional (2D) electromagnetic particle-in-cell (PIC) simulations in weakly magnetized plasma (Ω< ω, where Ω and ω are the electron gyrofrequency and electron plasma frequency, respectively), which initially exists in the simulation domain. The electron hole is unstable to the transverse instability and broken into several 2D electron holes. They successfully explained the observations by THEMIS satellites based on the generated magnetic structures associated with these 2D electron holes. In this paper, 2D electromagnetic particle-in-cell (PIC) simulations are performed in the x- y plane to investigate the nonlinear evolution of the electron two-stream instability in weakly magnetized plasma, where the background magnetic field $(\mathbf{B}_{0} =B_{0}\vec{\mathbf{e}} _{x})$ is along the x direction. Several 2D electron holes are formed during the nonlinear evolution, where the parallel cuts of E and E have bipolar and unipolar structures, respectively. Consistent with the results of Du et al. (), we found that the current along the z direction is generated by the electric field drift motion of the trapped electrons in the electron holes due to the existence of E, which produces the fluctuating magnetic field δB and δB in the electron holes. The parallel cuts of δB and δB in the electron holes have unipolar and bipolar structures, respectively. [ABSTRACT FROM AUTHOR]

Electron phase space holes (electron holes) are found to be unstable to the transverse instability. Two-dimensional (2D) electromagnetic particle-in-cell simulations are performed to investigate the structures of the fluctuating magnetic field associated with electron holes. The combined actions between the transverse instability and the stabilization by the background magnetic field (B0=B0evector x) lead a one-dimensional electron hole into several 2D electron holes which are isolated in both the x and y directions. The electrons trapped in these 2D electron holes suffer the electric field drift vE=E×B0/B02 due to the existence of the perpendicular electric field Ey, which generates the current along the z direction. Then, the unipolar and bipolar structures are formed for the parallel cut of the fluctuating magnetic field along the x and y directions, respectively. At the same time, these 2D electron holes move along the x direction, and the unipolar structures are formed for the parallel cut of the fluctuating magnetic field along the z direction. [ABSTRACT FROM AUTHOR]

Two-dimensional particle-in-cell simulations are performed to investigate the formation of electron density depletions in collisionless magnetic reconnection. In anti-parallel reconnection, the quadrupole structures of the out-of-plane magnetic field are formed, and four symmetric electron density depletion layers can be found along the separatrices due to the effects of magetic mirror. With the increase of the initial guide field, the symmetry of both the out-of-plane magnetic field and electron density depletion layers is distorted. When the initial guide field is sufficiently large, the electron density depletion layers along the lower left and upper right separatrices disappear. The parallel electric field in guide field reconnection is found to play an important role in forming such structures of the electron density depletion layers. The structures of the out-of-plane magnetic field By and electron depletion layers in anti-parallel and guide field reconnection are found to be related to electron flow or in-plane currents in the separatrix regions. In anti-parallel reconnection, electrons flow towards the X line along the separatrices, and are directed away from the X line along the magnetic field lines just inside the separatrices. In guide field reconnection, electrons can only flow towards the X line along the upper left and lower right separatrices due to the existence of the parallel electric field in these regions. [ABSTRACT FROM AUTHOR]

In collisionless reconnection, the magnetic field near the separatrix is stronger than that around the X-line, so an electron-beam can be formed and flows toward the X-line, which leads to a decrease of the electron density near the separatrix. Having been accelerated around the X-line, the electrons flow out along the magnetic field lines in the inner side of the separatrix. A quadruple structure of the Hall magnetic field By is formed by such a current system. A 2D particle-in-cell (PIC) simulation code is used in this paper to study the collisionless magnetic reconnection without an initial guide field. The current system described above is proved by the simulations. Furthermore, the position of the peak of the Hall magnetic field By is found to be between the separatrix and the center of the current sheet, which is verified by Cluster observations. [ABSTRACT FROM AUTHOR]

We perform 2.5-dimensional particle-in-cell simulations to investigate the nonlinear evolution of the lower hybrid drift instability (LHDI) in Harris current sheet. Due to the drift motion of electrons in the electric field of the excited low hybrid drift (LHD) waves, the electrons accumulate at the outer layer, and therefore there is net positive charge at the inner edge of the current sheet. This redistribution of charge can create an electrostatic field along the z direction, which then modifies the motions of the electrons along the y direction by E × B drift. This effect strongly changes the structure of the current sheet. [ABSTRACT FROM AUTHOR]

Using the two-fluid model in the case of a ? 1 (a = b/2Q, b is the ratio of thermal pressure to magnetic pressure, and Q = me/mi), we numerically investigate the interactions between two solitary kinetic Alfvén waves (SKAWs) and between an SKAW and a density discontinuity. The results show that the two SKAWs would remain in their original shapes and propagate at their initiating speeds, which indicates that SKAWs behave just like standard solitons. The simulation also shows that SKAWs will reflect and refract when crossing a discontinuity and propagating into a higher density region. The transmission wave is an SKAW with increasing density, and the reverberation is a disturbance with lower amplitude. [ABSTRACT FROM AUTHOR]

Previous particle-in-cell simulations have evidenced that supercritical, quasi-perpendicular shocks are non-stationary. By separating the incident ions into reflected (R) and directly transmitted (DT) parts, we investigate the ion distributions in a non-stationary perpendicular shock. The upstream ion distributions have two parts corresponding to the R and incident ions respectively, while the R ions have higher energy. The downstream ions have a corering distribution. The core and ring parts correspond to the DT and R, ions, respectively. The ion distributions depend largely on the non-stationary shock structure. The percentage of the reflected ions cyclically varies in time with a period equal to the shock self-reformation cycle, and the number of the R ions increases with the steepness of the shock ramp. [ABSTRACT FROM AUTHOR]