arXiv subjects

G. Qin

Publications and source records attributed to G. Qin.

At least 19 recordsLinked to original sources

arXiv2026

Moment-based formulas for the transport coefficients

The coefficients of the transport equations, describing the propagation features, are particularly important in astrophysics, interplanetary physics, and experimental plasma physics. In this paper, the variable-dependent diffusion coefficients are investigated. For the momentum transport equations, we find that the coefficients are related to statistical quantities, some of which are new. In addition, these coefficients take logarithmic forms, which are different from the results derived in previous papers. For isotropic pitch-angle scattering, we also obtain a formula that takes a logarithmic form. For fractional transport equations, the coefficients expressed in terms of the moments are obtained from the governing equations.

astro-ph.SR↗

arXiv2025

A Cosmic Ray Acceleration Mechanism Based on Background Flow Velocity Inhomogeneities Yielding Power-Law Spectra

In this article, momentum transport generated by the combined effects of pitch-angle diffusion and Background Flow Velocity Inhomogeneities (BFVIs) is proposed to obtain a cosmic rays acceleration mechanism, starting from the well-known focusing equation describing particle diffusion and acceleration. The inhomogeneities of background flow velocity is ubiquitous in astrophysical environment. The isotropic distribution function equation of charged energetic particles is derived, and its solution is obtained, demonstrating the form of momentum power laws of cosmic rays. In addition, if it is assumed that cosmic rays penetrate compressible plasma waves or turbulence, for quasi-steady states, the spectral index $\delta$ of the momentum power law spectrum of cosmic rays is found to be in the range $[-5, -3]$, which includes the observed power law indices of galactic cosmic rays. The results obtained in this article demonstrate that the mechanism proposed in this article, along with shock acceleration, may also contribute to the acceleration of galactic cosmic rays. Furthermore, when momentum convection effect and higher-order momentum derivative terms are considered, the indices of power laws should be smaller than $-5$. This may explain the power laws of solar energetic particle events.

astro-ph.HE↗

arXiv2023

The effect of solar wind on the charged particles' diffusion coefficients

The transport of energetic charged particles through magnetized plasmas is ubiquitous in interplanetary space and astrophysics, and the important physical quantities are the along-field and cross-field spatial diffusion coefficients of energetic charged particles. In this paper, the influence of solar wind on particle transport is investigated. Using the focusing equation, we obtain along- and cross-field diffusion coefficient accounting for the solar wind effect. For different conditions, the relative importance of solar wind effect to diffusion are investigated. It is shown that when energetic charged particles are close to the sun, for along-field diffusion the solar wind effect needs to be taken into account. These results are important for studying energetic charged particle transport processes in the vicinity of the sun.

physics.space-ph↗

arXiv2023

Relationship of transport coefficients with statistical quantities of charged particles

In the previous studies, from the Fokker-Planck equation the general spatial transport equation, which contains an infinite number of spatial derivative terms $T_n=κ_{nz}\partial^n{F}/ \partial{z^n}$ with $n=1, 2, 3, \cdots$, was derived. Due to the complexity of the general equation, some simplified equations with finite spatial derivative terms have been used in astrophysical researches, e.g., the diffusion equation, the hyperdiffusion one, subdiffusion transport one, etc. In this paper, the simplified equations with the highest order spatial derivative terms up to the first-, second-, third-, fourth-, and fifth-order are listed, and their transport coefficient formulas are derived, respectively. We find that most of the transport coefficients are determined by the corresponding statistical quantities. In addition, we find that the well-known statistical quantities, skewness $\mathcal{S}$ and kurtosis $\mathcal{K}$, are determined by some transport coefficients. The results can help one to use different transport coefficients determined by the statistical quantities, including many that are relatively new found in this paper, to study charged particle parallel transport processes.

astro-ph.SR↗

arXiv2022

The efficiency of electron acceleration by ICME-driven shocks

We present a study of the acceleration efficiency of suprathermal electrons at collisionless shock waves driven by interplanetary coronal mass ejections (ICMEs), with the data analysis from both the spacecraft observations and test-particle simulations. The observations are from the 3DP/EESA instrument onboard \emph{Wind} during the 74 shock events listed in Yang et al. 2019, ApJ, and the test-particle simulations are carried out through 315 cases with different shock parameters. A total of seven energy channels ranging from 0.428 to 4.161 keV are selected. In the simulations, using a backward-in-time method, we calculate the average downstream flux in the $90^\circ$ pitch angle. On the other hand, the average downstream and upstream fluxes in the $90^\circ$ pitch angle can also be directly obtained from the 74 observational shock events. In addition, the variation of the event number ratio with downstream to upstream flux ratio above a threshold value in terms of the shock angle (the angle between the shock normal and upstream magnetic field), upstream Alfv$\acute{\text e}$n Mach number, and shock compression ratio is statistically obtained. It is shown from both the observations and simulations that a large shock angle, upstream Alfv$\acute{\text e}$n Mach number, and shock compression ratio can enhance the shock acceleration efficiency. Our results suggest that shock drift acceleration is more efficient in the electron acceleration by ICME-driven shocks, which confirms the findings of Yang et al. 2018.

physics.space-ph↗

arXiv2021

Study of momentum diffusion with the effect of adiabatic focusing

Momentum diffusion of the energetic charged particles is an important mechanism of the transport process in astrophysics, physics of the fusion devices, and laboratory plasmas. In addition to the uniform field momentum diffusion, we obtain the modifying term due to the focusing effect of the large-scale magnetic field. After evaluating the modifying term, we find that it is determined by the sign of the focusing characteristic length and the Fokker-Planck coefficients $D_{μμ}$, $D_{μp}$, $D_{pμ}$, and $D_{pp}$. It is shown that we get a new second order acceleration mechanism in this work.

physics.plasm-ph↗

arXiv2021

Predicting Sunspot Numbers for Solar Cycles 25 and 26

The prediction of solar activity is important for advanced technologies and space activities. The peak sunspot number (SSN), which can represent the solar activity, has declined continuously in the past four solar cycles (21$-$24), and the Sun would experience a Dalton-like minimum, or even the Maunder-like minimum, if the declining trend continues in the following several cycles, so that the predictions of solar activity for cycles 25 and 26 are crucial. In Qin & Wu, 2018, ApJ, we established an SSN prediction model denoted as two-parameter modified logistic prediction (TMLP) model, which can predict the variation of SSNs in a solar cycle if the start time of the cycle has been determined. In this work, we obtain a new model denoted as TMLP-extension (TMLP-E), which can predict the solar cycle nearly two cycles in advance, so that the predictions of cycles 25 and 26 are made. It is found that the predicted solar maximum, ascent time, and cycle length are 115.1, 4.84 yr, and 11.06 yr, respectively, for cycle 25, and 107.3, 4.80 yr, and 10.97 yr, respectively, for cycle 26. The solar activities of cycles 25 and 26 are predicted to be at the same level as that of cycle 24, but will not decrease further. We therefore suggest that the cycles 24$-$26 are at a minimum of Gleissberg cycle.

astro-ph.SR↗

arXiv2020

Magnetic Cloud and Sheath in the Ground-Level Enhancement Event of 2000 July 14. II. Effects on the Forbush Decrease

Forbush decreases (Fds) in galactic cosmic ray intensity are related to interplanetary coronal mass ejections (ICMEs). The parallel diffusion of particles is reduced because the magnetic turbulence level in sheath region bounded by ICME's leading edge and shock is high. Besides, in sheath and magnetic cloud (MC) energetic particles would feel enhanced magnetic focusing effect caused by the strong inhomogeneity of the background magnetic field. Therefore, particles would be partially blocked in sheath-MC structure. Here, we study two-step Fds by considering the magnetic turbulence and background magnetic field in sheath-MC structure with diffusion coefficients calculated with theoretical models, to reproduce the Fd associated with the ground-level enhancement event on 2000 July 14 by solving the focused transport equation. The sheath and MC are set to spherical caps that are portions of spherical shells with enhanced background magnetic field. Besides, the magnetic turbulence levels in sheath and MC are set to higher and lower than that in ambient solar wind, respectively. In general, the simulation result conforms to the main characteristics of the Fd observation, such as the pre-increase precursor, amplitude, total recovery time, and the two-step decrease of the flux at the arrival of sheath and MC. It is suggested that sheath played an important role in the amplitude of Fd while MC contributed to the formation of the second step decrease and prolonged the recovery time. It is also inferred that both magnetic turbulence and background magnetic field in sheath-MC structure are important for reproducing the observed two-step Fd.

physics.space-ph↗

arXiv2020

Magnetic Cloud and Sheath in the Ground-Level Enhancement Event of 2000 July 14. I. Effects on the Solar Energetic Particles

Ground-level enhancements (GLEs) generally accompany with fast interplanetary coronal mass ejections (ICMEs), the shocks driven by which are the effective source of solar energetic particles (SEPs). In the GLE event of 2000 July 14, observations show that a very fast and strong magnetic cloud (MC) is behind the ICME shock and the proton intensity-time profiles observed at 1 au had a rapid two-step decrease near the sheath and MC. Therefore, we study the effect of sheath and MC on SEPs accelerated by an ICME shock through numerically solving the focused transport equation. The shock is regarded as a moving source of SEPs with an assumed particle distribution function. The sheath and MC are set to thick spherical caps with enhanced magnetic field, and the turbulence levels in sheath and MC are set to be higher and lower than that of the ambient solar wind, respectively. The simulation results of proton intensity-time profiles agree well with the observations in energies ranging from $\sim$1 to $\sim$100 MeV, and the two-step decrease is reproduced when the sheath and MC arrived at the Earth. The simulation results show that the sheath-MC structure reduced the proton intensities for about 2 days after shock passing through the Earth. It is found that the sheath contributed most of the decrease while the MC facilitated the formation of the second step decrease. The simulation also infers that the coordination of magnetic field and turbulence in sheath-MC structure can produce a stronger effect of reducing SEP intensities.

physics.space-ph↗

arXiv2020

Competing magnetic orders in quantum critical Sr$_3$Ru$_2$O$_7$

We investigated Sr$_3$Ru$_2$O$_7$, a quantum critical metal that shows a metamagnetic quantum phase transition and electronic nematicity, through density functional calculations. These predict a ferromagnetic ground state in contrast to the experimentally observed paramagnetism, raising the question of competing magnetic states and associated fluctuations that may suppress magnetic order. We did a search to identify such low energy antiferromagnetically ordered metastable states. We find that the lowest energy antiferromagnetic state has a striped order. This corresponds to the E-type order that has been shown to be induced by Mn alloying. We also note significant transport anisotropy in this E-type ordered state. These results are discussed in relation to experimental observations.

cond-mat.str-el↗

arXiv2020

The invariance of the diffusion coefficient with the iterative operations of charged particles' transport equation

The Spatial Parallel Diffusion Coefficient (SPDC) is one of the important quantities describing energetic charged particle transport. There are three different definitions for the SPDC, i.e., the Displacement Variance definition $κ_{zz}^{DV}=\lim_{t\rightarrow t_{\infty}}dσ^2/(2dt)$, the Fick's Law definition $κ_{zz}^{FL}=J/X$ with $X=\partial{F}/\partial{z}$, and the TGK formula definition $κ_{zz}^{TGK}=\int_0^{\infty}dt \langle v_z(t)v_z(0) \rangle$. For constant mean magnetic field, the three different definitions of the SPDC give the same result. However, for focusing field it is demonstrated that the results of the different definitions are not the same. In this paper, from the Fokker-Planck equation we find that different methods, e.g., the general Fourier expansion and perturbation theory, can give the different Equations of the Isotropic Distribution Function (EIDFs). But it is shown that one EIDF can be transformed into another by some Derivative Iterative Operations (DIOs). If one definition of the SPDC is invariant for the DIOs, it is clear that the definition is also an invariance for different EIDFs, therewith it is an invariant quantity for the different Derivation Methods of EIDF (DMEs). For the focusing field we suggest that the TGK definition $κ_{zz}^{TGK}$ is only the approximate formula, and the Fick's Law definition $κ_{zz}^{FL}$ is not invariant to some DIOs. However, at least for the special condition, in this paper we show that the definition $κ_{zz}^{DV}$ is the invariant quantity to the kinds of the DIOs. Therefore, for spatially varying field the displacement variance definition $κ_{zz}^{DV}$, rather than the Fick's law definition $κ_{zz}^{FL}$ and TGK formula definition $κ_{zz}^{TGK}$, is the most appropriate definition of the SPDCs.

math-ph↗

arXiv2019

Suprathermal electron acceleration by a quasi-perpendicular shock: simulations and observations

The acceleration of suprathermal electrons in the solar wind is mainly associated with shocks driven by interplanetary coronal mass ejections (ICMEs). It is well known that the acceleration of electrons is much more efficient at quasi-perpendicular shocks than at quasi-parallel ones. Yang et al. (2018, ApJ, 853, 89) (hereafter YEA2018) studied the acceleration of suprathermal electrons at a quasi-perpendicular ICME-driven shock event to claim the important role of shock drift acceleration (SDA). Here, we perform test-particle simulations to study the acceleration of electrons in this event, by calculating the downstream electron intensity distribution for all energy channels assuming an initial distribution based on the averaged upstream intensities. We obtain simulation results similar to the observations from YEA2018 as follows. It is shown that the ratio of downstream to upstream intensities peaks at about 90$^\circ$ pitch angle. In addition, in each pitch angle direction the downstream electron energy spectral index is much larger than the theoretical index of diffusive shock acceleration. Furthermore, considering SDA, the estimated drift length is proportional to the electron energy but the drift time is almost energy independent. Finally, we use a theoretical model based on SDA to describe the drift length and time, especially, to explain their energy dependence. These results indicate the importance of SDA in the acceleration of electrons by quasi-perpendicular shocks.

physics.space-ph↗

arXiv2019

Diffusion coefficient with displacement variance of energetic particles with adiabatic focusing

The equation $κ_{zz}=dσ^2/(2dt)$ (hereafter DCDV) is a well-known formula of energetic particles describing the relation of parallel diffusion coefficient $κ_{zz}$ with the parallel displacement variance $σ^2$. In this study, we find that DCDV is only applicable to two kinds of transport equations of isotropic distribution function, one is without cross terms, the other is without convection term. Here, by employing the more general transport equation, i.e., the variable coefficient differential equation derived from the Fokker-Planck equation, a new equation of $κ_{zz}$ as a function of $σ^2$ is obtained. We find that DCDV is the special case of the new equation. In addition, another equation of $κ_{zz}$ as a function of $σ^2$ corresponding to the telegraph equation is also investigated preliminarily.

astro-ph.HE↗

arXiv2019

Study of time evolution of the bend-over energy in the energetic particle spectrum at a parallel shock

Shock acceleration is considered one of the most important mechanisms for the acceleration of astrophysical energetic particles. In this work, we calculate the trajectories of a large number of test charged particles accurately in a parallel shock with magnetic turbulence. We investigate the time evolution of the accelerated-particle energy spectrum in the downstream of the shock in order to understand the acceleration mechanism of energetic particles. From simulation results we obtain power-law energy spectra with a bend-over energy, $E_0$, increasing with time. With the particle mean acceleration time and mean momentum change during each cycle of the shock crossing from diffusive shock acceleration model (following Drury), a time-dependent differential equation for the maximum energy, $E_{acc}$, of particles accelerated at the shock, can be approximately obtained. We assume the theoretical bend-over energy as $E_{acc}$. It is found that the bend-over energy from simulations agrees well with the theoretical bend-over energy using the non-linear diffusion theory, NLGCE-F, in contrast to that using the classic quasi-linear theory (QLT).

astro-ph.HE↗

arXiv2018

A Model of Sunspot Number with Modified Logistic Function

Solar cycles are studied with the Version 2 monthly smoothed international sunspot number, the variations of which are found to be well represented by the modified logistic differential equation with four parameters: maximum cumulative sunspot number or total sunspot number $x_m$, initial cumulative sunspot number $x_0$, maximum emergence rate $r_0$, and asymmetry $α$. A two-parameter function is obtained by taking $α$ and $r_0$ as fixed value. In addition, it is found that $x_m$ and $x_0$ can be well determined at the start of a cycle. Therefore, a prediction model of sunspot number is established based on the two-parameter function. The prediction for cycles $4-23$ shows that the solar maximum can be predicted with average relative error being 8.8\% and maximum relative error being 22\% in cycle 15 at the start of solar cycles if solar minima are already known. The quasi-online method for determining solar minimum moment shows that we can obtain the solar minimum 14 months after the start of a cycle. Besides, our model can predict the cycle length with the average relative error being 9.5\% and maximum relative error being 22\% in cycle 4. Furthermore, we predict the sunspot number variations of cycle 24 with the relative errors of the solar maximum and ascent time being 1.4\% and 12\%, respectively, and the predicted cycle length is 11.0 (95\% confidence interval is 8.3$-$12.9) years. The comparison to the observation of cycle 24 shows that our prediction model has good effectiveness.

astro-ph.SR↗

arXiv2018

Parallel and perpendicular diffusion coefficients of energetic charged particles with adiabatic focusing

It is very important to understand stochastic diffusion of energetic charged particles in non-uniform background magnetic field in plasmas of astrophysics and fusion devices. Using different methods considering along-field adiabatic focusing effect, various authors derived parallel diffusion coefficient $κ_\parallel$ and its correction $T$ to $κ_{\parallel 0}$, where $κ_{\parallel 0}$ is the parallel diffusion coefficient without adiabatic focusing effect. In this paper, using the improved perturbation method developed by He \& Schlickeiser and iteration process, we obtain a new correction $T'$ to $κ_{\parallel 0}$. Furthermore, by employing the isotropic pitch-angle scattering model $D_{μμ}=D(1-μ^2)$, we find that $T'$ has the different sign as that of $T$. In this paper the spatial perpendicular diffusion coefficient $κ_\bot$ with the adiabatic focusing effect is also obtained.

physics.space-ph↗

arXiv2018

Effects of shock and turbulence properties on electron acceleration

Using test particle simulations we study electron acceleration at collisionless shocks with a two-component model turbulent magnetic field with slab component including dissipation range. We investigate the importance of shock normal angle $θ_{Bn}$, magnetic turbulence level $\left(b/B_0\right)^2$, and shock thickness on the acceleration efficiency of electrons. It is shown that at perpendicular shocks the electron acceleration efficiency is enhanced with the decreasing of $\left(b/B_0\right)^2$, and at $\left(b/B_0\right)^2=0.01$ the acceleration becomes significant due to strong drift electric field with long time particles staying near the shock front for shock drift acceleration (SDA). In addition, at parallel shocks the electron acceleration efficiency is increasing with the increasing of $\left(b/B_0\right)^2$, and at $\left(b/B_0\right)^2=10.0$ the acceleration is very strong due to sufficient pitch-angle scattering for first-order Fermi acceleration, as well as due to large local component of magnetic field perpendicular to shock normal angle for SDA. On the other hand, the high perpendicular shock acceleration with $\left(b/B_0\right)^2=0.01$ is stronger than the high parallel shock acceleration with ($\left(b/B_0\right)^2=10.0$), the reason might be the assumption that SDA is more efficient than first-order Fermi acceleration. Furthermore, for oblique shocks, the acceleration efficiency is small no matter the turbulence level is low or high. Moreover, for the effect of shock thickness on electron acceleration at perpendicular shocks, we show that there exists the bend-over thickness, $L_{\text{diff,b}}$. The acceleration efficiency does not change evidently if the shock thickness is much smaller than $L_{\text{diff,b}}$. However, if the shock thickness is much larger than $L_{\text{diff,b}}$, the acceleration efficiency starts to drop abruptly.

astro-ph.HE↗

arXiv2018

Modulation of Galactic Cosmic Rays in the Inner Heliosphere over Solar Cycles

The 11-year and 22-year modulation of galactic cosmic rays (GCRs) in the inner heliosphere are studied using a numerical model developed by Qin and Shen in 2017. Based on the numerical solutions of Parker's transport equations, the model incorporates a modified Parker heliospheric magnetic field, a locally static time delayed heliosphere, and a time-dependent diffusion coefficients model in which an analytical expression of the variation of magnetic turbulence magnitude throughout the inner heliosphere is applied. Furthermore, during solar maximum, the solar magnetic polarity is determined randomly with the possibility of $A>0$ decided by the percentage of the north solar polar magnetic field being outward and the south solar polar magnetic field being inward. The computed results are compared with several GCR observations, e.g., IMP 8, SOHO/EPHIN, Ulysses, Voyager 1 \& 2, at various energies and show good agreement. It is shown that our model has successfully reproduced the 11-year and 22-year modulation cycles.

physics.space-ph↗