arXiv subjects

F. -J. Kong

Publications and source records attributed to F. -J. Kong.

5 recordsLinked to original sources

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↗

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

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

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↗

arXiv2017

Numerical simulations of particle acceleration at interplanetary quasi-perpendicular shocks

Using test particle simulations we study particle acceleration at highly perpendicular ($θ_{Bn}\geq 75^\circ$) shocks under conditions of modeling magnetic turbulence. We adopt a backward-in-time method to solve the Newton-Lorentz equation using the observed shock parameters for quasi-perpendicular interplanetary shocks, and compare the simulation results with $ACE$/EPAM observations to obtain the injection energy and timescale of particle acceleration. With our modeling and observations we find that a large upstream speed is responsible for efficient particle acceleration. Our results also show that the quasi-perpendicular shocks are capable of accelerating thermal particles to high energies of the order of MeV for both kappa and Maxwellian upstream distributions, which may originate from the fact that in our model the local background magnetic field has a component parallel to the shock normal.

physics.space-ph↗