arXiv subjects

S. -S. Wu

Publications and source records attributed to S. -S. Wu.

7 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↗

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↗

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↗

arXiv2017

Model of energy spectrum parameters of ground level enhancement events in solar cycle 23

Mewaldt et al. 2012 fitted the observations of the ground level enhancement (GLE) events during solar cycle 23 to the double power-law equation to obtain the four energy spectra parameters, the normalization parameter $C$, low-energy power-law slope $γ_1$, high-energy power-law slope $γ_2$, and break energy $E_0$. There are 16 GLEs from which we select $13$ for study by excluding some events with complicated situation. We analyze the four parameters with conditions of the corresponding solar events. According to solar event conditions we divide the GLEs into two groups, one with strong acceleration by interplanetary (IP) shocks and another one without strong acceleration. By fitting the four parameters with solar event conditions we obtain models of the parameters for the two groups of GLEs separately. Therefore, we establish a model of energy spectrum of solar cycle 23 GLEs which may be used in prediction in the future.

physics.space-ph↗