| 2012 Top |
| 7. | Fan Guo, Effects of Turbulent Magnetic Fields on the Transport and Acceleration of Energetic Charged Particles: Numerical Simulations with Application to Heliospheric Physics, Ph.D Dissertation, , 2012-11. [ (3.09MB)] | Abstract. Turbulent magnetic fields are ubiquitous in space physics and astrophysics. The
influence of magnetic turbulence on the motions of charged particles contains the
essential physics of the transport and acceleration of energetic charged particles in
the heliosphere, which is to be explored in this thesis. After a brief introduction
on the energetic charged particles and magnetic fields in the heliosphere, the rest
of this dissertation focuses on three specific topics: 1. the transport of energetic
charged particles in the inner heliosphere, 2. the acceleration of ions at collisionless
shocks, and 3. the acceleration of electrons at collisionless shocks. We utilize various
numerical techniques to study these topics. In Chapter 2 we study the propagation
of charged particles in turbulent magnetic fields similar to the propagation of solar
energetic particles in the inner heliosphere. The trajectories of energetic charged
particles in the turbulent magnetic field are numerically integrated. The turbulence
model includes a Kolmogorov-like magnetic field power spectrum containing a broad
range of scales from those that lead to large-scale field-line random walk to small
scales leading to resonant pitch-angle scattering of energetic particles. We show that
small-scale variations in particle intensities (the so-called “dropouts”) and velocity
dispersions observed by spacecraft can be reproduced using this method. Our study
gives a new constraint on the error of “onset analysis”, which is a technique com-
monly used to infer information about the initial release of energetic particles. We
also find that the dropouts are rarely produced in the simulations using the so-called
“two-component” magnetic turbulence model (Matthaeus et al., 1990). The result
questions the validity of this model in studying particle transport. In the first part
of Chapter 3 we study the acceleration of ions in the existence of turbulent mag-
netic fields. We use 3-D self-consistent hybrid simulations (kinetic ions and fluid
electrons) to investigate the acceleration of low-energy particles (often termed as
“injection problem”) at parallel shocks. We find that the accelerated particles al-
ways gain the first amount of energy by reflection and acceleration at the shock layer.
The protons can move off their original field lines in the 3-D electric and magnetic
fields. The results are consistent with the acceleration mechanism found in previous
1-D and 2-D simulations. In the second part of Chapter 3, we use a stochastic inte-
gration method to study diffusive shock acceleration in the existence of large-scale
magnetic variations. We show that the 1-D steady state solution of diffusive shock
acceleration can be significantly modified in this situation. The results suggest that
the observations of anomalous cosmic rays by Voyager spacecraft can be explained
by a 2-D shock that includes the large-scale magnetic field variations. In Chapter
4 we study electron acceleration at a shock passing into a turbulent magnetic field
by using a combination of hybrid simulations and test-particle electron simulations.
We find that the acceleration of electrons is greatly enhanced by including the ef-
fect of large-scale magnetic turbulence. Since the electrons mainly follow along the
magnetic lines of force, the large-scale braiding of field lines in space allows the
fast-moving electrons interacting with the shock front multiple times. Ripples in
the shock front occurring at various scales also contribute to the acceleration by
mirroring the electrons. Our calculation shows that this process favors electron ac-
celeration at perpendicular shocks. We discuss the application of this process in
interplanetary shocks and flare termination shocks. We also discuss the implication
of this study to solar energetic particles (SEPs) by comparing the acceleration of
electrons with that of protons. The intensity correlation of electrons and ions in SEP
events indicates that perpendicular or quasi-perpendicular shocks play an important
role in accelerating charged particles. In Chapter 5 we summarize the results of this
thesis and discuss possible future work. |
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| 2011 Top |
| 6. | Fan Guo, Shengtai Li, Hui Li, Joe Giacalone, and J. R. Jokipii, On the physics of supernova blast shock waves propagating in a turbulent density medium, Specify the Journal, 7, 106, 2011-09. [(8.92MB)] | |
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| 2010 Top |
| 5. | F. Guo, J. R. Jokipii, and J. Kota, Particle acceleration by collisionless shocks containing large-scale magnetic field variations, Astrophys. J., 725, 128-133, 2010-12. [(1.78MB)] | | Abstract. Diffusive shock acceleration at collisionless shocks is thought to be the source of many of the energetic particles observed in space. Large-scale spatial variations of the magnetic field has been shown to be important in understanding observations. The effects are complex, so here we consider a simple, illustrative model. Here, we solve numerically the Parker transport equation for a shock in the presence of large-scale sinusoidal magnetic-field variations. We demonstrate that the familiar planar-shock results can be significantly altered as a consequence of large-scale, meandering magnetic lines of force. Because perpendicular diffusion coefficient $\kappa_\perp$ is generally much smaller than parallel diffusion coefficient $\kappa_\parallel$, the energetic charged particles are trapped and preferentially accelerated along the shock front in the regions where the connection points of magnetic field lines intersecting the shock surface converge, and thus create the ``hot spots" of the accelerated particles. For the regions where the connection points separate from each other, the acceleration to high energies will be suppressed. Further, the particles diffuse away from the ``hot spot" regions and modify the spectra of downstream particle distribution. These features are qualitatively similar to the recent Voyager's observation in the Heliosheath. These results are potentially important for particle acceleration at shocks propagating in turbulent magnetized plasmas as well as those which contain large-scale nonplanar structures. Examples include anomalous cosmic rays accelerated by the solar wind termination shock, energetic particles observed in propagating heliospheric shocks, and galactic cosmic rays accelerated by supernova blast waves, etc. |
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| 4. | Fan Guo and Joe Giacalone, The effect of Large Scale Magnetic Turbulence on the Acceleration of Electrons by Perpendicular Collisionless Shocks, Astrophys. J., 715, 406-411, 2010-05. [(3.21MB)] | | Abstract. We study the physics of electron acceleration at collisionless shocks that move through a plasma containing large-scale magnetic fluctuations. We numerically integrate the trajectories of a large number of electrons, which are treated as test particles moving in the time-dependent electric and magnetic fields determined from two-dimensional hybrid simulations (kinetic ions and fluid electron). The large-scale magnetic fluctuations effect the electrons in a number of ways and lead to efficient and rapid energization at the shock front. Since the electrons mainly follow along magnetic lines of force, the large-scale braiding of field lines in space allows the fast-moving electrons to cross the shock front several times, leading to efficient acceleration. Ripples in the shock front occurring at various scales will also contribute to the acceleration by mirroring the electrons. Our calculation shows that this process favors electron acceleration at perpendicular shocks. The current study is also helpful in understanding the injection problem for electron acceleration by collisionless shocks. It is also shown that the spatial distribution of energetic electrons is similar to in situ observations. The process may be important to our understanding of energetic electrons in planetary bow shocks and interplanetary shocks, and explaining herringbone structures seen in some type II solar radio bursts. |
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| 2008 Top |
| 3. | GUO Fan(郭帆), LU Quan-Ming(陆全明), GUO Jun(郭俊) and WANG Shui(王水), Nonlinear Evolution of Lower-Hybrid Drift Instability in Harris Current Sheet, Chinese Phys. Lett., 25, 2725, 2008-07. [(2.27MB)] | | Abstract. http://adsabs.harvard.edu/abs/2008ChPhL..25.2725G |
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| 2006 Top |
| 2. | Lu, Q. M., F. Guo, and S. Wang, Magnetic spectral signatures in the terrestrial plasma depletion layer: Hybrid simulations, J. Geophys. Res., 111, A04207, doi:10.1029/2005JA011405., 2006-06. [(266KB)] | |
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| 2005 Top |
| 1. | Guo Fan, Lu Quanming, and Wang Shui, Excited low frequency waves by beam plasma in upstream of collisionless shock and their effect on dissipation of shock, Chinese J. Space Sci., 25(4), 248-253, 2005-09. [(457.5KB)] | |
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