Browsing by Author "Chan, Anthony Arthur"

Now showing 1 - 8 of 8
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    A model of bounce-averaged relativistic protons with emphasis on the March 1991 magnetospheric compression
    (1997) Braaten, Karsten Eric; Chan, Anthony Arthur
    We have derived relativistically correct gyro- and bounce-averaged Hamiltonian equations of motion to describe the motions of non-isotropic proton distributions in the Earth's inner and middle magnetosphere. We have focused on the case of equatorially-mirroring protons, and we have modified the Magnetospheric Specification Model (MSM) to trace these particles. We call the new particle simulation the Radiation Belt Test Code (RBTC). We have modeled the March 1991 magnetic storm, which was an extremely strong magnetospheric compression in which protons were energized to 1 to 100 MeV on time scales of a few minutes. We have compared our results with CRESS data collected during the event, and with the simulation results of Hudson et al. (3). We see a significant flux increase, but it is not as large as the increase observed by CRRES. We conclude that our model correctly describes the gross features of high-energy magnetospheric protons, but that the present algorithm of the MSM is too computationally intensive to model these equations in a reasonable time, especially for the highest energy particles that we were interested in. Suggestions for improvements and alternative methods are suggested.
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    Bounce-resonant ion interaction with hydromagnetic waves
    (1997) Klamczynski, Karen M.; Chan, Anthony Arthur
    Wave-particle interactions between hydromagnetic waves and bounce-resonant ring current ions may cause ions precipitation observed during geomagnetic storms. Uncovering mechanisms of ion loss is important to understanding the recovery phases of these storms. A computer model was developed to numerically solve Hamiltonian guiding center equations of motion for a test particle in a three-dimensional time-dependent electromagnetic field model. The background magnetic field is a simple dipole and hydromagnetic waves are modeled by time-dependent electromagnetic perturbations. Specifically, a single compressional Pc 5 wave well below the ion-cyclotron frequency was used in simulations; the amplitude of the perturbation has been varied up to the maximum observed. Bounce-resonant ring current ions near L = 3 undergo pitch angle scattering and ions are moved along the field line toward the loss cone. Wave perturbations which are superpositions of several different wave modes are also considered. Although bounce-resonant interactions alone cannot account for the observed precipitation, they may be an important part of a multi-step precipitation process.
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    Dynamics of relativistic electrons during magnetic storms
    (1999) Kim, Hee-Jeong; Chan, Anthony Arthur
    Our society increasingly relies on spacecraft operations in the Earth's inner magnetosphere, particularly for communications. Long-duration high-intensity fluxes of relativistic electron are hazardous to spacecraft operational systems. Adverse effects of these energetic electrons on spacecraft has resulted in significant public interest and renewed efforts to advance our understanding and predictive capabilities of relativistic electron flux variations in the inner magnetosphere. The flux variations are especially dynamic during geomagnetically disturbed times. It is often observed that fluxes of relativistic electrons in the Earth's inner magnetosphere decrease by orders of magnitude, followed by a substantial enhancement of up to two orders of magnitude above the pre-storm levels. This work primarily focuses on the investigation of two physical processes for the relativistic electron flux variations: The fully-adiabatic effect and the delayed substorm injection mechanism. We simulate fully-adiabatic variations of electron fluxes for the special case of equatorially mirroring electrons using Rice magnetic field models and a quiet-time electron flux model. The storm-time electron fluxes can be obtained by fully-adiabatically evolving pre-storm fluxes using Liouville's theorem. Our study shows that the fully-adiabatic effect can cause a flux decrease of up to almost two orders of magnitude for Dst = -100 nT. We also simulate acceleration and injection of plasma sheet electrons during substorm dipolarization using a 3-D MHD field model. The test particle simulation shows that tens-of-keV plasma sheet electrons may be accelerated up to relativistic energies during a rapid substorm injection followed by a slow radial diffusion to the inner magnetosphere. Comparison with measurements shows that the mechanisms may contribute significantly to the observed flux variations.
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    Hamiltonian theory and stochastic simulation methods for radiation belt dynamics
    (2009) Tao, Xin; Chan, Anthony Arthur
    This thesis describes theoretical studies of adiabatic motion of relativistic charged particles in the radiation belts and numerical modeling of multi-dimensional diffusion due to interactions between electrons and plasma waves. A general Hamiltonian theory for the adiabatic motion of relativistic charged particles confined by slowly-varying background electromagnetic fields is presented based on a unified Lie-transform perturbation analysis in extended phase space (which includes energy and time as independent coordinates) for all three adiabatic invariants. First, the guiding-center equations of motion for a relativistic particle are derived from the particle Lagrangian. Covariant aspects of the resulting relativistic guiding-center equations of motion are discussed and contrasted with previous works. Next, the second and third invariants for the bounce motion and drift motion, respectively, are obtained by successively removing the bounce phase and the drift phase from the guiding-center Lagrangian. First-order corrections to the second and third adiabatic invariants for a relativistic particle are derived. These results simplify and generalize previous works to all three adiabatic motions of relativistic magnetically-trapped particles. Interactions with small amplitude plasma waves are described using quasi-linear diffusion theory, and we note that in previous work numerical problems arise when solving the resulting multi-dimensional diffusion equations using standard finite difference methods. In this thesis we introduce two new methods based on stochastic differential equation theory to solve multi-dimensional radiation belt diffusion equations. We use our new codes to assess the importance of cross diffusion, which is often ignored in previous work, and effects of ignoring oblique waves, which are omitted in the parallel-propagation approximation of calculating diffusion coefficients. Using established wave models we show that ignoring cross diffusion or oblique waves may produce large errors at high energies. Results of this work are useful for understanding radiation belt dynamics, which is crucial for predictability of radiation in space.
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    Magnetospheric Model Performance During Conjugate Aurora
    (2014-04-23) Longley, William; Reiff, Patricia H.; Geurts, Frank; Chan, Anthony Arthur
    At 16:40 UT on August 17th, 2001, the IMAGE satellite was in position to view an auroral storm in the Northern Hemisphere, while the POLAR satellite was simultaneously in position to view the storm in the Southern Hemisphere. For many low-latitude auroras, the precipitation maps along field lines from the Southern Hemisphere to the Northern Hemisphere. However, in this case, the IMF had a very strong dawn-dusk component which has been shown to make the polar cap shift towards the dusk in one hemisphere and towards the dawn in the other, but this has not yet been confirmed by simultaneous auroral imaging. Using the satellite images in the 130 nm to 160 nm wavelength range, we have been able to identify the Polar Cap Boundary in both hemispheres throughout the event, and calculated the Dawn-Dusk Offset, ∆L, in both hemispheres. We then found correlations of 0.90 in the Northern Hemisphere and 0.83 in the Southern Hemisphere between ∆L, ranging from 4 to 12 degrees during the event, and By, ranging from 20 to 32 nT during the event. ∆L also correlated well against IMF Clock Angle (ϴC) and the Epsilon parameter (ϵ=vB2sin[ϴC/2]) when using specific time averages of these parameters. The same methods are then applied to the compute Polar Cap Boundaries in the BATSRUS, OpenGGCM, and LFM-MIX models that were run to simulate the event. We find that none of the models accurately describe the observed open-closed field line boundary during this event, with BATSRUS tending to produce boundaries that are too ideal and symmetric, OpenGGCM producing boundaries that are highly distorted and random, and the LFM-MIX model always yielding low correlations between ∆L and the various solar wind parameters.
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    Measurement of plasma parameters in the exhaust of a magnetoplasma rocket by gridded energy analyzer and emissive Langmuir probe
    (2002) Glover, Timothy Ward; Chan, Anthony Arthur
    The 10 kilowatt prototype of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) engine, abbreviated as VX-10, is designed to eject plasma at exhaust velocities of tens of kilometers per second. In this device, energy is imparted to the plasma ions by two mechanisms: ion cyclotron resonant heating (ICRH), and acceleration in an ambipolar electric field. Measurements from two different electrostatic probes are combined to determine how much each mechanism contributes to the total ion energy. The first probe is a gridded retarding potential analyzer (RPA) that incorporates a multi-channel collimator to obtain precise measurement of the ion and electron parallel energy distributions. The second is an emissive Langmuir probe that measures the DC and RF components of the plasma potential. The plasma potential obtained from the emitting probe allows calculation of the parallel velocity distribution once the parallel energy distribution is obtained from the energy analyzer data. Biasing the RPA housing is shown to minimize the plasma perturbation, as monitored by an auxiliary probe. When this minimization is done, the RPA measurements become compatible with the emissive probe's measurement of plasma potential. The collimated RPA and emissive probe have been used to examine the effects of a double dual half-turn (DDHT) antenna encircling the plasma. When power at the ion cyclotron frequency is applied, changes are seen in the saturation current and mean ion energy of the collimated RPA characteristic. The evolution of these changes as the RPA is moved downstream from the antenna is interpreted as firm evidence of ion cyclotron heating, albeit at absorbed energies of less than 1 electronvolt per ion. The emissive probe shows that, within experimental error, all of the increased ion energy is accounted for by an increase in the plasma potential that occurs when the ICRF power is applied. The combined RPA and emissive probe data also show that there is a jet of flowing plasma in the VX-10 when operated with the helicon source alone but that the signal from this jet is overwhelmed by a rapidly growing stationary plasma within the first second of the discharge.
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    Simulation of dynamics of radiation belt electrons during geomagnetic storms driven by high speed solar wind streams
    (2007) Yu, Bin; Chan, Anthony Arthur
    Satellite observations have shown that fluxes of relativistic electrons in the earth's radiation belts can vary by orders of magnitude during periods of high solar activity. Understanding the dynamic behavior of these particles is very important because they can disrupt wireless communication, impair space exploration and affect GPS navigation. We use two numerical methods to simulate the variations of energetic particles in the radiation belts. First, we develop a radial diffusion model with time-dependent boundary conditions and a Kp-dependent electron lifetime model. Using this model, we simulate a series of high-speed-stream declining-phase magnetic storm events. The results are consistent with spacecraft observations and show that radial diffusion can propagate the enhancement of phase space density from the outer boundary into the center of the outer radiation belt. The second part of the work adapts Nunn's Vlasov Hybrid Simulation method to an existing MHD-Particle simulation code, resulting in an efficient new method to calculate phase space density of energetic particles. We use the 1995 January storm event as a test case. Good agreement is obtained between the simulation results and measured phase space densities for this event. Simulating the dynamics of the radiation belts is one important part of global space weather modeling. The advance in radiation belt modeling can help us to better understand the physics behind these interesting and important phenomena.
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    Simulation of radiation belt electron diffusion
    (2007) Fei, Yue; Chan, Anthony Arthur
    This thesis presents theoretical and numerical studies of the radial diffusion of relativistic radiation belt electrons. The research has been focused particularly on the radiation belt phase space density profile, and radial diffusion due to particle drift resonance with ULF waves. Observations have shown a strong connection between magnetospheric ULF oscillations and electron flux enhancements. I investigate radial diffusion coefficients based on theoretical analysis of particle diffusion in ULF perturbation electric and magnetic fields. The analytical diffusion coefficients consist of two terms: a symmetric term and an asymmetric term. The symmetric term agrees with earlier works, and the asymmetric terms are new. Both terms show good agreement with numerical test particle simulations. The asymmetric terms have higher L dependence, which indicates they might be more important at higher L-shells or at times when the magnetospheric field is highly asymmetric. A numerical radial diffusion model has been developed which can take into account: dynamic boundary locations and values, plus effects of losses and sources. Several test cases are considered to study the effects of different diffusion coefficients, internal sources, external sources, and loss. A method of converting observational particle flux to phase space density is also presented. Identifying the source and loss processes using observational data is currently one of the key issues for understanding and modeling radiation belt dynamics. We present a new measurement technique which utilizes two GOES satellites located at different local times to calculate the radial gradient of phase space density at geostationary locations. The result shows positive gradient at geomagnetic quiet periods. To further study the high energy electron transport, especially the ULF related acceleration during storm times, I use the numerical radial diffusion model for the September 24-26, 1998 storm and compare the results with an MHD test particle simulation. The diffusion result using ULF-wave diffusion coefficients and a time-dependent outer-boundary condition can reproduce the main features of the MHD-particle results quite well. Using wave driven diffusion coefficients gives better results than using power law or Kp-dependent diffusion coefficients.