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Mapping of magnetospheric electric fields from Cluster into ionospheric convection from SuperDARN using the Tsyganenko field model.Maphaha, Fulufhelo Kenneth. January 2010 (has links)
The data from Super Dual Auroral Radar Network (SuperDARN) radar and Cluster was
used to investigate whether the processes observed by SuperDARN radar in the highlatitude
ionosphere leads to the processes that take place in the magnetosphere. SuperDARN
is a global network of high-frequency coherent-scatter radars operating in the
northern and southern hemispheres and these radars are capable of observing backscatter
from ionospheric irregularities. The Tsyganenko T96 field model was used to map the
footprint of the Cluster spacecraft into the ionosphere. The SuperDARN radar data analysis
software was used to generate time series, fi eld of view and convection plots. The
data from Syowa South, Syowa East SuperDARN radars, Advance Composition Explorer
(ACE) and Cluster spacecrafts was used in this study. Two events were found and
discussed, an event on 09 August 2002 and another event on 30 September 2002. The
SuperDARN radar and Cluster data for these events were compared. When a change in
electric field was observed by Cluster, SuperDARN radar observed a change in the convection
flow. There was a time delay between the change in the electric fi eld and the change
in convection flow. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2010.
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Linear properties of the cross-field ion acoustic instability in a double plasma device.Dempers, Clemens Arnold. January 1990 (has links)
This thesis deals with the dependence of the linear spatial growth rate of the
cross-field ion acoustic instability on various plasma parameters. A kinetic
theory model, with elastic and inelastic ion-neutral collisions included, is
presented and used to conduct a numerical survey of the instability. The
growth rate is computed as a function of distance into the plasma, taking
into account the attenuation of the ion beam by charge exchange collisions.
Further calculations show the variation in growth rate as a function of the
following quantities: electron and ion beam temperature, electron density,
beam velocity, background ion temperature, magnetic field, the angle between
magnetic field direction and wave vector and the finite width of the plasma.
The instability was observed in a double plasma device where an ion beam
was passed through a background of stationary magnetized electrons. The
magnetic field was sufficiently weak to allow approximately rectilinear ion
motion. The growth rate of the wave was studied using interferometer techniques.
It was identified by the dispersion relation as the cross-field ion
acoustic wave propagating as the slow mode of the beam. It was found that
the background ions play an important role in determining the phase velocity.
Experimental data of the growth rate dependence on wave number,
beam velocity and magnetic field strength were found to be well described by
the theoretical model. The growth rate dependence of magnetic field direction
on plasma width was furthermore found to be in qualitative agreement
with the model. / Thesis (M.Sc.)-University of Natal, Durban, 1990.
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The association between VLF and ELF chorus emissions and electron precipitation.Musumpuka, Remmy. January 2009 (has links)
This is an investigation into the association between ionospheric absorption caused
by electron precipitation and ELF:3 Hz-3 kHz and VLF:3 kHz-30 kHz, chorus.
Ionospheric absorption was measured using the chain of riometers in Finland and
related to chorus events recorded simultaneously at SANAE (L=4.2), Antarctica. The
displacement in longitude of the Finnish riometers from SANAE’s conjugate point made
it impossible to establish a clear relationship between chorus and riometer absorption.
The diurnal variation of chorus has been established for the years 2002, 2004 and
2005 and it is shown that chorus can occur at any Local Time(LT) but has a well
defined maximum probability of occurring between 0800 LT to 0900 LT.
To study the occurrence of chorus automatically we have developed an Index of
ELF/VLF activity which enables us to identify chorus and distinguish it from other
emissions such as hiss and whistlers.
This index of VLF Activity was established by computing the standard deviation
of the VLF signal amplitude and it has been observed that the index is larger for the
chorus signature as opposed to the hiss which is low and does not vary widely due to
the hiss’ steady signal. This index is called ASD index of “VLF Activity”. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2009.
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Analysing transient effects in the ionosphere using narrowband VLF data.Bremner, Sherry. January 2009 (has links)
Very Low Frequency (VLF) radio waves propagate within the Earth-ionosphere
waveguide with very little attenuation. Modifications of the waveguide geometry affect the propagation
conditions, and hence, the amplitude and phase of VLF signals. Changes in the ionosphere,
such as the presence of the D-region during the day, or the precipitation of energetic particles,
are the main causes of this modification. Using narrowband receivers monitoring remote VLF
transmitters, the amplitude and phase of these signals are recorded. A multivariate data analysis
technique, Principal Component Analysis (PCA), is applied to the data in order to determine parameters
such as seasonal and diurnal changes which affect the variation of these signals. Data was
then analysed for effects from extragalactic gamma ray bursts, terrestrial gamma ray flashes and
solar flares. Only X-rays from solar flares were shown to have an appreciable affect on ionospheric
propagation. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2009.
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Measure-preserving and time-reversible integration algorithms for constant temperature molecular dynamics.Obaga, Emmanuel Omboga. January 2011 (has links)
This thesis concerns the formulation of integration algorithms for non-Hamiltonian
molecular dynamics simulation at constant temperature. In particular, the
constant temperature dynamics of the Nosé-Hoover, Nosé-Hoover chain, and
Bulgac-Kusnezov thermostats are studied. In all cases, the equilibrium statistical
mechanics and the integration algorithms have been formulated using
non-Hamiltonian brackets in phase space. A systematic approach has been
followed in deriving numerically stable and efficient algorithms. Starting from
a set of equations of motion, time-reversible algorithms have been formulated
through the time-symmetric Trotter factorization of the Liouville propagator.
Such a time-symmetric factorization can be combined with the underlying non-
Hamiltonian bracket-structure of the Liouville operator, preserving the measure
of phase space. In this latter case, algorithms that are both time-reversible
and measure-preserving can be obtained. Constant temperature simulations of
low-dimensional harmonic systems have been performed in order to illustrate
the accuracy and the efficiency of the algorithms presented in this thesis. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.
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A study of the 90Zr(n,d)89Y reaction.Bawa, Ahmed Cassim. January 1986 (has links)
A study has been made of the 90 Zr(n,d) 89 Y reactlion at an
incident neutron energy of 22 MeV. The experimental
aspect of the study was performed at the Van der Graaf
facility at the National Accelerator Centre, using a
particle spectrometer developed by K Bharuth-Ram and
W R McMurray for the study of neutron-induced charged-particle
emissions. The spectrometer, which consists of
a telescope of three multiwire proportional counters and
a curved plastic scintillator, permits the simultaneous
accumulation of data over an angular range of 80°. Solid-angle-
calculations have been performed to correct for the
effect of the geometry of the system on the angular
distribution of the cross-sections. A review has been made
of the shell model of the nucleus, the optical potential model
and the distorted waves method (or DWBA) for the analysis
of direct nuclear reactions. A distorted-waves method
analysis of the reaction is performed with the code DWUCK 4
and the resulting angular distribution of the various
cross-sections are compared with the experimentally obtained
data. ThIs comparison produces spectroscopic factors which
are used to perform some analysis of the nuclear structure
of the 90 Zr nucleus. / Thesis (M.Sc.)-University of Durban-Westville, 1986.
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Simple qubit systems in bosonic baths.Pumulo, Nathan. 03 October 2013 (has links)
The study is focused on the thermal entanglement of spin chains. Chains consisting of two
and three qubits are considered. These chains are considered open because they are coupled to
bosonic baths at different temperatures. The baths represent the environment. The dynamics
of these open systems are examined as are the effects of different parameters - such as bath
temperature - on the entanglement of the spins. The measure of entanglement used in these
cases is the concurrence. Comparisons are made between a model that assumes a strong spinspin
interaction and one that assumes a weak one. In all these cases, analytical solutions for
the system dynamics are presented. It is found that at large times, all systems converge to
a state that depends only on bath temperature. It is also found that increases in bath temperatures
diminish the entanglement between spins and that at high enough temperatures the
entanglement vanishes altogether. The time and temperature dependence of the entanglement
is different for the two models that are studied. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2010.
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Sugar crystal size characterization using digital image processing.Argaw, Getachew Abebe. January 2007 (has links)
The measurement of the crystal size distribution is a key prerequisite in optimising the growth of sugar crystals in crystalisation pans or for quality control of the final product. Traditionally, crystal size measurements are carried out by inspection or using mechanical sieves. Apart from being time consuming, these techniques can only provide limited quantitative information. For this reason, a more quantitative automatic system is required. In our project, software routines for the automated measurement of crystal size using classical image analysis techniques were developed. A digital imaging technique involves automatically analyzing a captured image of a representative sample of ~ 100 crystals for the automated measurement of crystal size has been developed. The main problem of crystals size measurements using image processing is the lack of an efficient algorithm to identify and separate overlapping and touching crystals which otherwise compromise the accuracy of size measurement. This problem of overlapping and touching crystals was addressed in two ways. First, 5 algorithms which identify and separate overlapping and touching crystals, using mathematical morphology as a tool, were evaluated. The accuracy of the algorithms depends on the technique used to mark every crystal in the image. Secondly, another algorithm which used convexity measures of the crystals based on area and perimeter, to identify and reject overlapping and touching crystals, have been developed. Finally, the two crystal sizing algorithms, the one applies ultimate erosion followed by a distance transformation and the second uses convexity measures to identify overlapping crystals, were compared with well established mechanical sieving technique. Using samples obtained from a sugar refinery, the parameters of interest, including mean aperture (MA) and coefficient of variance (CV), were calculated and compared with those obtained from the sieving method. The imaging technique is faster, more reliable than sieving and can be used to measure the full crystal size distributions of both massecuite and dry product. / Thesis (PhD)-University of KwaZulu-Natal, Durban, 2007.
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Computational studies of bond-site percolation.Nduwayo, Léonard. January 2007 (has links)
Percolation theory enters in various areas of research including critical phenomena
and phase transitions. Bond-site percolation is a generalization of pure percolation
motivated by the fact that bond-site is close to many physical realities. This work
relies on a numerical study of percolation in lattices. A lattice is a regular pattern
of sites also known as nodes or vertices connected by bonds also known as links
or edges. Sites may be occupied or unoccupied, where the concentration ps is the fraction of occupied sites. The quantity pb is the fraction of open bonds. A cluster
is a set of occupied sites connected by opened bonds.
The bond-site percolation problem is formulated as follows: we consider an infinite
lattice whose sites and bonds are at random or correlated and either allowed or
forbidden with probabilities ps and pb that any site and any bond are occupied and
open respectively. If those probabilities are small, there appears a sprinkling of isolated
clusters each consisting of occupied sites connected by open bonds surrounded
by numerous unoccupied sites. As the probabilities increase, reaching critical values
above which there is an infinitely large cluster, then percolation is taking place. This
means that one can cross the entire lattice by going successively from one occupied
site connected by a opened bond to a neighbouring occupied site. The sudden onset
of a spanning cluster happens at particular values of ps and pb, called the critical concentrations. Quantities related to cluster configuration (mean cluster and correlation length) and
individual cluster structure (size and gyration radius of clusters ) are determined
and compared for different models. In our studies, the Monte Carlo approach is applied
while some authors used series expansion and renormalization group methods.
The contribution of this work is the application of models in which the probability of
opening a bond depends on the occupancy of sites. Compared with models in which
probabilities of opening bonds are uncorrelated with the occupancy of sites, in the
suppressed bond-site percolation, the higher site occupancy is needed to reach percolation.
The approach of suppressed bond-site percolation is extended by considering
direction of percolation along bonds (directed suppressed bond-site percolation).
Fundamental results for models of suppressed bond-site percolation and directed
suppressed bond-site percolation are the numerical determination of phase boundary between the percolating and non-percolating regions. Also, it appears that the spanning cluster around critical concentration is independent on models. This is an intrinsic property of a system. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2007.
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Synthesis, structural and magnetic properties of bulk and nanosized (Zn, Cd, Cu)0.5Ni0.5Fe2o4 and NiFe204 ferritesJanuary 2007 (has links)
We present a study of the synthesis, structural and magnetic properties of
bulk and nanosized (Zn, Cd, Cu)0:5Ni0:5Fe2O4 and NiFe2O4 compounds. The
e®ects of electronic con¯guration and atomic sizes of Zn, Cd, Cu and Ni on
the magnetic properties of the ferrites are the primary focus of the study.
Di®erent synthesis routes, preparation conditions and how they a®ect single
phase formation are explored.
The synthesis was undertaken by solid{state reaction, combustion, hydrothermal
and glycothermal techniques. The structure determination was by Xray
di®raction. The magnetic measurements were performed using MÄossbauer
spectroscopy (from 79 K to about 850 K) and a vibrating sample magnetometer
(at about 300 K). The bulk densities of the sintered pellets were deduced
by Archimedes principle. The bulk oxides were produced by solid{state reaction
and combustion techniques. Fine powders with grain sizes of about 10
nm were produced from bulk compounds by a Retsch planetary ball mill and
by the hydrothermal and glycothermal processes. The e®ects of the applied
pressure used to make pellets (related to green density of the raw pellets) and
the sintering temperature on the properties were investigated.
An anomalous variation of bulk densities of (Zn, Cd)0:5Ni0:5Fe2O4 oxides
with increase in pelletizing pressure was observed which appears to suggest evidence
for trapped porosity. Di®erent states of pelletizing the samples appear
to be related to a systematic change of the hyper¯ne ¯eld distributions derived
from the MÄossbauer spectra. The temperature dependence of the magnetic hyper
¯ne ¯elds at tetrahedral (A) and octahedral (B) sites were observed to vary
with temperature according to the equations Bhf (T) = Bhf (0)[1 ¡ (T=TC)n]¯n
where n = 1 (based on the Landau{Ginzburg theory) and n = 2 (based on the
Stoner theory). The equation Bhf (T) = Bhf (0)[1¡(T=TC)2]¯2 appears to ¯t the
hyper¯ne ¯eld data over a wider temperature range. The Zn{ and Cd{based
oxides were found to be ferrimagnetic with Curie temperature TC = 548 § 3 K (measured by zero velocity technique). The Cu{based compound exhibited antiferromagnetic
behavior with a magnetic transition temperature of 825 § 3 K.
The di®erence in behavior between Zn{, Cd{ and Cu{based compounds is due
to di®erence in electronic con¯guration and atomic or ionic sizes. The stronger
magnetic coupling between spins in the Cu{based sample can be explained by
the presence of RKKY interactions in addition to superexchange interactions.
The larger ionic size for Cd appears to favour smaller grain sizes in Cd{based
oxides. An anomalous increase in TC is obtained in the Zn0:5Ni0:5Fe2O4 compound
with reduction in grain size. This increase in TC is attributed to a
distribution of Zn ions on both A and B sites.
The MÄossbauer spectra of the milled nanosized samples show a combination
of ferrimagnetic and paramagnetic behavior. The coercive ¯eld (HC) at room
temperature was found to increase with reduction in grain size (G) according to
the equation HC = am+bm=G, which is consistent with multidomain particles.
With further reduction in grain sizes, the coercive ¯eld reduced according to the
equation HC = as ¡bs=G2. This equation is associated with the onset of single
domain particles. The samples produced by hydrothermal and glycothermal
processes show evidence of transformation from single domain to multidomain
structure with increasing sintering temperature.
The ease of single{phase formation in the compounds studied is shown to
depend on the technique used to prepare the samples. Single phase formation of
the spinel structure was easier to achieve in samples prepared by wet chemical
methods because lower sintering temperatures (T < 1000 oC) were required. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2007.
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