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Cosmic ray modulation processes in the heliosphere / Vos E.E.Vos, Etienne Eben January 2011 (has links)
The solar minimum of 2009 has been identified as an exceptional event with regard to
cosmic ray (CR)modulation, since conditions in the heliosphere have reached unprecedented
quiet levels. This unique minimum has been observed by the Earth–orbiting
satellite, PAMELA, launched in June, 2006, from which vast sets of accurate proton
and electron preliminary observations have been made available. These simultaneous
measurements from PAMELA provide the ideal opportunity to conduct an in–depth
study of CR modulation, in particular charge–sign dependent modulation. In utilizing
this opportunity, a three–dimensional, steady–state modulation model was used to reproduce
a selection of consecutive PAMELA proton and electron spectra from 2006 to
2009. Thiswas done by assuming full drifts and simplified diffusion coefficients, where
the rigidity dependence and absolute value of themean free paths for protons and electrons
were sequentially adjusted below 3 GV and 300 MV, respectively. Care has
been taken in calculating yearly–averaged current–sheet tilt angle and magnetic field
values that correspond to the PAMELA spectra. Following this study where the numerical
model was used to investigate the individual effects resulting from changes in
the tilt angle, diffusion coefficients, and global drifts, it was found that all these modulation
processes played significant roles in contributing to the total increase in CR
intensities from 2006 to 2009, as was observed by PAMELA. Furthermore, the effect
that drifts has on oppositely charged particles was also evident from the difference
between the peak–shaped time profiles of protons and the flatter time profiles of electrons,
as is expected for an A < 0 polarity cycle. Since protons, which drift into the
heliosphere along the heliospheric current–sheet, haven’t yet reached maximum intensity
levels by 2008, their intensities increased notably more than electrons toward the
end of 2009. The time and energy dependence of the electron to proton ratios were
also studied in order to further illustrate and quantify the effect of drifts during this
remarkable solar minimum period. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2012.
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Cosmic ray modulation processes in the heliosphere / Vos E.E.Vos, Etienne Eben January 2011 (has links)
The solar minimum of 2009 has been identified as an exceptional event with regard to
cosmic ray (CR)modulation, since conditions in the heliosphere have reached unprecedented
quiet levels. This unique minimum has been observed by the Earth–orbiting
satellite, PAMELA, launched in June, 2006, from which vast sets of accurate proton
and electron preliminary observations have been made available. These simultaneous
measurements from PAMELA provide the ideal opportunity to conduct an in–depth
study of CR modulation, in particular charge–sign dependent modulation. In utilizing
this opportunity, a three–dimensional, steady–state modulation model was used to reproduce
a selection of consecutive PAMELA proton and electron spectra from 2006 to
2009. Thiswas done by assuming full drifts and simplified diffusion coefficients, where
the rigidity dependence and absolute value of themean free paths for protons and electrons
were sequentially adjusted below 3 GV and 300 MV, respectively. Care has
been taken in calculating yearly–averaged current–sheet tilt angle and magnetic field
values that correspond to the PAMELA spectra. Following this study where the numerical
model was used to investigate the individual effects resulting from changes in
the tilt angle, diffusion coefficients, and global drifts, it was found that all these modulation
processes played significant roles in contributing to the total increase in CR
intensities from 2006 to 2009, as was observed by PAMELA. Furthermore, the effect
that drifts has on oppositely charged particles was also evident from the difference
between the peak–shaped time profiles of protons and the flatter time profiles of electrons,
as is expected for an A < 0 polarity cycle. Since protons, which drift into the
heliosphere along the heliospheric current–sheet, haven’t yet reached maximum intensity
levels by 2008, their intensities increased notably more than electrons toward the
end of 2009. The time and energy dependence of the electron to proton ratios were
also studied in order to further illustrate and quantify the effect of drifts during this
remarkable solar minimum period. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2012.
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Modelling the evolution of pulsar wind nebulae / Michael Johannes VorsterVorster, Michael Johannes January 2014 (has links)
This study focusses on modelling important aspects of the evolution of pulsar wind nebulae
using two different approaches. The first uses a hydrodynamic model to simulate the morphological
evolution of a spherically-symmetric composite supernova remnant that is expanding
into a homogeneous interstellar medium. In order to extend this model, a magnetic field is
included in a kinematic fashion, implying that the reaction of the fluid on the magnetic field
is taken into account, while neglecting any counter-reaction of the field on the fluid. This approach
is valid provided that the ratio of electromagnetic to particle energy in the nebula is
small, or equivalently, for a large plasma β environment. This model therefore allows one to
not only calculate the evolution of the convection velocity but also, for example, the evolution
of the average magnetic field.
The second part of this study focusses on calculating the evolution of the energy spectra of
the particles in the nebula using a number of particle evolution models. The first of these is
a spatially independent temporal evolution model, similar to the models that can be found
in the literature. While spatially independent models are useful, a large part of this study
is devoted to developing spatially dependent models based on the Fokker-Planck transport
equation. Two such models are developed, the first being a spherically-symmetric model that
includes the processes of convection, diffusion, adiabatic losses, as well as the non-thermal
energy loss processes of synchrotron radiation and inverse Compton scattering. As the magnetic
field geometry can lead to the additional transport process of drift, the previous model is
extended to an axisymmetric geometry, thereby allowing one to also include this process. / PhD (Space Physics), North-West University, Potchefstroom Campus, 2014
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Modelling of cosmic ray modulation in the heliosphere by stochastic processes / Roelf du Toit StraussStrauss, Roelf du Toit January 2013 (has links)
The transport of cosmic rays in the heliosphere is studied by making use of a newly developed
modulation model. This model employes stochastic differential equations to numerically solve
the relevant transport equation, making use of this approach’s numerical advantages as well
as the opportunity to extract additional information regarding cosmic ray transport and the
processes responsible for it. The propagation times and energy losses of galactic electrons
and protons are calculated for different drift cycles. It is confirmed that protons and electrons
lose the same amount of rigidity when they experience the same transport processes. These
particles spend more time in the heliosphere, and also lose more energy, in the drift cycle
where they drift towards Earth mainly along the heliospheric current sheet. The propagation
times of galactic protons from the heliopause to Earth are calculated for increasing heliospheric
tilt angles and it is found that current sheet drift becomes less effective with increasing solar
activity. Comparing calculated propagation times of Jovian electrons with observations, the
transport parameters are constrained to find that 50% of 6 MeV electrons measured at Earth
are of Jovian origin. Charge-sign dependent modulation is modelled by simulating the proton
to anti-proton ratio at Earth and comparing the results to recent PAMELA observations.
A hybrid cosmic ray modulation model is constructed by coupling the numerical modulation
model to the heliospheric environment as simulated by a magneto-hydrodynamic model. Using
this model, it is shown that cosmic ray modulation persists beyond the heliopause. The
level of modulation in this region is found to exhibit solar cycle related changes and, more
importantly, is independent of the magnitude of the individual diffusion coefficients, but is
rather determined by the ratio of parallel to perpendicular diffusion. / PhD (Space Physics), North-West University, Potchefstroom Campus, 2013
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Modelling the evolution of pulsar wind nebulae / Michael Johannes VorsterVorster, Michael Johannes January 2014 (has links)
This study focusses on modelling important aspects of the evolution of pulsar wind nebulae
using two different approaches. The first uses a hydrodynamic model to simulate the morphological
evolution of a spherically-symmetric composite supernova remnant that is expanding
into a homogeneous interstellar medium. In order to extend this model, a magnetic field is
included in a kinematic fashion, implying that the reaction of the fluid on the magnetic field
is taken into account, while neglecting any counter-reaction of the field on the fluid. This approach
is valid provided that the ratio of electromagnetic to particle energy in the nebula is
small, or equivalently, for a large plasma β environment. This model therefore allows one to
not only calculate the evolution of the convection velocity but also, for example, the evolution
of the average magnetic field.
The second part of this study focusses on calculating the evolution of the energy spectra of
the particles in the nebula using a number of particle evolution models. The first of these is
a spatially independent temporal evolution model, similar to the models that can be found
in the literature. While spatially independent models are useful, a large part of this study
is devoted to developing spatially dependent models based on the Fokker-Planck transport
equation. Two such models are developed, the first being a spherically-symmetric model that
includes the processes of convection, diffusion, adiabatic losses, as well as the non-thermal
energy loss processes of synchrotron radiation and inverse Compton scattering. As the magnetic
field geometry can lead to the additional transport process of drift, the previous model is
extended to an axisymmetric geometry, thereby allowing one to also include this process. / PhD (Space Physics), North-West University, Potchefstroom Campus, 2014
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Modelling of cosmic ray modulation in the heliosphere by stochastic processes / Roelf du Toit StraussStrauss, Roelf du Toit January 2013 (has links)
The transport of cosmic rays in the heliosphere is studied by making use of a newly developed
modulation model. This model employes stochastic differential equations to numerically solve
the relevant transport equation, making use of this approach’s numerical advantages as well
as the opportunity to extract additional information regarding cosmic ray transport and the
processes responsible for it. The propagation times and energy losses of galactic electrons
and protons are calculated for different drift cycles. It is confirmed that protons and electrons
lose the same amount of rigidity when they experience the same transport processes. These
particles spend more time in the heliosphere, and also lose more energy, in the drift cycle
where they drift towards Earth mainly along the heliospheric current sheet. The propagation
times of galactic protons from the heliopause to Earth are calculated for increasing heliospheric
tilt angles and it is found that current sheet drift becomes less effective with increasing solar
activity. Comparing calculated propagation times of Jovian electrons with observations, the
transport parameters are constrained to find that 50% of 6 MeV electrons measured at Earth
are of Jovian origin. Charge-sign dependent modulation is modelled by simulating the proton
to anti-proton ratio at Earth and comparing the results to recent PAMELA observations.
A hybrid cosmic ray modulation model is constructed by coupling the numerical modulation
model to the heliospheric environment as simulated by a magneto-hydrodynamic model. Using
this model, it is shown that cosmic ray modulation persists beyond the heliopause. The
level of modulation in this region is found to exhibit solar cycle related changes and, more
importantly, is independent of the magnitude of the individual diffusion coefficients, but is
rather determined by the ratio of parallel to perpendicular diffusion. / PhD (Space Physics), North-West University, Potchefstroom Campus, 2013
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