Spelling suggestions: "subject:"galactic electrons"" "subject:"alactic electrons""
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Modelling of galactic and jovian electrons in the heliosphere / Daniel M. MoeketsiMoeketsi, Daniel Mojalefa January 2004 (has links)
A three-dimensional (3D) steady-state electron modulation model based on Parker (1965) transport
equation is applied to study the modelling of – 7 MeV galactic and Jovian electrons in the inner
heliosphere. The latter is produced within Jupiter's magnetosphere which is situated at - 5 AU in the
ecliptic plane. The heliospheric propagation of these particles is mainly described by the heliospheric
diffusion tensor. Some elements of the tensor, such as the diffusion coefficient in the azimuthal direction,
which were neglected in the previous two-dimensional modulation studies are investigated to account for
the three-dimensional transport of Jovian electrons. Different anisotropic solar wind speed profiles that
could represent solar minimum conditions were modelled and their effects were illustrated by computing
the distribution of 7 MeV Jovian electrons in the equatorial regions. In particular, the electron intensity
time-profile along the Ulysses spacecraft trajectory was calculated for these speed profiles and compared
to the 3-10 MeV electron flux observed by the Kiel Electron Telescope (KET) on board the Ulysses
spacecraft from launch (1990) up to end of its first out-of-ecliptic orbit (2000). It was found that the
model solution computed with the solar wind profile previously assumed for typical solar minimum
conditions produced good compatibility with observations up to 1998. After 1998 all model solutions
deviated completely from the observations. In this study, as a further attempt to model KET observations
more realistically, a new relation is established between the latitudinal dependence of the solar wind
speed and the perpendicular polar diffusion. Based on this relation, a transition of an average solar wind
speed from solar minimum conditions to intermediate solar activity and to solar maximum conditions
was modelled based on the assumption of the time-evolution of large polar coronal holes and were
correlated to different scenarios of the enhancement of perpendicular polar diffusion. Effects of these
scenarios were illustrated, as a series of steady-state solutions, on the computed 7 MeV Jovian and
galactic electrons in comparison with the 3-10 MeV electron observed by the KET instrument from the
period 1998 up to the end of 2003. Subsequent effects of these scenarios were also shown on electron
modulation in general. It was found that this approach improved modelling of the post-1998 discrepancy
between the model and KET observations but it also suggested the need for a time-dependent 3D
electron modulation model to describe modulation during moderate to extreme solar maximum
conditions. / Thesis (M.Sc.)--North-West University, Potchefstroom Campus, 2004.
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Modelling of galactic and jovian electrons in the heliosphere / Daniel M. MoeketsiMoeketsi, Daniel Mojalefa January 2004 (has links)
A three-dimensional (3D) steady-state electron modulation model based on Parker (1965) transport
equation is applied to study the modelling of – 7 MeV galactic and Jovian electrons in the inner
heliosphere. The latter is produced within Jupiter's magnetosphere which is situated at - 5 AU in the
ecliptic plane. The heliospheric propagation of these particles is mainly described by the heliospheric
diffusion tensor. Some elements of the tensor, such as the diffusion coefficient in the azimuthal direction,
which were neglected in the previous two-dimensional modulation studies are investigated to account for
the three-dimensional transport of Jovian electrons. Different anisotropic solar wind speed profiles that
could represent solar minimum conditions were modelled and their effects were illustrated by computing
the distribution of 7 MeV Jovian electrons in the equatorial regions. In particular, the electron intensity
time-profile along the Ulysses spacecraft trajectory was calculated for these speed profiles and compared
to the 3-10 MeV electron flux observed by the Kiel Electron Telescope (KET) on board the Ulysses
spacecraft from launch (1990) up to end of its first out-of-ecliptic orbit (2000). It was found that the
model solution computed with the solar wind profile previously assumed for typical solar minimum
conditions produced good compatibility with observations up to 1998. After 1998 all model solutions
deviated completely from the observations. In this study, as a further attempt to model KET observations
more realistically, a new relation is established between the latitudinal dependence of the solar wind
speed and the perpendicular polar diffusion. Based on this relation, a transition of an average solar wind
speed from solar minimum conditions to intermediate solar activity and to solar maximum conditions
was modelled based on the assumption of the time-evolution of large polar coronal holes and were
correlated to different scenarios of the enhancement of perpendicular polar diffusion. Effects of these
scenarios were illustrated, as a series of steady-state solutions, on the computed 7 MeV Jovian and
galactic electrons in comparison with the 3-10 MeV electron observed by the KET instrument from the
period 1998 up to the end of 2003. Subsequent effects of these scenarios were also shown on electron
modulation in general. It was found that this approach improved modelling of the post-1998 discrepancy
between the model and KET observations but it also suggested the need for a time-dependent 3D
electron modulation model to describe modulation during moderate to extreme solar maximum
conditions. / Thesis (M.Sc.)--North-West University, Potchefstroom Campus, 2004.
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A study of cosmic ray anisotropies in the heliosphere / Godfrey Sibusiso NkosiNkosi, Godfrey Sibusiso January 2006 (has links)
Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2007.
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A study of cosmic ray anisotropies in the heliosphere / Godfrey Sibusiso NkosiNkosi, Godfrey Sibusiso January 2006 (has links)
The three-dimensional (3D) steady-state electron modulation model of Ferreira (2002),
based on Parker (1965) transport equation, is used to study the modulation of the 7 MeV
galactic and Jovian electron anisotropies in the inner heliosphere. The Jovian electrons
are produced in Jupiter's magnetosphere which is situated at ~ 5 AU in the ecliptic plane.
The propagation of these particles is mainly described by the diffusion tensor applicable
for the inner heliosphere. Some of the elements of the diffusion tensor are revisited in
order to establish what contribution they make to the three-dimensional anisotropy vector
and its components in the inner heliosphere. The 'drift' term is neglected since the focus
of this study is on low-energy electrons. The effects on the electron anisotropy of
different scenarios when changing the solar wind speed from minimum to maximum
activity is illustrated. The effects on both the galactic and Jovian electron anisotropy of
changing the polar perpendicular coefficient, in particular, are illustrated. It is shown that
the computed Jovian electron anisotropy dominates the galactic anisotropy close to the
Jovian electron source at ~5 AU, as expected, testifying to the validity of the3D-model.
For the latitudinal anisotropy, the polar perpendicular diffusion plays a dominant role for
Jovian electrons close to the source, with the polar gradient becoming the dominant factor
away from the electron source. Of all three anisotropy components, the azimuthal
anisotropy is dominant in the equatorial plane close to the source. It is found that there is
a large azimuthal gradient close to the source because the low-energy electrons tend to
follow the heliospheric magnetic field more closely than higher energy particles. The
transition of the solar wind speed from minimum to intermediate to maximum solar
activity condition was used to illustrate the modulation of the magnitude of the 7 MeV
total anisotropy vector along the Ulysses trajectory. It was found that during the two
encounters with the planet a maximum anisotropy of 38% was computed but with
different anisotropy-timepeaks as the approach to Jupiter was different. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2007.
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A study of cosmic ray anisotropies in the heliosphere / Godfrey Sibusiso NkosiNkosi, Godfrey Sibusiso January 2006 (has links)
The three-dimensional (3D) steady-state electron modulation model of Ferreira (2002),
based on Parker (1965) transport equation, is used to study the modulation of the 7 MeV
galactic and Jovian electron anisotropies in the inner heliosphere. The Jovian electrons
are produced in Jupiter's magnetosphere which is situated at ~ 5 AU in the ecliptic plane.
The propagation of these particles is mainly described by the diffusion tensor applicable
for the inner heliosphere. Some of the elements of the diffusion tensor are revisited in
order to establish what contribution they make to the three-dimensional anisotropy vector
and its components in the inner heliosphere. The 'drift' term is neglected since the focus
of this study is on low-energy electrons. The effects on the electron anisotropy of
different scenarios when changing the solar wind speed from minimum to maximum
activity is illustrated. The effects on both the galactic and Jovian electron anisotropy of
changing the polar perpendicular coefficient, in particular, are illustrated. It is shown that
the computed Jovian electron anisotropy dominates the galactic anisotropy close to the
Jovian electron source at ~5 AU, as expected, testifying to the validity of the3D-model.
For the latitudinal anisotropy, the polar perpendicular diffusion plays a dominant role for
Jovian electrons close to the source, with the polar gradient becoming the dominant factor
away from the electron source. Of all three anisotropy components, the azimuthal
anisotropy is dominant in the equatorial plane close to the source. It is found that there is
a large azimuthal gradient close to the source because the low-energy electrons tend to
follow the heliospheric magnetic field more closely than higher energy particles. The
transition of the solar wind speed from minimum to intermediate to maximum solar
activity condition was used to illustrate the modulation of the magnitude of the 7 MeV
total anisotropy vector along the Ulysses trajectory. It was found that during the two
encounters with the planet a maximum anisotropy of 38% was computed but with
different anisotropy-timepeaks as the approach to Jupiter was different. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2007.
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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 of galactic cosmic ray electrons in the heliosphere / Nndanganeni, R.R.Nndanganeni, Rendani Rejoyce January 2012 (has links)
The Voyager 1 spacecraft is now about 25 AU beyond the heliospheric termination shock and
soon it should encounter the outer boundary of the heliosphere, the heliopause. This is set to
be at 120 AU in the modulation model used for this study. This implies that Voyager 1, and
soon afterwards also Voyager 2, should be able to measure the heliopause spectrum, to be
interpreted as the lowest possible local interstellar spectrum, for low energy galactic electrons
(1 MeV to 120 MeV). This could give an answer to a long outstanding question about the
spectral shape (energy dependence) of the galactic electron spectrum at these low energies.
These in situ electron observations from Voyager 1, until the year 2010 when it was already
beyond 112 AU, are used for a comparative study with a comprehensive three dimensional
numerical model for the solar modulation of galactic electrons from the inner to the outer
heliosphere.
A locally developed steady state modulation model which numerically solves the relevant
heliospheric transport equation is used to compute and study modulated electron spectra from
Earth up to the heliopause. The issue of the spectral shape of the local interstellar spectrum at
these low energies is specifically addressed, taking into account modulation in the inner
heliosheath, up to the heliopause, including the effects of the transition of the solar wind
speed from supersonic to subsonic in the heliosheath. Modulated electron spectra from the
inner to the outer heliosphere are computed, together with radial and latitudinal profiles,
focusing on 12 MeV electrons. This is compared to Voyager 1 observations for the energy
range 6–14 MeV. A heliopause electron spectrum is computed and presented as a new
plausible local interstellar spectrum from 30 GeV down to 10 MeV.
The comparisons between model predictions and observations from Voyager 1 and at Earth
(e.g. from the PAMELA mission and from balloon flights) and in the inner heliosphere (e.g.
from the Ulysses mission) are made. This enables one to make conclusions about diffusion
theory applicable to electrons in the heliosphere, in particular the rigidity dependence of
diffusion perpendicular and parallel to the local background solar magnetic field. A general
result is that the rigidity dependence of both parallel and perpendicular diffusion coefficients
needs to be constant below P < 0.4 GV and only be allowed to increase above this rigidity to
assure compatibility between the modeling and observations at Earth and especially in the outer heliosphere. A modification in the radial dependence of the diffusion coefficients in the
inner heliosheath is required to compute realistic modulation in this region. With this study,
estimates of the intensity of low energy galactic electrons at Earth can be made. A new local
interstellar spectrum is computed for these low energies to improve understanding of the
modulation galactic electrons as compared to previous results described in the literature. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2012.
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Modelling of galactic cosmic ray electrons in the heliosphere / Nndanganeni, R.R.Nndanganeni, Rendani Rejoyce January 2012 (has links)
The Voyager 1 spacecraft is now about 25 AU beyond the heliospheric termination shock and
soon it should encounter the outer boundary of the heliosphere, the heliopause. This is set to
be at 120 AU in the modulation model used for this study. This implies that Voyager 1, and
soon afterwards also Voyager 2, should be able to measure the heliopause spectrum, to be
interpreted as the lowest possible local interstellar spectrum, for low energy galactic electrons
(1 MeV to 120 MeV). This could give an answer to a long outstanding question about the
spectral shape (energy dependence) of the galactic electron spectrum at these low energies.
These in situ electron observations from Voyager 1, until the year 2010 when it was already
beyond 112 AU, are used for a comparative study with a comprehensive three dimensional
numerical model for the solar modulation of galactic electrons from the inner to the outer
heliosphere.
A locally developed steady state modulation model which numerically solves the relevant
heliospheric transport equation is used to compute and study modulated electron spectra from
Earth up to the heliopause. The issue of the spectral shape of the local interstellar spectrum at
these low energies is specifically addressed, taking into account modulation in the inner
heliosheath, up to the heliopause, including the effects of the transition of the solar wind
speed from supersonic to subsonic in the heliosheath. Modulated electron spectra from the
inner to the outer heliosphere are computed, together with radial and latitudinal profiles,
focusing on 12 MeV electrons. This is compared to Voyager 1 observations for the energy
range 6–14 MeV. A heliopause electron spectrum is computed and presented as a new
plausible local interstellar spectrum from 30 GeV down to 10 MeV.
The comparisons between model predictions and observations from Voyager 1 and at Earth
(e.g. from the PAMELA mission and from balloon flights) and in the inner heliosphere (e.g.
from the Ulysses mission) are made. This enables one to make conclusions about diffusion
theory applicable to electrons in the heliosphere, in particular the rigidity dependence of
diffusion perpendicular and parallel to the local background solar magnetic field. A general
result is that the rigidity dependence of both parallel and perpendicular diffusion coefficients
needs to be constant below P < 0.4 GV and only be allowed to increase above this rigidity to
assure compatibility between the modeling and observations at Earth and especially in the outer heliosphere. A modification in the radial dependence of the diffusion coefficients in the
inner heliosheath is required to compute realistic modulation in this region. With this study,
estimates of the intensity of low energy galactic electrons at Earth can be made. A new local
interstellar spectrum is computed for these low energies to improve understanding of the
modulation galactic electrons as compared to previous results described in the literature. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2012.
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