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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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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 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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