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Transient Cosmic-ray Events beyond the Heliopause: Interpreting Voyager-1 ObservationsKóta, J., Jokipii, J. R. 25 April 2017 (has links)
In 2013 March and 2014 May, Voyager-1 (V1) experienced small but significant increases in the flux of galactic cosmic rays (GCRs) in the hundred MeV/n range. Additionally, V1 also saw episodic depletion of GCR flux around perpendicular pitch angles. We discuss the pitch-angle distribution and the time profiles of these events. In a previous paper, we interpreted the 2013 "bump" as the GCRs remotely sensing a shock that reached the magnetic field line passing through V1: particles gained energy as they were reflected on the approaching region of the stronger magnetic field of the disturbance. Here, we point out that energy gain is not restricted to reflected particles -GCRs passing through the disturbance also gain energy. The effect should be present in a broad range of pitch angles with the maximum increase of GCR intensity predicted to occur at the critical reflection angle. In this paper, the shock is not step-like, but a gradual increase of the magnetic field strength, B, taking a few days, in agreement with V1 measurements. This smoothens the profile of the predicted bump in the GCR flux. We also address the linear episodic decreases seen around perpendicular pitch angles. These events are interpreted in terms of adiabatic cooling behind the shock due to the slow weakening of B. We present simple numerical model calculations and find that a gradual shock followed by a slow decrease of B, as observed, may account for both the episodic increases and the anisotropic depletion of GCR fluxes.
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Variations of solar wind parameters over a solar cycle : expectations for NASA's Solar TErrestrial RElations Observatory (STEREO) mission /Walker, Catherine C. January 2007 (has links) (PDF)
Undergraduate honors paper--Mount Holyoke College, 2007. Dept. of Astronomy. / Includes bibliographical references (leaves i-vi).
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Ultraviolet resonance radiation and the structure of the heliosphere.Hall, Doyle Thomas. January 1992 (has links)
The solar system and heliosphere are embedded in a partially ionized medium flowing past the Sun at about 22 km s⁻¹. The Voyager and Pioneer 10 spacecraft are travelling upstream and downstream respectively, detecting Lyα radiation resonantly scattered from heliospheric hydrogen. None of the probes has encountered the solar wind termination shock, where the supersonic solar wind is believed to decelerate to subsonic speeds. Penetration of H atoms from the local interstellar flow is the principal source of heliospheric H. Solar gravitation, radiation pressure, and ionization processes largely control the H distribution. However, the presence of the solar wind termination shock is predicted to have two additional effects. H-p charge exchange reactions occurring in the hot, post-shock solar wind plasma should both reduce the number of penetrating H atoms and create a population of suprathermal H atoms. Therefore, heliospheric Lyα emission lines should be composed of narrow and wide components, which should be diagnostic of outer heliospheric structure. Previously unpublished Voyager Cruise Maneuver observations obtained between 15 and 40 AU reveal that upstream Lyα intensities fall as r⁽⁻⁰·⁷⁵ ⁺/⁻ ⁰·⁰⁵⁾. Beyond 15 AU downstream, Pioneer 10 Lyα falls as r⁽⁻¹·⁰⁷ ⁺/⁻ ⁰·¹⁾. These trends cannot be simultaneously reproduced using models which do not include the termination shock. The Voyager data suggest an additional source of Lyα in the upstream region beyond 40 AU. This may be due to suprathermal H gas and/or gradients in the H density, both predicted to be associated with the termination shock. A new method of estimating the heliospheric H density between the two Voyager spacecraft is introduced. The results are ambiguous and suffer due to the uncertainty in relative instrumental Lyα sensitivities.
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Magnetohydrodynamic discontinuities and the structure of coronal mass ejectionsKilmurray, Richard Ian January 1999 (has links)
No description available.
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A study of the time-dependent modulation of cosmic rays in the inner heliosphere / E. MagidimishaMagidimisha, Edwin January 2010 (has links)
A two-dimensional (2-D) time-dependent cosmic ray modulation model is used to calculate the
modulation of cosmic-ray protons and electrons for 11-and 22-year modulation cycles using a
compound approach to describe solar cycle related changes in the transport parameters. The
compound approach was developed by Ferreira and Potgieter (2004) and incorporates the concept
of propagation diffusion barriers, global changes in the magnetic field, time-dependent
gradient, curvature and current-sheet drifts, and other basic modulation mechanisms. By comparing
model results with 2.5 GV Ulysses observations, for both protons and electrons, it is
shown that the compound approach results in computed intensities on a global scale compatible
to observations. The model also computes the expected latitudinal dependence, as
measured by the Ulysses spacecraft, for both protons and electrons. This is especially highlighted
when computed intensities are compared to observations for the different fast latitude
scan (FLS) periods. For cosmic ray protons a significant latitude dependence was observed for
the first FLS period which corresponded to solar minimum conditions. For the second, which
corresponded to solar maximum, no latitude dependence was observed as was the case for the
third FLS period, which again corresponded to moderate to minimum solar activity. For the
electrons the opposite occurred with only an observable latitude dependence in intensities for
the third FLS period. It is shown that the model results in compatible intensities when compared
to observations for these periods. Due to the success of the compound approach, it is
also possible to compute charge-sign dependent modulation for 2.5 GV protons and electrons.
The electron to proton ratio is presented at Earth and along the Ulysses trajectory. Lastly, it is
also shown how the modulation amplitude between solar minimum and maximum depends
on rigidity. This is investigated by computing cosmic ray intensities for both protons and electrons,
not only at 2:5 GV, but also up to 7:5 GV. A refinement for the compound approach
at higher rigidities is proposed. / Thesis (M.Sc. (Space Physics))--North-West University, Potchefstroom Campus, 2011.
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A study of the time-dependent modulation of cosmic rays in the inner heliosphere / E. MagidimishaMagidimisha, Edwin January 2010 (has links)
A two-dimensional (2-D) time-dependent cosmic ray modulation model is used to calculate the
modulation of cosmic-ray protons and electrons for 11-and 22-year modulation cycles using a
compound approach to describe solar cycle related changes in the transport parameters. The
compound approach was developed by Ferreira and Potgieter (2004) and incorporates the concept
of propagation diffusion barriers, global changes in the magnetic field, time-dependent
gradient, curvature and current-sheet drifts, and other basic modulation mechanisms. By comparing
model results with 2.5 GV Ulysses observations, for both protons and electrons, it is
shown that the compound approach results in computed intensities on a global scale compatible
to observations. The model also computes the expected latitudinal dependence, as
measured by the Ulysses spacecraft, for both protons and electrons. This is especially highlighted
when computed intensities are compared to observations for the different fast latitude
scan (FLS) periods. For cosmic ray protons a significant latitude dependence was observed for
the first FLS period which corresponded to solar minimum conditions. For the second, which
corresponded to solar maximum, no latitude dependence was observed as was the case for the
third FLS period, which again corresponded to moderate to minimum solar activity. For the
electrons the opposite occurred with only an observable latitude dependence in intensities for
the third FLS period. It is shown that the model results in compatible intensities when compared
to observations for these periods. Due to the success of the compound approach, it is
also possible to compute charge-sign dependent modulation for 2.5 GV protons and electrons.
The electron to proton ratio is presented at Earth and along the Ulysses trajectory. Lastly, it is
also shown how the modulation amplitude between solar minimum and maximum depends
on rigidity. This is investigated by computing cosmic ray intensities for both protons and electrons,
not only at 2:5 GV, but also up to 7:5 GV. A refinement for the compound approach
at higher rigidities is proposed. / Thesis (M.Sc. (Space Physics))--North-West University, Potchefstroom Campus, 2011.
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A study of the time-dependent modulation of galactic cosmic rays in the heliosphere / Dzivhuluwani C. NdiitwaniNdiitwani, Dzivhuluwani Christopher January 2005 (has links)
Time-dependent cosmic ray modulation in the heliosphere is studied by using a two-dimensional
time dependent modulation model. To compute realistic cosmic ray modulation a compound
approach is used, which combines the effect of the global changes in the heliospheric magnetic
field magnitude and the current sheet tilt angle to establish realistic time dependent diffusion
and drift coefficients. This approach is refined by scaling down drifts additionally (compared
to diffusion) towards solar maximum. The amount of drifts needed in the model to realistically
compute 2.5 GV proton and electron and 1.2GV electron and helium intensities, as measured
by Ulysses from 1990 to 2004, is established. It is shown that the model produces the correct
latitudinal gradients evident from the observations during both the Ulysses fast latitude scan
periods. Also, much can be learned on the magnitude of perpendicular diffusion in the polar
direction, K┴θ, especially for solar minimum conditions and for polarity cycles when particles
drift in from the poles. For these periods K┴θ = 0.12K║ in the polar regions (with K║ the parallel
diffusion coefficient)and K┴θ /K║ can vary between 0.01 to even 0.04 in the equatorial
regions depending on the enhancement factor toward the poles. The model is also applied to
compute radial gradients for 2.5 GV cosmic ray electrons and protons in the inner heliosphere.
It is shown that, for solar minimum, and in the equatorial regions, the protons (electrons) have
a radial gradient of 1.9 %/AU (2.9 %/AU), increasing for both species to a very fluctuating
gradient varying between 3 to 4 %/AU at solar maximum. Furthermore, the model also computes
realistic electron to proton and electron to helium ratios when compared to Ulysses observations,
and charge-sign dependent modulation is predicted up to the next solar minimum
expected in 2007. Lastly the model is also applied to model simultaneously galactic cosmic
ray modulation at Earth and along the Voyager 1 trajectory, and results are compared with> 70
MeV count rates from Voyager 1 and IMP8. To produce realistic modulation, this model gives
the magnitude of perpendicular diffusion in the radial direction as K┴r/K║= 0.035 and that
the modulation boundary seemed to be situated between at 120 AU and 140 AU. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2005.
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On the development and applications of a three-dimensional ab initio cosmic-ray modulation model / Nicholas Eugéne EngelbrechtEngelbrecht, Nicholas Eugéne January 2012 (has links)
A proper understanding of the effects of turbulence on the diffusion and drift of cosmic-rays in
the heliosphere is imperative for a better understanding of cosmic-ray modulation. This study
presents an ab initio model for cosmic-ray modulation, incorporating for the first time the results
yielded by a two-component turbulence transport model. The latter model is solved for
solar minimum heliospheric conditions, utilizing boundary values chosen in such a way that
the results of this model are in fair to good agreement with spacecraft observations of turbulence
quantities, not only in the ecliptic plane, but also along the out-of-ecliptic trajectory of the
Ulysses spacecraft. These results are employed as inputs for modelled slab and 2D turbulence
energy spectra, which in turn are used as inputs for parallel mean free paths based on those
derived from quasi-linear theory, and perpendicularmean free paths from extended nonlinear
guiding center theory. The modelled 2D spectrum is chosen based on physical considerations,
with a drop-off at the very lowest wavenumbers commencing at the 2D outerscale. There currently
exist no models or observations for this quantity, and it is the only free parameter in this
study. The use of such a spectrum yields a non-divergent 2D ultrascale, which is used as an
input for the reduction terms proposed to model the effects of turbulence on cosmic-ray drifts.
The resulting diffusion and drift coefficients are applied to the study of galactic cosmic-ray
protons, electrons, antiprotons, and positrons using a three-dimensional, steady-state numerical
cosmic-ray modulation code. The magnitude and spatial dependence of the 2D outerscale
is demonstrated to have a significant effect on computed cosmic-ray intensities. A form for the
2D outerscale was found that resulted in computed cosmic-ray intensities, for all species considered,
in reasonable agreement with multiple spacecraft observations. Computed galactic
electron intensities are shown to be particularly sensitive to choices of parameters pertaining
to the dissipation range of the slab turbulence spectrum, and certain models for the onset
wavenumber of the dissipation range could be eliminated in this study. / Thesis (PhD (Physics))--North-West University, Potchefstroom Campus, 2013
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On the development and applications of a three-dimensional ab initio cosmic-ray modulation model / Nicholas Eugéne EngelbrechtEngelbrecht, Nicholas Eugéne January 2012 (has links)
A proper understanding of the effects of turbulence on the diffusion and drift of cosmic-rays in
the heliosphere is imperative for a better understanding of cosmic-ray modulation. This study
presents an ab initio model for cosmic-ray modulation, incorporating for the first time the results
yielded by a two-component turbulence transport model. The latter model is solved for
solar minimum heliospheric conditions, utilizing boundary values chosen in such a way that
the results of this model are in fair to good agreement with spacecraft observations of turbulence
quantities, not only in the ecliptic plane, but also along the out-of-ecliptic trajectory of the
Ulysses spacecraft. These results are employed as inputs for modelled slab and 2D turbulence
energy spectra, which in turn are used as inputs for parallel mean free paths based on those
derived from quasi-linear theory, and perpendicularmean free paths from extended nonlinear
guiding center theory. The modelled 2D spectrum is chosen based on physical considerations,
with a drop-off at the very lowest wavenumbers commencing at the 2D outerscale. There currently
exist no models or observations for this quantity, and it is the only free parameter in this
study. The use of such a spectrum yields a non-divergent 2D ultrascale, which is used as an
input for the reduction terms proposed to model the effects of turbulence on cosmic-ray drifts.
The resulting diffusion and drift coefficients are applied to the study of galactic cosmic-ray
protons, electrons, antiprotons, and positrons using a three-dimensional, steady-state numerical
cosmic-ray modulation code. The magnitude and spatial dependence of the 2D outerscale
is demonstrated to have a significant effect on computed cosmic-ray intensities. A form for the
2D outerscale was found that resulted in computed cosmic-ray intensities, for all species considered,
in reasonable agreement with multiple spacecraft observations. Computed galactic
electron intensities are shown to be particularly sensitive to choices of parameters pertaining
to the dissipation range of the slab turbulence spectrum, and certain models for the onset
wavenumber of the dissipation range could be eliminated in this study. / Thesis (PhD (Physics))--North-West University, Potchefstroom Campus, 2013
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A study of the time-dependent modulation of galactic cosmic rays in the heliosphere / Dzivhuluwani C. NdiitwaniNdiitwani, Dzivhuluwani Christopher January 2005 (has links)
Time-dependent cosmic ray modulation in the heliosphere is studied by using a two-dimensional
time dependent modulation model. To compute realistic cosmic ray modulation a compound
approach is used, which combines the effect of the global changes in the heliospheric magnetic
field magnitude and the current sheet tilt angle to establish realistic time dependent diffusion
and drift coefficients. This approach is refined by scaling down drifts additionally (compared
to diffusion) towards solar maximum. The amount of drifts needed in the model to realistically
compute 2.5 GV proton and electron and 1.2GV electron and helium intensities, as measured
by Ulysses from 1990 to 2004, is established. It is shown that the model produces the correct
latitudinal gradients evident from the observations during both the Ulysses fast latitude scan
periods. Also, much can be learned on the magnitude of perpendicular diffusion in the polar
direction, K┴θ, especially for solar minimum conditions and for polarity cycles when particles
drift in from the poles. For these periods K┴θ = 0.12K║ in the polar regions (with K║ the parallel
diffusion coefficient)and K┴θ /K║ can vary between 0.01 to even 0.04 in the equatorial
regions depending on the enhancement factor toward the poles. The model is also applied to
compute radial gradients for 2.5 GV cosmic ray electrons and protons in the inner heliosphere.
It is shown that, for solar minimum, and in the equatorial regions, the protons (electrons) have
a radial gradient of 1.9 %/AU (2.9 %/AU), increasing for both species to a very fluctuating
gradient varying between 3 to 4 %/AU at solar maximum. Furthermore, the model also computes
realistic electron to proton and electron to helium ratios when compared to Ulysses observations,
and charge-sign dependent modulation is predicted up to the next solar minimum
expected in 2007. Lastly the model is also applied to model simultaneously galactic cosmic
ray modulation at Earth and along the Voyager 1 trajectory, and results are compared with> 70
MeV count rates from Voyager 1 and IMP8. To produce realistic modulation, this model gives
the magnitude of perpendicular diffusion in the radial direction as K┴r/K║= 0.035 and that
the modulation boundary seemed to be situated between at 120 AU and 140 AU. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2005.
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