The time-dependent modulation of galactic cosmic rays in the heliosphere is studied by computing
intensities using a two-dimensional, time-dependent modulation model. The compound
approach of Ferreira and Potgieter (2004), which describes changes in the cosmic ray
transport coefficients over a solar cycle, is improved by introducing recent theoretical advances
in the model. Computed intensities are compared with Voyager 1 and 2, IMP 8 and Ulysses
proton observations in search of compatibility. It is shown that this approach gives realistic
cosmic ray proton intensities on a global scale at Earth and along both Voyager spacecraft
trajectories. The results show that cosmic ray modulation, in particular during the present
polarity cycle, is not just determined by changes in the drift coefficient but is also dependent
on changes in the diffusion coefficients. Furthermore, a comparison of computations to observations
along the Voyager 1 and Voyager 2 trajectories illustrates that the heliosphere is
asymmetrical. Assuming the latter, E > 70 MeV and 133-242 MeV cosmic ray proton intensities
along Voyager 1 and 2 trajectories are predicted from 2012 onwards. It is shown that
the computed intensities along Voyager 1 can increase with an almost constant rate since the
spacecraft is close to the heliopause. However, the model shows that Voyager 2 is still under
the influence of temporal solar activity changes because of the relatively large distance to
the heliopause when compared to Voyager 1. Along the Voyager 2 trajectory the intensities
should remain generally constant for the next few years and then should start to steadily increase.
It is also found that without knowing the exact location of heliopause and transport
parameters one cannot conclude anything about local interstellar spectra. The effect of a dynamic
inner heliosheath width on cosmic ray modulation is also studied by implementing a
time-dependent termination shock position in the model. This does not lead to improved compatibility
with spacecraft observations so that a time-dependent termination shock along with
a time-dependent heliopause position is required. The variation of the heliopause position
over a solar cycle is found to be smaller compared to that of the termination shock. The model
predicts the heliopause and termination shock positions along Voyager 1 in 2012 at 119 AU
and 88 AU respectively and along Voyager 2 at 100 AU and 84 AU respectively. / Thesis (PhD (Space Physics))--North-West University, Potchefstroom Campus, 2013
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/8740 |
| Date | January 2013 |
| Creators | Manuel, Rex |
| Publisher | North-West University |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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