The interest in the role of the solar wind termination shock (TS) and heliosheath in cosmic ray (CR) modulation
studies has increased sigm6cantly as the Voyager 1 and 2 spacecraft approach the estimated position of the TS. For
this work the modulation of galactic CR protons, anti-protons, electrons with a Jovian source, positrons, Helium,
and anomalous protons and Helium, and the consequent charge-sign dependence, are studied with an improved
and extended two-dimensional numerical CR modulation model including a TS with diffusive shock acceleration, a
heliosheath and drifts. The modulation is computed using improved local interstellar spectra (LIS) for almost all
the species of interest to this study and new fundamentally derived diffusion coefficients, applicable to a number of
CR species during both magnetic polarity cycles of the Sun. The model also allows comparisons of modulation with
and without a TS and between solar minimum and moderate maximum conditions. The modulation of protons
and Helium with their respective anomalous components are also studied to establish the consequent charge-sign
dependence at low energies and the influence on the computed p/p, e-/p, and e-/He. The level of modulation in
the simulated heliosheath, and the importance of this modulation 'barrier' and the TS for the different species are
illustrated. From the computations it is possible to estimate the ratio of modulation occurring in the heliosheath
to the total modulation between the heliopause and Earth for the mentioned species. It has been found that the
modulation in the heliosheath depends on the particle species, is strongly dependent on the energy of the CRs, on
the polarity cycle and is enhanced by the inclusion of the TS. The computed modulation for the considered species
is surprisingly different and the heliosheath is important for CR modulation, although 'barrier' modulation is more
prominent for protons, anti-protons and Helium, while the heliosheath cannot really be considered a modulation
'barrier' for electrons and positrons above energies of ~150 MeV. The effects of the TS on modulation are more
pronounced for polarity cycles when particles are drifting primarily in the equatorial regions of the heliosphere
along the heliospheric current sheet to the Sun, e.g. the A < 0 polarity cycle for protons, positrons, and Helium,
and the A > 0 polarity cycle for electrons and anti-protons. This study also shows that the proton and Helium
LIS may not be known at energies <~ 200 MeV until a spacecraft actually approaches the heliopause because of the
strong modulation that occurs in the heliosheath, the effect of the TS, and the presence of anomalous protons and
Helium. For anti-protons, in contrast, these effects are less pronounced. For positrons, with a completely different
shape LIS, the modulated spectra have very mild energy dependencies <~ 300 MeV, even at Earth, in contrast to the
other species. These characteristic spectral features may be helpful to distinguish between electron and positron
spectra when they are measured near and at Earth. These simulations can be of use for future missions to the
outer heliosphere and beyond. / Thesis (Ph.D. (Physics))--North-West University, Potchefstroom Campus, 2004.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/346 |
| Date | January 2004 |
| Creators | Langner, Ulrich Wilhelm |
| Publisher | North-West University |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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