The modulation of galactic cosmic ray (GCR) Carbon in a north-south asymmetrical heliosphere
is studied, using a two-dimensional numerical model that contains a solar wind termination
shock (TS), a heliosheath, as well as particle drifts and diffusive shock re-acceleration
of GCRs. The asymmetry in the geometry of the heliosphere is incorporated in the model by
assuming a significant dependence on heliolatitude of the thickness of the heliosheath. As a
result, the model allows comparisons of modulation in the north and south hemispheres during
both magnetic polarity cycles of the Sun, and from solar minimum to moderate maximum
conditions. When comparing the computed spectra between polar angles of 55o (approximating
the Voyager 1 direction) and 125o (approximating the Voyager 2 direction), it is found that
at kinetic energies E < 1:0 GeV/nuc the effects of the assumed asymmetry in the geometry
of the heliosphere on the modulated spectra are insignificant up to 60 AU from the Sun,
but become increasingly more significant with larger radial distances to reach a maximum
inside the heliosheath. In contrast, with E > 1:0 GeV/nuc, these effects remained insignificant
throughout the heliosphere even very close to the heliopause (HP). However, when the
enhancement of both polar and radial perpendicular diffusion coefficients off the equatorial
plane is assumed to differ from heliographic pole to pole, reflecting different modulation conditions
between the two hemispheres, major differences in the computed intensities between
the two Voyager directions are obtained throughout the heliosphere. The model is further improved
by incorporating new information about the HP location and the relevant heliopause
spectrum for GCR Carbon at E < 200 MeV/nuc based on the recent Voyager 1 observations.
When comparing the computed solutions at the Earth with ACE observations taken during
different solar modulation conditions, it is found that it is possible for the level of modulation
at the Earth, when solar activity changes from moderate maximum conditions to solar minimum
conditions, to exceed the total modulation between the HP and the Earth during solar
minimum periods. In the outer heliosphere, reasonable compatibility with the corresponding
Voyager observations is established when drifts are scaled down to zero in the heliosheath in
both polarity cycles. The effects of neglecting drifts in the heliosheath are found to be more
significant than neglecting the enhancement of polar perpendicular diffusion. Theoretical expressions
for the scattering function required for the reduction of the drift coefficient in modulation
studies are illustrated and implemented in the numerical model. It is found that when
this scattering function decreases rapidly over the poles, the computed A < 0 spectra are higher
than the A > 0 spectra at all energies at Earth primarily because of drifts, which is unexpected
from a classical drift modeling point of view. Scenarios of this function with strong decreases
over the polar regions seem realistic at and beyond the TS, where the solar wind must have a
larger latitudinal dependence. / PhD (Space Physics), North-West University, Potchefstroom Campus, 2015
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/15476 |
| Date | January 2015 |
| Creators | Ngobeni, Mabedle Donald |
| Source Sets | North-West University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
Page generated in 0.002 seconds