A three-dimensional (3D) steady-state electron modulation model based on Parker (1965) transport
equation is applied to study the modelling of – 7 MeV galactic and Jovian electrons in the inner
heliosphere. The latter is produced within Jupiter's magnetosphere which is situated at - 5 AU in the
ecliptic plane. The heliospheric propagation of these particles is mainly described by the heliospheric
diffusion tensor. Some elements of the tensor, such as the diffusion coefficient in the azimuthal direction,
which were neglected in the previous two-dimensional modulation studies are investigated to account for
the three-dimensional transport of Jovian electrons. Different anisotropic solar wind speed profiles that
could represent solar minimum conditions were modelled and their effects were illustrated by computing
the distribution of 7 MeV Jovian electrons in the equatorial regions. In particular, the electron intensity
time-profile along the Ulysses spacecraft trajectory was calculated for these speed profiles and compared
to the 3-10 MeV electron flux observed by the Kiel Electron Telescope (KET) on board the Ulysses
spacecraft from launch (1990) up to end of its first out-of-ecliptic orbit (2000). It was found that the
model solution computed with the solar wind profile previously assumed for typical solar minimum
conditions produced good compatibility with observations up to 1998. After 1998 all model solutions
deviated completely from the observations. In this study, as a further attempt to model KET observations
more realistically, a new relation is established between the latitudinal dependence of the solar wind
speed and the perpendicular polar diffusion. Based on this relation, a transition of an average solar wind
speed from solar minimum conditions to intermediate solar activity and to solar maximum conditions
was modelled based on the assumption of the time-evolution of large polar coronal holes and were
correlated to different scenarios of the enhancement of perpendicular polar diffusion. Effects of these
scenarios were illustrated, as a series of steady-state solutions, on the computed 7 MeV Jovian and
galactic electrons in comparison with the 3-10 MeV electron observed by the KET instrument from the
period 1998 up to the end of 2003. Subsequent effects of these scenarios were also shown on electron
modulation in general. It was found that this approach improved modelling of the post-1998 discrepancy
between the model and KET observations but it also suggested the need for a time-dependent 3D
electron modulation model to describe modulation during moderate to extreme solar maximum
conditions. / Thesis (M.Sc.)--North-West University, Potchefstroom Campus, 2004.
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/194 |
| Date | January 2004 |
| Creators | Moeketsi, Daniel Mojalefa |
| Publisher | North-West University |
| Source Sets | North-West University |
| Detected Language | English |
| Type | Thesis |
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