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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/843 |
| Date | January 2005 |
| Creators | Ndiitwani, Dzivhuluwani Christopher |
| Publisher | North-West University |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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