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dc.contributor.advisorPotgieter, M.S.
dc.contributor.advisorStrauss, R.D.
dc.contributor.authorRaath, Jan Louis
dc.date.accessioned2015-12-03T12:47:03Z
dc.date.available2015-12-03T12:47:03Z
dc.date.issued2015
dc.identifier.urihttp://hdl.handle.net/10394/15516
dc.descriptionMSc (Space Physics), North-West University, Potchefstroom Campus, 2015en_US
dc.description.abstractUntil recently, numerical modulation models for the solar modulation of cosmic rays have been based primarily on finite difference approaches; however, models based on the solution of an appropriate set of stochastic differential equations have become increasingly popular. This study utilises such a spatially three-dimensional and time-stationary model, based on that of Strauss et al. (2011b). The remarkable numerical stability and powerful illustrative capabilities of this model are utilised extensively and in a distinctly comparative fashion to enable new insights into the processes of modulation. The model is refined to provide for both the Smith-Bieber (Smith and Bieber, 1991) and Jokipii-Kota (Jokipii and Kota, 1989) modifcations to the Parker heliospheric magnetic field (Parker, 1958) and the implications for modulation are investigated. During this investigation it is conclusively illustrated that the Parker field is most conducive to drift dominated modulation, while the Jokipii-Kota and Smith-Bieber modifcations are seen to induce successively larger contributions from diffusive processes. A further refinement to the model is the incorporation of a different profile for the heliospheric current sheet. This profile is defined by its latitudinal extent given by Kota and Jokipii (1983), as opposed to the profile given by Jokipii and Thomas (1981). An extensive investigation into current sheet related matters is launched, illustrating the difference between these current sheet geometries, the associated drift velocity fields and the effect on modulation. At high levels of solar activity, such that the current sheet enters deep enough into the polar regions, the profile of Kota and Jokipii (1983) is found to significantly reduce the effective inward (outward) drifts of positively (negatively) charged particles during A > 0 polarity cycles. The analogous effect is true for A < 0 polarity cycles and the overall effect is of such an extent that the A > 0 and A < 0 solutions are found to coincide at the highest levels of solar activity to form a closed loop. This is a result that has never before been achieved without having to scale down the drift coefficient to zero at solar maximum, as was done by e.g. Ndiitwani et al. (2005). Furthermore, it is found that the drift velocity fields associated with these two current sheet profiles lead to significant differences in modulation even at such low levels of solar activity where no difference in the geometries of these profiles are yet in evidence. The model is finally applied to reproduce four observed galactic proton spectra, selected from PAMELA measurements (Adriani et al., 2013) during the atypical solar minimum of 2006 to 2009; a new proton local interstellar spectrum was employed. The results are found to be in accordance with that found by other authors and in particular Vos (2011), i.e. the diffusion was required to consistently increase from 2006 to 2009 and, in addition, the rigidity dependence below ~ 3 GV was required to change over this time so that the spectra became increasingly softer.en_US
dc.language.isoenen_US
dc.publisherNorth-West University
dc.subjectSolar modulationen_US
dc.subjectCosmic raysen_US
dc.subjectStochastic diffirential equationsen_US
dc.subjectDiffusive processesen_US
dc.subjectDrift coefficienten_US
dc.subjectHeliospheric current sheeten_US
dc.subjectHeliospheric magnetic fielden_US
dc.subjectSolar activityen_US
dc.subjectMagnetic polarity cyclesen_US
dc.subjectLocal interstellar proton spectrumen_US
dc.titleA comparative study of cosmic ray modulation modelsen
dc.typeThesisen_US
dc.description.thesistypeMastersen_US
dc.contributor.researchID10060014 - Potgieter, Marthinus Steenkamp (Supervisor)
dc.contributor.researchID13065440 - Strauss, Roelf Du Toit (Supervisor)


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