Long-term cosmic-ray modulation studies using ab initio numerical modulation models require
an understanding of the solar-cycle dependence of the heliospheric diffusion tensor.
Such an understanding requires information as to possible solar-cycle dependences of various
basic turbulence quantities. In this study, 1-minute resolution data for the N-component
of the heliospheric magnetic field spanning from 1974 to 2012 is analysed using second-order
structure functions constructed assuming a simple three-stage power-law frequency spectrum.
This spectrum is motivated observationally and theoretically, and has an inertial, an energycontaining
and a cutoff-range at small frequencies to ensure a finite energy density. Of the
turbulence quantities calculated from 27-day averaged second-order structure functions, only
the magnetic variance and the spectral level show a significant solar-cycle dependence, much
less so the spectral index in the energy range. The spectral indices in the inertial range, as well
as the turnover and cutoff scales do not appear to depend on the level of solar activity. The
ratio of the variance to the square of the magnetic field also appears to be solar-cycle independent.
These results suggest that the dominant change in the spectrum over several solar-cycles
is its level. Comparisons of the results found in this study with relevant published observations
of turbulence quantities are very favourable. Furthermore, when the magnetic variances
and heliospheric magnetic magnitudes calculated in this study are used as inputs for theoretically
motivated expressions for the mean free paths and turbulence-reduced drift lengthscale,
clear solar-cycle dependencies in these quantities are seen. Values for the diffusion and drift
lengthscales during the recent unusual solar minimum are found to be significantly higher
than during previous solar minima. / MSc (Space Physics), North-West University, Potchefstroom Campus, 2015
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/16076 |
| Date | January 2015 |
| Creators | Nel, Amoré Elsje |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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