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Cosmic ray modulation in the heliosphere


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Baker, Christopher Paul 1993 , 'Cosmic ray modulation in the heliosphere', PhD thesis, University of Tasmania.

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Models describing the modulation of galactic cosmic rays in the heliosphere have been
developed and investigated. These models are numerical solutions of the cosmic ray
transport equation under idealized heliospheric conditions. Primarily, the models were
used to predict the radial gradient g r of galactic cosmic rays in the vicinity of the Earth
during solar-minimum conditions specified by a flat neutral sheet and no propagating shock
disturbances. It was found that g r was not sensitive to the assumed value of the cosmic
ray distribution at the Sun, but was very sensitive to (a) the diffusion coefficients used to
specify the rate of diffusion of the cosmic ray distribution near the Sun, and (b) the choice
of the integrating algorithm used to determine the distribution from the transport equation.
The stability of each integrating algorithm used was also investigated. There have been
many measurements of the value of g,. at Earth in the past, mainly derived from data
collected by polar-based neutron monitors or detectors on board satellites. From all sources
of data and over a wide range of energy (tens of MeV to ~100 GeV) the general consensus
is that g,. at Earth is less than 5% AU-1 and probably around 1-2 % AU-1 . The models
investigated in this thesis predict a gradient in this range but only if certain diffusion
coefficients are specified.
The models were also used to investigate the relative importance of shocks, or propagating
solar disturbances, to the total eleven year cycle of cosmic ray modulation for medium
energy (1-10 GeV) particles. It was found that such disturbances were unable to produce
the observed intensity difference seen from solar minimum to solar maximum. It was
concluded that these, and higher energy particles, are modulated significantly by changes in
the overall global average heliospheric magnetic field through bulk drift motion. An
example of this bulk-drift modulation was investigated by looking at the correlation of
neutral-sheet position with isotropic intensity wave variations seen in the cosmic ray data.
For this analysis, a generalized model of the neutral sheet was developed..
An independent measurement of g r was also calculated from data collected by the network
of cosmic ray detectors operated by the Hobart cosmic ray group during the years 1975-78
and 1982-85. This group consists of the Cosmic Ray Section of the Australian Antarctic
Division and the Cosmic Ray Section of the University of Tasmania (Physics Department).
The gradient g r was determined by applying a north-south anisotropy analysis to the data.
The value of g r obtained was sensitive to the assumed energy spectrum of the north-south
anisotropy but was not inconsistent with other experimentally determined values. The
gradient was generally less than 5% AU -1 . It was necessary to calculate coupling
coefficients and atmospheric correction coefficients for the Mawson muon telescopes in
order to use the data from these detectors in the north-south anisotropy analysis.

Item Type: Thesis - PhD
Authors/Creators:Baker, Christopher Paul
Keywords: Cosmic rays, Heliosphere
Copyright Holders: The Author
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Copyright 1993 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1994. Includes bibliographical references

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