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Modulation of high energy cosmic rays in the heliosphere


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Hall, DL (1995) Modulation of high energy cosmic rays in the heliosphere. PhD thesis, University of Tasmania.

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The distribution of galactic cosmic ray particles in the heliosphere is influenced (modulated)
by the Sun's interplanetary magnetic field (IMF) and the solar wind. The particles diffuse
inward, convect outward and have drifts in the motion of their gyro-centres. They are also
scattered from their gyro-orbits by irregularities in the IMF. These processes are the
components of solar modulation and produce streaming (anisotropies) of particles in the
heliosphere. The anisotropies can be investigated at Earth by examining the count rates of
cosmic ray detectors. The anisotropic streams appear as diurnal and semi-diurnal variations
in the count rates of cosmic ray recorders in solar and sidereal time. Theoretical models of
solar modulation predict effects which are dependent on the polarity of the Sun's magnetic
dipole (A >0 or A <0). The solar diurnal and North-South anisotropy can be used to test
these predictions.
The yearly averaged solar and sidereal diurnal variations in data recorded by seven neutron
monitors and ten muon telescopes for the period 1957 to 1990 have been deduced by Fourier
analysis methods. The rigidities of the galactic cosmic rays to which these instruments
respond encompass the range 10 to 1400 Giga volts (GV). The rigidity spectrum of the solar
diurnal anisotropy has been inferred to have a mean spectral index extremely close to zero
and an idealised upper limiting rigidity of 100± 25 GV. This is in good agreement with
previous determinations. It is shown that this upper limit has a temporal variation between 50
GV and 180 GV and is correlated with the magnitude of the IMF. The rigidity spectrum is
likely to be dependent on the polarity of the Sun's magnetic dipole, the spectral index being
determined as positive in the A >0 magnetic polarity state and negative in the A <0 polarity
state. It is also shown that the amplitude of the anisotropy varies with an 11-year variation
and the time of maximum varies with 22-year variation. Both of these variations are shown
to be independent of any change in the rigidity spectrum. The solar diurnal anisotropy is also used as a tool to calculate the modulation parameters λ∥
Gτ, (the product of the parallel mean-free path and radial density gradient) and Glzl (an
indicator of the symmetric latitudinal density gradient). λ∥
Gτ is found to have a 22-year
variation at all rigidities studied and furthermore to only have rigidity dependence when the λ∥Gτ
heliosphere is in the A >0 magnetic polarity state. It is unlikely that has any rigidity
dependence in the A <0 polarity state. Glzl indicates that below 50 GV the symmetric
latitudinal density gradient behaves in accordance with the predictions of current modulation
theories. Between 50 and 195 GV however, the predicted behaviour is only observed when
the rigidity spectrum of the solar diurnal anisotropy is assumed to be flat, static and have an
upper limiting rigidity of 100 GV.
The sidereal diurnal variation in the data recorded by the instruments has been deduced and
used to study the North-South anisotropy. The results indicate that this anisotropy has only
a small variation in amplitude. There is strong evidence for heliospheric asymmetric
modulation (with respect to above and below the neutral sheet) of a galactic anisotropy in the
sense proposed by Nagashima et al. (1982) and that this modulation may have a 22-year
variation. From the examination of the North-South anisotropy the radial density gradients
(Gτ) at 1 AU of 17 to 195 GV particles were determined. The gradient is slightly smaller around times of solar minimum. No magnetic polarity dependence of the radial gradient was
observed, in direct conflict with conventional theoretical predictions.
The modulation parameters have been used to determine the parallel mean-free path (λ∥) of
galactic cosmic rays with rigidities between 17 and 195 GV near the Earth. It was found that
this parameter depends on magnetic polarity at all the rigidities examined and has a linear
relationship with rigidity. Perpendicular diffusion has been examined and shown to have
very little contribution to the values of the modulation parameters except for years near solar

Item Type: Thesis (PhD)
Keywords: Galactic cosmic rays, Galactic cosmic rays
Copyright Holders: The Author
Copyright Information:

Copyright 1995 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, 1995.

Date Deposited: 09 Dec 2014 00:01
Last Modified: 27 Mar 2017 23:44
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