Abstract. We describe a new method for calculating geomagnetic cutoffs that incorporates obliquely incident primaries, and we use it to interpret a sea level neutron monitor latitude survey. Effects due to obliquely incident primaries are significant and may be responsible for anomalies observed in this and other latitude surveys. We define an "apparent" cutoff that takes these obliquely incident particles into account. Use of our apparent cutoff in a Dorrnan function fit to the 1994-1995 Bartol Research Institute-University of Tasmania latitude survey data results in a significant improvement over use of the more conventional effective vertical cutoff.

We have analyzed the yearly averaged sidereal daily variations in the count rates of 46 underground muon telescopes by fitting Gaussian functions to the data. These functions represent the loss cone and tail‐in anisotropies of the sidereal anisotropies model proposed by Nagashima et al. [l995a, b]. The underground muon telescopes cover the median rigidity range 143–1400 GV and the viewing latitude range 73°N–76°S. From the Gaussian amplitudes and positions we have confirmed that the tail‐in anisotropy is more prominent in the southern hemisphere with its reference axis located at declination (δ) ∼14°S and right ascension (α) ∼4.7 sidereal hours. The tail‐in anisotropy is asymmetric about its reference axis, and the observed time of maximum intensity depends on the viewing latitude of the underground muon telescopes. We also find that the declination of the reference axis may be related to the rigidity of the cosmic rays. We show that the loss cone anisotropy is symmetric and has a reference axis located on the celestial equator (δ ∼ 0°) and α ∼ 13 sidereal hours. We have used the parameters of the Gaussian fits to devise an empirical model of the sidereal anisotropies. The model implies that the above characteristics of the anisotropies can explain the observed north‐south asymmetry in the amplitude of the sidereal diurnal variation. Furthermore, we find that the anisotropies should cause the phase of the sidereal semidiurnal variation of cosmic rays to be observed at later times from the northern hemisphere compared to observations from the southern hemisphere. We present these results and discuss them in relation to current models of the heliosphere.

DECLARATIONSI certify that this thesis does not incorporate without acknowledgment any material previously submitted for a degree or diploma in any university; and to the best of my knowledge and belief it does not contain any material previously published or written by another person where due reference is not made in the text. Damian Lindsay Hallll This thesis may be made available for loan and limited copying in accordance with the Copyright Act 1968. Damian Lindsay Hall Ill ABSTRACTThe 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 ?Lip, (the product of the parallel mean-free path and radial density gradient) and Gtzl (an indicator of the symmetric latitudinal density gradient). X G r is found to have a 22-year II variation at all rigidities studied and furthermore to only have rigidity dependence when the heliosphere is in the A >0 magnetic polarity state. It is unlikely that X IIG r has any rigidity dependence in the A <0 polarity s...

The Sun's interplanetary magnetic field and the solar wind modulate the distrzbution of galactic cosmic-ray particles in the heliosphere. The particles diffuse inward, convect outward and have drifts in the motion of their gyro-centres. Irregularities in the IMF also scatter particles from their gyro-orbits. These processes are the components of solar modulation and produce streaming (and higher-order anisotropies) of particles in the heliosphere. The anisotropies can be investigated at the Earth by examining the count rates of cosmic-ray detectors. The anisotropic streams appear as dmrnal variations m solar and sidereal time in the count rates. Higher-order anisotroples produce generally much smaller semi-diurnal and higher-order variations. Theoretical models of solar modulation predict effects that depend on the polarity of the Sun's magnetic dipole. The solar diurnal and north-south anisotropies can be used to test these predictions. This paper is a short review of analyses of 60 years of cosmic-ray data collected at the Earth for the solar and sMereal diurnal variations present. Past analyses have yielded interesting and controversial results regarding the rigidity spectra and components of these anisotropies. Some of the controversy remains today. Analyses of these anisotropies have also yielded quantitative information about parameters important to solar modulation, such as latitudinal and radial density gradients. The relatively new techniques used for these determinations are explained here. Calculations of these modulation parameters from Earthbased cosmic-ray detectors are reviewed and compared to spaceprobe measurements and theoretical predictions of their values. Recently, investigations of the sidereal and solar diurnal anisotropies have been combined to calculate mean-free-paths of cosmic rays in the heliosphere. The latest conclusions from these analyses are that the parallel mean-free-paths of cosmic rays may depend on the polarity of the Sun's magnetic field. The results of these investigations are included in this paper to indicate the present state of knowledge concerning this facet of cosmic-ray research.

From an analysis of the solar diurnal variation in Galactic cosmic rays with rigidities between 17 and 195 GV from 1957 to 1990, we have derived the product of the parallel mean free path and the radial density gradient and the bidirectional latitudinal gradient indicator We Ðnd the rigidity spec-(j A G r ) G @z@ . trum of the solar diurnal anisotropy averaged over the entire time span to have a spectral index of [0.1^0.2 and an upper limiting rigidity of 100^25 GV. The yearly averaged spectrum may depend on the heliospheric polarity state, having a slightly positive spectral index during the A [ 0 epoch.We Ðnd that the parameter depends inversely on rigidity, more so during the A [ 0 polarity j A G r state. Around solar minimum of this epoch, is also much lower than for corresponding years in the j A G r A \ 0 polarity state. Combining the derived values of with results from previous studies of the j A G r radial gradient, we Ðnd evidence for the values of (at all the rigidities examined) having a variation j A related to the magnetic polarity state of the heliosphere.indicates that at rigidities up to 185 GV a G @z@ bidirectional latitudinal gradient exists and changes direction following solar magnetic polarity reversals, in agreement with drift theories. This gradient attains its largest magnitudes around times of solar minimum and also appears to be larger during A \ 0 polarity states.

Abstract. By using the two-hemisphere network of underground muon telescopes we have examined the average sidereal daily variations in the count rates recorded by 48-component muon telescopes. The telescopes respond to primary cosmic rays with rigidities between -140 and 1700 GV and view almost the entire celestial sphere. We have modeled the data by using Gaussian functions, and we have related the Gaussian parameters to the recent tail-in and loss cone anisotropy model proposed by Nagashima et al. [1995a, b] to explain the sidereal daily variations. We have used the model parameters to derive the rigidity and latitude spectra of the galactic anisotropies and find them to be qualitatively in agreement with Nagashima et al.'s predictions. The results indicate, however, that the tail-in anisotropy is asymmetric about its reference axis, whereas the loss cone anisotropy is more symmetric. We show that these characteristics of the galactic anisotropies may explain the north-south asymmetry observed in the amplitude of the sidereal diurnal variation derived from Fourier analysis techniques. Method of Analysis Telescopes in the THNThe THN of underground muon telescopes is composed of the multidirectional underground muon telescopes located at 367

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We have deduced the yearly averaged value of the solar diurnal variation as observed by a surface muon telescope and three underground muon telescopes over the years 1957 to 1985. This has allowed us to examine the temporal variation in both the latitudinal gradient Gz and the product of the parallel mean free path and the radial gradient of galactic cosmic rays during three consecutive solar cycles. The median rigidities of the primary particles being detected by the telescopes are 50 GV in the case of the surface muon telescope and greater than 150 GV in the case of the underground muon telescopes. We have compared our results with those of a similar study made from observations of the solar diurnal variation by neutron monitors and an ion chamber, which have median rigidities of response between 17 and 70 GV (Bieber and Chen 1991a). The product has a solar magnetic cycle dependence and our values are lower than those observed by neutron monitors, in agreement with the Bieber and Chen observation that reverses after a solar magnetic field reversal, in accordance with drift theories.