Cosmic ray modulation in the heliosphere

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.