Modulation of high-energy cosmic rays in the heliosphere

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 minimum.