Long-term modulation of cosmic rays in interplanetary magnetic turbulence

Long-term modulation of galactic cosmic rays in interplanetary magnetic field (IMF) turbulence is a fundamental subject for understanding the connection between cosmic ray transport and solar activity. The discovery of a new cosmic ray modulation phenomenon is reported. Analysis of neutron monitor data has revealed that the difference in amplitude of the cosmic ray diurnal anisotropy for IMF sectors directed toward and away from the Sun displays a solar cycle variation. Neutron monitor data recorded at Climax, Deep River, Hyancayo, Kiel, Mt. Washington, and Swarthmore/Newark over the period 1957 to 1988 show that the amplitude difference varied between approximately 0.1 to -0.1 percent, with peaks in 1960, 1972, and 1982. A theoretical expression for this difference was derived from a three-dimensional model. Analysis also showed that the latitudinal density gradient of cosmic rays changed between 1.6 and -1.6 percent/AU with a solar cycle variation, which may produce the variation of the amplitude difference of the cosmic ray diurnal anisotropy. In order to investigate the underlying physical mechanism which produces the observed effect, it was necessary to develop a new analytical technique for calculating energy spectra of the IMF turbulence. The cosmic ray scattering mean free path was calculated from the measured spectrum, solar wind speed, and mean magnetic field by using the quasilinear theory in the slab model. This transport parameter displays a solar cycle variation and is proportional to the amplitude difference of the cosmic-ray diurnal anisotropy