Power spectral analysis of cosmic ray intensity recorded by neutron monitors at Calgary and Deep River, Alberta, was carried out over a wide range of frequencies from 3×10−9 Hz to 6×10−6 Hz (periodicities from 2 days to 11 years) during the epoch 1965–1989 and revealed different behavior of the power spectral density for the three ranges of frequency domains. At low frequencies corresponding to the periodicity T ≈ 20 months the power spectrum shows an abrupt change in the level and profile of power spectral density (PSD). This indicates that the processes responsible for the long‐period variations are different from the one which causes short‐period variations. At middle frequencies corresponding to periodicities between 6 and 18 months, the PSD indicates that the periodicities are not stable and show a transient character over the entire epoch of analysis. The PSD for periods T ≈ 27 days indicates an 11 year recurring tendency related to solar activity. The PSD for periods T > 27 days indicates a depression during the minimum of solar activity showing a 22 year recurring tendency and suggests that it is controlled by drift effects which are correlated to the solar polar field reversal. For higher frequencies f > 2×10−6 Hz corresponding to periods T < 6 days, the slope of the PSD is found to be similar to that of the interplanetary magnetic field inhomogeneties measured at different radial distances between 1 and 30 AU.

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The relationship between the solar and geomagnetic activity has been investigated. The results show clearly that the geomagnetic activity represented by Kp index reveals two maxima for each single maximum of solar activity as represented by sunspot numbers and indicate that the geomagnetic activity has two discrete components attributed to solar flare effects and corotating streams.

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A series of spectacular cosmic ray events, which included two relativistic solar particle enhancements and three major Forbush decreases, was registered by ground-based cosmic ray monitors beginning on August 4, 1972. Among these, the Forbush decrease that occurred on August 4-5 exhibited extremely interesting and complex behavior, the prominent features of which are a preincrease (PI-1) prior to the largest Forbush decrease (FD-2) during the recovery of which an abrupt universal time increase (PI-2) occurred. Large N-S and E-W anisotropies were observed during the entire Forbush decrease event. The rigidity spectra for both FD-2 and PI-2 had practically the same exponent of -1.2 + 0.2 and an upper cutoff rigidity of about 50-60 GV, and the anisotropy during both PI-1 and PI-2 was from the sunward direction. This paper describes the detailed observational features and•presents-a unified model to explain these in terms of a transient modulating region associated with the passage of a shock front. In this model, the reflection of particles from the approaching shock front accounts for the preincrease (PI-1), whereas the early onset of FD-2 from the antisun direction is caused by the occultation of particle trajectories reaching the earth from that direction while the detectors looking along the sunward direction are still sampling albedo particles reflected from the shock front. The main Forbush decrease occurs as the shock front, containing tangled magnetic fields with large-scale tangential discontinuities, sweeps past the earth. The particles, diffusing into the cavity as they are swept by the solar wind, get 'piled up' behind the tangled field region causing the abrupt increase (PI-2). Evidence from interplanetary plasma, radio, and field measurements is provided in support of the model wherever possible. Duringthe declining phase of the current solar cycle, a series of intense solar flares erupted in August 1972 from an active region (McMath plage region 11976) on the solar disk; as a result, severe cosmic ray disturbances on the earth [Pomerantz and Duggal, 1973a] accompanied by quite spectacular visual auroras, geomagnetic storms, radio blackout, and a host of other terrestrial effects were caused. The time