Cosmic-ray solar flare effect of January 28, 1967, as recorded by neutron monitors

On January 28, 1967, at 0830 UT, sea-level neutron monitors at cutoff rigidities Pc ≤ 4.5 Gv recorded an increase presumably associated with a solar flare. From geophysical data and the characteristics of the cosmic-ray increase at earth, it is concluded that this increase was produced by a solar flare located about 60° beyond the west limb of the sun. The primary rigidity spectrum in the rigidity interval 1.5 < P <5.0 Gv at 1200 UT can be represented as exp (−P/Po) with Po = 0.6 Gv. The event was marked by an absence of any large anisotropies in the flux during the onset phase. The intensity-time profile for a sea-level neutron monitor at Pc = 1.0 Gv is consistent with anisotropic diffusion of the solar particles to the earth, with an absorbing boundary at about 3 AU (Burlaga, 1967). The solar flare increase at sea level was followed by large anisotropies in the galactic cosmic-ray flux, which characteristically occur after large solar outbursts.