Evidence for a cosmic ray gradient perpendicular to the solar equatorial plane

Abstract

A study has been made of the yearly variation of the cosmic ray intensity for the years 1961–67 inclusive using pressure corrected neutron monitor data from both hemispheres to minimize seasonal meteorological effects. An annual wave is found in the data with an amplitude which varied between 0.2 and 1.0 per cent during the period but which had a sensibly constant phase, the time of maximum being in March. These observations, which are shown to be consistent with the observed heliolatitude distribution of coronal 5303Å emission, indicate the existence of a southerly directed asymmetrical gradient of up to 8 per cent perpendicular to the solar equatorial plane. It is found that the cosmic ray intensity at the Earth is controlled by the solar activity in a narrow band of heliolatitudes ±10° or ±20° centred at the heliolatitude of the Earth. Also, the results indicate that there was a phase lag of 1 ± 1 month between solar activity and the resulting changes in the cosmic ray intensity at the Earth giving a radius for the modulating region of ≲ 10 A.U. during the period of low solar activity considered.