Unidirectional cosmic-ray intensities and their variation with latitude

Abstract

Geomagnetic cosmic-ray effects are of especial interest because of their bearing upon the problem of the nature of the primary cosmic radiation. As the writer has previously pointed out (Phys. Rev., v. 45, p. 569, 1934, and v. 48, p. 287, 1935), the variations of intensity with latitude depend upon the sum of the positive and negative electrically-charged components and the west-east asymmetry depends upon their difference. From measurements of both effects it is possible to determine the intensity of each of the two charged components. The accuracy of the determination depends upon the accuracy of the measurements of the two magnetic effects and upon the accuracy of the assumptions underlying the theory of the effects, from which the energy-limits are determined. If the assumption is made that the Earth's field is that of an eccentric dipole, the theory of Lemaitre and Vallarta (Phys. Rev., v. 49, p. 719, 1936, and previous articles referred to therein) may be applied, but on this basis inconsistencies in the estimates of the composition of the primary cosmic radiation arise when the experimental results from different longitudes are considered. Such inconsistencies appear to be greater than can be accounted for by the experimental errors and they lead to the suspicion that local anomalies in the Earth's magnetic field, not specified by the field of the dipole, are having an important influence on the observed geomagnetic cosmic-ray effects. This suspicion constituted one of the reasons for making unidirectional measurements of the cosmic-ray latitude-effect, for it was realized that rays from the vertical direction should be more subject to such local effects than those incident from other directions. There was also the need for quantitative measurements of the latitude-effect from particular directions, notably the vertical direction, in terms of the sensitivity of the coincidence-counter train, for with this instrument there is no correction to be made for the absorption of the cosmic rays in a lead shield such as is used in the electro scope-technique and it is with this instrument that asymmetries are measured.