Coupling functions applicable to the underground meson telescopes

A new procedure is proposed for determining the effective primary threshold energy (Eo) applicable to an underground meson telescope located at a moderate depth. Eo so derived is used in Dorman's procedure for calculating the coupling functions for the underground meson telescopes from coupling functions applicable to a sea-level meson telescope. In this manner we have derived the coupling functions for the underground meson telescopes located at 40 and 60 mwe under rock below sea level. The practical usefulness of these functions is demonstrated for the study of the rigidity dependence of the mean solar diurnal variation for the period January 1, 1968, to June 30, 1969, and the Forbush decrease of July 13, 1961. We conclude that the median primary energy of response of an underground meson telescope is much lower than has been believed.