Directional dependence of atmospheric temperature effects on cosmic-ray muons at sea-level

Abstract

The coefficient of atmospheric temperature effects on the hard component of cosmic ray at sea level is derived as a function of the cut-off energy and the aperture of measuring instrument and of the atmospheric depth of reference level. The derivation of these coefficients is primarily based upon the solutions of diffusion equations for cosmic-ray mesons in the standard atmosphere, taking the curvature of isobar levels into account. Secondly, the influence of geomagnetic deflexion of muons in the atmosphere is also considered. The main feature of these coefficients is generally in agreement with Dorman's results, except for large zenith angle. Among the results, the following points are to be noted: (i) There is a direction of maximum negative temperature effect around the zenith angle of 75°, which shifts slightly towards larger angle for higher energy and decreases with the height of reference level, (ii) The coefficient of total effect (positive and negative temperature effects) for small zenith angle is nearly constant against atmospheric depth, (iii) Influence of geomagnetic deflexion of muons upon the positive temperature effect is negligible even at the geomagnetic equator, but it is not negligible for the negative effect; the absolute value of the coefficient becomes larger for positive muons than for negative muons arriving from the east direction, and the change is reversed for the muons from west, (iv) Contribution of cosmic ray K-mesons to the atmospheric effect is to suppress the increase of positive temperature coefficient with increase of energy, but the effect seems undetectable due to the existence of an upper limit of magnitude of this effect.