Vitaly A. Kudryavtsev

The relationship between the sidereal variation at 570 hg/cm2 underground and different levels of solar activity

Abstract This paper presents data on the sidereal variation of muons recorded at 570 hg/cm 2 , corresponding to primary rigidities around 1.8 TV. The record investigated covers a period of five years, from September 1985, near solar minimum, to September 1990, near solar maximum. The features of the observed sidereal variation histogram are related to the changing heliospheric conditions. Three conclusions emerge: (1) The Fourier reconstruction of the 24-h histogram requires a large number of harmonics, since the first and the second (diurnal and semidiurnal) are often outranked by higher order Fourier harmonics; (2) the anisotropy observed during the period of minimum solar activity has galactic origin; (3) as the level of solar activity increases the information on this anisotropy is altered by solar modulation processes.