Enhanced sidereal diurnal variation of galactic cosmic rays observed by the two-hemisphere network of surface level muon telescopes

Significant enhancements of the cosmic ray sidereal diurnal variation were observed during the period 1992-1995 by the two-hemisphere network of surfa ce-level multidirectional muon telescopes at Hobart (Tasmania, Australia) a nd Nagoya (Aichi, Japan). The telescopes cover the primary cosmic ray rigid ity range of 50-120 GV. Since the enhancement is less prominent in the high er rigidity range (150-550 GV) covered by the shallow underground observati ons at Misato and Sakashita, it is concluded that the enhancement was cause d by significant solar modulation in the lower energy region. Observed side real diurnal variations, corrected for spurious variations by a procedure p roposed by Nagashima, give a space harmonic vector with amplitude of 0.104 +/- 0.008% at 60 GV and maximum at 6.9 +/- 0.3 hour local sidereal time. Th e time of maximum is consistent with northward streaming of cosmic rays per pendicular to the ecliptic plane. Such a north-south anisotropy is expected from cross-field xi(NS) = -lambda(perpendicular to) G(theta) diffusion if both the cross-field mean-free-path lambda(perpendicular to) and the southw ard directed unidirectional latitudinal density gradient G(theta) have larg e enough magnitudes. It is shown that the sector-dependent solar diurnal va riations are also enhanced in the period, consistent with G(theta) being di rected south of the ecliptic plane. Magnitudes of G(theta) and lambda(perpe ndicular to) derived from the observations are discussed.