Long term variation of cosmic ray latitude gradient in the heliosphere

We examine the long-term change in the unidirectional latitude gradient (G(theta)) of galactic cosmic-rays in the heliosphere, by analyzing the "Toward-Away" solar diurnal variation (SDV) of cosmic-ray intensity recorded by a network of Japanese multi-directional muon telescopes during 18 years from 1978 to 1995. In our analysis, we take into account not only the north-south (NS) symmetric SDV (S-sym) but also the NS antisymmetric SDV(Santi-sym), which was first observed by the Nagoya surface muon telescope in 1971-1979 and well confirmed by the two hemisphere observations at Nagoya and Hobart in 1992-1995. The phase of the yearly mean S-sym in space is found at similar to 0500 or similar to 1700 hours local solar time depending on the year, while the phase of Santi-sym is always found at similar to 1700 hours in the northern hemisphere. G(theta) derived from the component of S-sym perpendicular to the interplanetary magnetic field shows no clear variation related to the 11-year solar activity- or 22-year solar magnetic-cycles, but it remains positive after the late 80's implying a higher density of cosmic-rays in the southern hemisphere below the heliospheric current sheet. (C) 1999 COSPAR. Published by Elsevier Science Ltd.