Long term variation of cosmic ray latitude gradient in the heliosphere

Abstract

We examine the long-term change in the unidirectional latitude gradient ( G θ ) 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 anti-symmetric SDV ( S anti- 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 ∼0500 or ∼1700 hours local solar time depending on the year, while the phase of S anti- sym is always found at ∼1700 hours in the northern hemisphere. G θ 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.