The November 1970 Forbush decrease investigation at the cut-off rigidity where the latitude curve changes its curvature

The first recorded and analysed cosmic ray event since the Athens 3 NM-64 came into operation, that of 7 November 1970, is presented. The relative amplitude of the Forbush Decrease reached a value of (4.5 ± 0.3)% peak to peak which is comparatively large at a cut-off rigidity of 8.7 GV (Shea et al., 1968).According to the differential method (Amaldi et al., 1963 ; Bachelet et al, 1972) it has been computed that the slope of the primary differential rigidity spectrum varied at the Athens cut-off rigidity during the above mentioned F.D. by (6.3 ± 0.8)%.This information repairs the omission which was in the threshold rigidity region (8 ± 2) GV, which is quite important because the latitude curves change the algebraic sign of their curvatures (Geranios, 1971).