Longterm variation in the observed mean multiplicity in 9-NM-64 Oulu, Finland

The measurements of the mean multiplicity made by 9-NM-64 Neutron Monitor in Oulu, Finland, during the period November 1965 - April 1968 have been used in a longterm study of the primary cosmic ray spectral modulation. In calculating the mean multiplicities, the usual procedure (correlations for finite gating time and overlapping events) has been employed, and the values have also been corrected for the atmospheric pressure and the atmospheric temperature (at 100 mb pressure level), using the correction factors -0.00023 1/mb and 0.00022 1/0C, respectively. The latter factor has been estimated from the contribution of the stopping muons to the counting rate of the NM. A trend of the mean multiplicity towards higher values has been noted, which indicates a hardening of the primary cosmic ray spectrum with the increasing solar activity. The mean multiplicity, however, also shows some variation in the opposite phase with the atmospheric temperature. (author)