Long-term decline of South Pole neutron rates

The count rate recorded by a neutron monitor at South Pole, Antarctica, displays a long-term decline over the 32-year span from 1965 to 1997. The neutron rate follows an 11-year cycle with maxima at times of low solar activity, but the 1997 peak rate was approximately 8% lower than the 1965 peak rate based on 27-d averages. This change is much larger than that recorded by any other neutron monitor. We suggest that the South Pole monitor, owing to its unique position at both high latitude and high altitude (2820 m), has enhanced sensitivity at similar to 1-3 GV relative to a sea level monitor and may be responding to a change in the intensity of primary cosmic rays in this rigidity region. Measurements of cosmic rays made aboard stratospheric balloons and on the IMP-8 spacecraft support the possibility of a long-term change in cosmic ray intensity.