Summary
After an unusually (for recent solar cycles) long absence, sunspots of solar cycle 24 started to appear late in 2009. Cosmic ray activity monitored at Earth has an inverse relationship with the sunspot cycle, being maximum at solar minimum. Earth-surface Neutron Monitors record flux variations of up to 20% or more between solar maximum and solar minimum. By the end of March 2010 the Mawson and Kingston monitors were reporting about a 2.5% decrease in measured flux since the peak values they reached in December 2009. The observation is supported by monitors elsewhere, such as that at McMurdo. This decrease has been particularly fast, and follows the advent of new sunspots more rapidly than has been the case in previous solar cycles. Neutron monitors are subject to several environmental effects and also to instrumental drifts, both in the detectors themselves and in the pulse-level sensitivity of the primary amplifiers. No external calibration techniques are available and consequently long-term stability of absolute detection efficiency depends on intra- and inter-monitor comparisons. These introduce a degree of subjectivity into the estimation of efficiency variations. In spite of these difficulties it appears that the cosmic ray flux at Earth during the recent solar minimum was several percent higher than the maxima recorded at any solar minimum since neutron monitoring began in 1953. Existing 'wisdom' has held that the level reached in previous solar minima represented the maximum cosmic ray density achievable in the inner heliosphere. This clearly requires revision.