Cosmic ray modulation during the solar activity growth phase of cycle 24

Abstract

Recent years allowed us to study long-term variations in the cosmic ray (CR) intensity at an unusually deep solar activity (SA) minimum between cycles 23 and 24 and during the SA growth phase in cycle 24, which was the cycle when SA was the lowest for the epoch of regular ground-based CR observations since 1951. The intensity maximum, the value of which depends on the particle energy, was observed in CR variations during the period of an unusually prolonged SA minimum: the CR density during the aformentioned period (2009) is higher than this density at previous CR maxima in cycles 19–23 for low-energy particles (observed on spacecraft and in the stratosphere) and medium-energy particles (observed with neutron monitors). After 2009 CR modulation at the SA growth phase was much weaker over three years (2010–2012) than during the corresponding SA growth periods in the previous cycles. The possible causes of this anomaly in CR variations, which are related to the CR residual modulation value at a minimum between cycles 23 and 24 and to variations in SA characteristics during this period, were examined. The contribution of different solar magnetic field characteristics and indices, taking into account sporadic solar activity, has been estimated.