Hale cycle in solar-rotation related recurrence of galactic cosmic rays
A. Gil, K. Mursula
We study the long-termchange of the amplitude of the first (27-day, A27) and second (14-day, A14) harmonics of the galactic cosmic rays (GCR) recurrent variation connected to the solar rotation period. Using two high-latitude neutron monitors, Oulu and Apatity, we verify the dominant 11-year cycle in A27 and A14 and their systematic 22-year variation (Hale cycle) during sunspot minima, with higher values during the positive polarity (A > 0) minima than during negative (A< 0). We show that the A27 and A14 amplitudes during sunspot minima exhibit a slight declining trend. We suggest that this declining trend is due to the weakening of the solar polar magnetic field during the previous four solar cycles and the ensuing latitudinal widening of the heliospheric current sheet (HCS) region. After trend removal the amplitudes of the 22-year variation increase, and are typically about 15-30 % for A27 and about 30-45% for A14. Here we quantify the 22-year cyclicity in A27 and A14 in a robust way for last five solar minima and give a physical explanation to the earlier ideas about the effect of drifts to produce the observed phase of the 22-year cyclicity.