... The main periodicity in the Sun 's activity is the 11-year cycle called the solar cycle. ... During the cycle, changes occur in the Sun's internal magnetic field and in the surface disturbance level. ... At the beginning of a cycle the solar magnetic field resembles a dipole which axis is aligned with Sun's rotation axis. ... The sunspot number - and Sun's activity - is at maximum in the middle of a cycle, when the Sun's internal magnetic field is most chaotic. ... Space Physics Text Book of Oulu ...