Understanding the solar cycle remains as one of the biggest problems in solar physics. It is also one of the
oldest. Several key features of the solar cycle have been reviewed here and must be explained by any viable
theory or model.
- The solar cycle has a period of about 11 years but varies in length with a standard deviation
of about 14 months.
- Each cycle appears as an outburst of activity that overlaps with both the preceding and
following cycles by about 18 months.
- Solar cycles are asymmetric with respect to their maxima – the rise to maximum is shorter
than the decline to minimum and the rise time is shorter for larger amplitude cycles.
- Big cycles usually start early and leave behind a short preceding cycle and a high minimum of
- Sunspots erupt in low latitude bands on either side of the equator and these bands drift toward
the equator as each cycle progresses.
- The activity bands widen during the rise to maximum and narrow during the decline to
- At any time one hemisphere may dominate over the other but the northern and southern
hemispheres never get completely out of phase.
- Sunspots erupt in groups extended in longitude but more constrained in latitude with one
magnetic polarity associated with the leading (in the direction of rotation) spots and the
opposite polarity associated with the following spots.
- The magnetic polarities of active regions reverse from northern to southern hemispheres and
from one cycle to the next.
- The polar fields reverse polarity during each cycle at about the time of cycle maximum.
- The leading spots in a group are positioned slightly equatorward of the following spots and
this tilt increases with latitude.
- Cycle amplitudes exhibit weak quasi-periodicities like the 7 to 8-cycle Gleissberg Cycle.
- Cycle amplitudes exhibit extended periods of inactivity like the Maunder Minimum.
- Solar activity exhibits quasi-periodicities at time scales shorter than 11 years.
- Predicting the level of solar activity for the remainder of a cycle is reliable 2 – 3 years after
- Predictions for the amplitude of a cycle based on the Sun’s polar field strength or on
geomagnetic activity near cycle minimum are significantly better than using the climatological