The Maunder minimum: a mixed-parity dynamo mode?

It is well-known that the number of active regions (sunspots and facula) in each solar hemisphere is not the same at a given time. Such north-south asymmetries have been recently considered by Jennings & Weiss (1991) and Jennings (1991). In a non linear regime, an azimuthal field can be generated from a dominant dipole and a weak quadrupole mode. This leads to small north-south asymmetries in the field strength. A very interesting aspect of the nonlinear dynamo equations is the existence of another solution for which the dipole and the quadrupole component are similar in strength. This leads to a situation with almost no sunspot activity in one hemisphere and a butterfly diagram restricted to a single hemisphere. Such a situation did occur during the Maunder minimum, from about 1660 to 1704, where sunspots were sighted in a single hemisphere and within a narrow latitude band hardly exceeding 20deg.