The relative intensity of the preferred zone varied from cycle to cycle and, in some individual cycles the preference was not evident. These findings suggest a long-term memory built into the mechanism generating the solar magnetic field responsible for the activity. The solar magnetic field is generated by a dynamo located near the bottom of the convection zone. A particular role is played by a thin tachocline layer on the border between the convection zone and the radiative core. The structure and motions inferred from helioseismic data and models do not, however, indicate any preferred longitude.
Preferred longitudes can arise due to excitation of non-axisymmetric magnetic modes.
We will discuss the conditions of excitation and persistence of non-axisymmetric modes on the Sun implied by the results of our data analyses. The work was performed in part for the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.