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A. Ruzmaikin, J. Feynman, M. Neugebauer, E.J. Smith (JPL)
Several studies of the distribution of activity on the solar surface indicate preferred longitude patterns sometimes lasting longer than a solar cycle. We have analyzed 150 years of daily sunspot number and find evidence for a preferred longitude zone that persisted over 13 solar cycles. The relative intensity of the preferred zone varied from cycle to cycle and, in some individual cycles the preference was not evident. This complements the recent analysis of the measured solar wind speed and interplanetary magnetic field over more than three solar cycles, which implied that the solar magnetic moment returned to the same longitude after each reversal (Neugebauer et al., JGR, 105, 2315, 2000).
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. These modes have been invoked for explanation of clustering of emerging activity (Ruzmaikin, Solar Phys., 182, 1, 1998). The solar magnetic moment is determined by the sum of the axisymmetric (m = 0) and the first non-axisymmetric (m = 1) mode. 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.