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J. Collord, H. B. Snodgrass (Lewis \& Clark College)
We do a multipole expansion of the photospheric magnetic field, determined by least-squares fits to the radial component plotted on Carrington maps of Mount Wilson magnetograph data. We study these moments in each hemisphere separately and also for the Sun as a whole, and follow their evolution over three solar cycles. The axial and equatorial components of the dipole each have a roughly 21-yr. sinusoidal variation and, as expected, are 90o out of phase. The equatorial component is strongest around solar maximum, and its direction can suddenly shift. Between such shifts, it rotates at a rate that varies during the cycle. The north and south axial components reflect the different times of the north and south polar field reversals, and also a persistent north-south asymmetry, for the southern axial dipole lags in its time development, but is stronger at maximum. Although the polar reversals seem to involve some `redirecting' of the hemispheric dipoles, our evidence suggests that the field reversals cannot be thought of as rotations of these dipoles. The behavior of the quadrupole moments is also discussed, as is the overall method for doing this expansion.
This work was supported by NSF Grant AST9416999.
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