Tilt of sunspot bipoles in Solar Cycles 15 to 24

Abstract

We use recently digitized sunspot drawings from Mount Wilson Observatory to investigate the latitudinal dependence of tilt angles of active regions and its change with solar cycle. The drawings cover the period from 1917 to present and contain information about polarity and strength of magnetic field in sunspots. We identify clusters of sunspots of same polarity, and used these clusters to form ``bipole pairs''. The orientation of these bipole pairs was used to measure their tilts. We find that the latitudinal profile of tilts does not monotonically increase with latitude as most previous studies assumed, but instead, it shows a clear maximum at about 25--30 degree latitudes. Functional dependence of tilt ($\gamma$) on latitude ($\phi$) was found to be $\gamma =(0.20\pm 0.08)\sin (2.80\phi )+(-0.00\pm 0.06)$. We also find that latitudinal dependence of tilts varies from one solar cycle to another, but larger tilts do not seem to result in stronger solar cycles. Finally, we find the presence of a systematic offset in tilt of active regions (non-zero tilts at the equator), with odd cycles exhibiting negative offset and even cycles showing the positive offset.

14 pages, 5 figures, accepted for publication in Solar Physics

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• Reconstructing solar magnetic fields from historical observations V. Sunspot magnetic field measurements at Mount Wilson Observatory
• Evolution of the Sun's Polar-fields and the Poleward Transport of Remnant Magnetic Flux
• The effect of telescope aperture, scattered light, and human vision on early measurements of sunspot and group numbers
• The mean tilt of sunspot bipolar regions: theory, simulations and comparison with observations
• Magnetic fluxes of solar active regions of different magneto-morphological classes: I. Cyclic variations
• Active-Region Tilt Angles from White-Light Images and Magnetograms: The Role of Magnetic Tongues
• Correcting the effect of magnetic tongues on the tilt angle of bipolar active regions
• Bayesian approach for modeling solar active region global magnetic parameters
• Large Scale Properties of Tilt of Sunspot Groups and Joy's Law Near The Solar Equator
• Modeling Magnetic Flux Emergence in Bipolar Active Regions
• Relationship Between the Tilt Angle of Sunspot Group and the Properties of the Next Solar Cycle
• Solar Interior
• Solar active region scaling laws revisited
• Comparison Between Cycle-to-Cycle Variations in the Coefficient of Joy's Law and Covariance of Rotation Residuals and Meridional Motions of Sunspot Groups
• Low-Latitude Sunspot Group Tilt Angles in 15-24 Activity Cycles