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Comparisons between theory and observation of active region tilts
Fisher, G. H.; Fan, Y.; Howard, R. F.
AA(University of California, Berkeley, CA), AB(National Solar Observatory, Tucson, AZ), AC(National Solar Observatory, Tucson, AZ)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 438, no. 1, p. 463-471 (ApJ Homepage)
Publication Date:
Solar Physics
NASA/STI Keywords:
Magnetohydrodynamics, Solar Magnetic Field, Sunspots, Computerized Simulation, Numerical Analysis, Solar Convection (Astronomy), Solar Oscillations
Bibliographic Code:


Active regions in the Sun are generally tilted relative to the azimuthal direction, with the leading side being closer to the equator than the following side. This tilts is known to increase with latitude. Recently, theoretical calculations of the dynamics of emerging, initially toroidal active-region flux tubes have been done, showing that the observed tilts can be explained by the Coriolis force acting on a diverging flow field in emerging flux loops. The calculations of Fan, Fisher, & McClymont predict that alpha proportional to Phi1/4 B0 exp -5/4 sin theta, where alpha is the tilt angle of the active region, B0 is the magnetic field strength of the active-region flux tube near the base of the convection zone, and phi is the amount of magnetic flux in the tube. We compare these theoretical predictions with the behavior of a sample of 24,701 sunspot groups observed at Mount Wilson over a period of 68 yr, using the polarity separation distance d as a proxy for phi. Our major findings are given.

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