Tilt Angle of Bipolar Active Regions from Solar Cycle 23

Abstract

Active regions (ARs) are the photospheric manifestation of magnetic flux ropes (FRs)emerging from the solar interior. A key parameter in their evolution is the tilt angle, de-fined as the inclination of the AR polarity axis relative to the solar equator. Despite itscentral role in flux-transport dynamo models, tilt measurements are affected by systematicbiases—most notably magnetic tongues—particularly during the emergence phase. In thiswork we analyze tilt estimates for 108 bipolar active regions from Solar Cycle 23 usingtwo methodologies: (1) the conventional magnetic barycenter method and (2) a BayesianFR model that fits observed magnetograms with a synthetic model of an emerging twistedtoroidal flux tube. We find notable differences between the two estimates, ranging fromabout 30◦ at early stages of emergence to ∼ 10◦ near the end. Our analysis shows that Joy’slaw is already present at the earliest stages of AR emergence, although the tilt dispersionis significantly larger at these times. Tilt rotations are strongest during the early emergencephase, with no preferred sense of rotation, and decrease as ARs approach their maximummagnetic flux, suggesting an important role of near-surface convective motions. We also findthat the quantitative characterization of Joy’s law depends on both the magnetic flux of ARsand the method used to estimate their tilt. The Bayesian framework adopted here provides arobust basis for future analyses by allowing known sources of scatter to be explicitly mod-eled within a consistent statistical formalism.