Observed Joy’s Law of Bipolar Magnetic Region Tilts at Emergence Supports the Thin Flux Tube Model

Abstract Bipolar sunspots, or more generally, bipolar magnetic regions (BMRs), are the dynamic magnetic regions that appear on the solar surface and are central to solar activity. One striking feature of these regions is that they are often tilted with respect to the equator, and this tilt increases with the latitude of appearance, popularly known as Joy’s law. Although this law has been examined for over a century through various observations, its physical origin is still not established. An attractive theory that has been put forward behind Joy’s law is the Coriolis force acting on the rising flux tube in the convection zone, which has been studied using the thin flux tube model. However, observational support for this theory is limited. If the Coriolis force is the cause of the tilt, then we expect BMRs to hold to Joy’s law at their initial emergence on the surface. By automatically identifying the BMRs over the last two solar cycles from high-resolution magnetic observations, we robustly capture their initial emergence signatures on the surface. We find that from their appearance, BMRs exhibit tilts consistent with Joy’s law. This early tilt signature of BMRs suggests that the tilt is developed underneath the photosphere, driven by the Coriolis force and helical convection, as predicted by the thin flux tube model. Considerable scatter around Joy’s law observed during the emergence phase, which reduces in the postemergence phase, reflects the interaction of the vigorous turbulent convection with the rising flux tubes in the near-surface layer.