Observed Joys law of Bipolar Magnetic Region tilts at the emergence supports the thin flux tube model
arXiv:2511.03558 · doi:10.3847/2041-8213/ae1c1b
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 Joys 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 Joys 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 Joys 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 Joys 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 Joys law observed during the emergence phase, which reduces in the post emergence phase, reflects the interaction of the vigorous turbulent convection with the rising flux tubes in the near surface layer.
arXiv comment12 pages, 9 figures. Accepted for publication in ApJL. Comments are welcome
References in corpus (21)
- Solar cycle prediction
- The crucial role of surface magnetic fields for the solar dynamo
- Bipolar Magnetic Regions on the Sun: Global Analysis of the SOHO/MDI Data Set
- The Rise of Active Region Flux Tubes in the Turbulent Solar Convective Envelope
- Three dimensional non-linear evolution of a magnetic flux tube in a spherical shell: Influence of turbulent convection and associated mean flows
- Comparing Simulations of Rising Flux Tubes Through the Solar Convection Zone with Observations of Solar Active Regions: Constraining the Dynamo Field Strength
- Models for the long-term variations of solar activity
- Forward modelling of brightness variations in Sun-like stars I. Emergence and surface transport of magnetic flux
- Magnetic field dependence of bipolar magnetic region tilts on the Sun: Indication of tilt quenching
- A low upper limit on the subsurface rise speed of solar active regions
- On rising magnetic flux tube and formation of sunspots in a deep domain
- Recovery from Maunder-like Grand Minima in a Babcock--Leighton Solar Dynamo Model
- Tilt Angle and Footpoint Separation of Small and Large Bipolar Sunspot Regions Observed with HMI
- Measurements of Solar Differential Rotation Using the Century Long Kodaikanal Sunspot Data
- Average motion of emerging solar active region polarities II: Joy's law
- Analysis of BMR tilt from AutoTAB catalog: Hinting towards the thin flux tube model?
- AutoTAB: Automatic Tracking Algorithm for Bipolar Magnetic Regions
- Active-Region Tilt Angles from White-Light Images and Magnetograms: The Role of Magnetic Tongues
- Low-latitude magnetic flux emergence on rapidly rotating solar-type stars
- Coriolis force acting on near-surface horizontal flows during simulations of flux emergence produces a tilt angle consistent with Joy's law on the Sun
- Average solar active region: I. Intensities, velocities, and the photospheric magnetic field