Magnetic Twist and Writhe of Active Regions: On the Origin of Deformed Flux Tubes

Abstract

We study the long term evolution of a set of 22 bipolar active regions (ARs) in which the main photospheric polarities are seen to rotate one around the other during several solar rotations. We first show that differential rotation is not at the origin of this large change in the tilt angle. A possible origin of this distortion is the nonlinear development of a kink-instability at the base of the convective zone; this would imply the formation of a non-planar flux tube which, while emerging across the photosphere, would show a rotation of its photospheric polarities as observed. A characteristic of the flux tubes deformed by this mechanism is that their magnetic twist and writhe should have the same sign. From the observed evolution of the tilt of the bipoles, we derive the sign of the writhe of the flux tube forming each AR; while we compute the sign of the twist from transverse field measurements. Comparing the handedness of the magnetic twist and writhe, we find that the presence of kink-unstable flux tubes is coherent with no more than 35\% of the 20 cases for which the sign of the twist can be unambiguously determined. Since at most only a fraction of the tilt evolution can be explained by this process, we discuss the role that other mechanisms may play in the inferred deformation. We find that 36\% of the 22 cases may result from the action of the Coriolis force as the flux tube travels through the convection zone. Furthermore, because several bipoles overpass in their rotation the mean toroidal (East-West) direction or rotate away from it, we propose that a possible explanation for the deformation of all these flux tubes may lie in the interaction with large-scale vortical motions of the plasma in the convection zone, including also photospheric or shallow sub-photospheric large scale flows.

Published in Astronomy and Astrophysics

References in corpus (3)

• The physics of twisted magnetic tubes rising in a stratified medium: two dimensional results
• The Counter-kink Rotation of a Non-Hale Active Region
• The Three-dimensional Evolution of Rising, Twisted Magnetic Flux Tubes in a Gravitationally Stratified Model Convection Zone

Cited by in corpus (30)

• Flare-productive active regions
• The solar magnetic field
• Eruption of a Kink-Unstable Filament in Active Region NOAA 10696
• The Magnetic Field in the Solar Atmosphere
• Linking two consecutive nonmerging magnetic clouds with their solar sources
• Non-neutralized Electric Current Patterns in Solar Active Regions: Origin of the Shear-Generating Lorentz Force
• Numerical Simulations of Flare-productive Active Regions: delta-sunspots, Sheared Polarity Inversion Lines, Energy Storage, and Predictions
• Flux cancellation and the evolution of the eruptive filament of 2011 June 7
• Evolution of solar and stellar dynamo theory
• Numerical Study on Emergence of Kinked Flux Tube for Understanding of Possible Origin of Delta-spot Regions
• 3D MHD modeling of twisted coronal loops
• Reconstruction of an Inductive Velocity Field Vector from Doppler Motions and a Pair of Solar Vector Magnetograms
• Evidence of Twisted flux-tube Emergence in Active Regions
• Average motion of emerging solar active region polarities II: Joy's law
• Evolution of Anemone AR NOAA 10798 and the Related Geo-Effective Flares and CMEs
• The Role of Twist in Kinked Flux Rope Emergence and Delta-Spot Formation
• A Catalog of Bipolar Active Regions Violating the Hale Polarity Law, 1989-2018
• Subsurface Flows in and Around Active Regions with Rotating and Non-rotating Sunspots
• Synthetic solar cycle for active regions violating the Hale's polarity law
• On Asymmetry of Magnetic Helicity in Emerging Active Regions: High Resolution Observations
• Active-Region Tilt Angles from White-Light Images and Magnetograms: The Role of Magnetic Tongues
• Divergent Horizontal Sub-surface Flows within Active Region 11158
• The extremely strong non-neutralized electric currents of the unique solar active region NOAA 13664
• Correcting the effect of magnetic tongues on the tilt angle of bipolar active regions
• Solar active region evolution and imminent flaring activity through a color-coded visualization of photospheric vector magnetograms
• Modeling Magnetic Flux Emergence in Bipolar Active Regions
• Bayesian approach for modeling solar active region global magnetic parameters
• Eruption of the magnetic flux rope in a fast decayed active region
• Solar Interior
• Thermal Shielding of an Emerging Active Region