[Paper Review] Wreathes of Magnetism in Rapidly Rotating Suns

Benjamin P. Brown, Matthew K. Browning+3 more|ArXiv.org|Jun 12, 2009

Solar and Space Plasma Dynamics1 references3 citations

TL;DR

This study uses 3D MHD simulations with the ASH code to investigate dynamo action in rapidly rotating solar-type stars, revealing that organized global-scale magnetic 'wreathes' form in the convection zone without a tachocline. Surprisingly, these wreathes sustain persistent dipolar fields and exhibit cyclic activity, challenging the long-held belief that a tachocline is essential for large-scale dynamo action.

ABSTRACT

When our Sun was young it rotated much more rapidly than now. Observations of young, rapidly rotating stars indicate that many possess substantial magnetic activity and strong axisymmetric magnetic fields. We conduct simulations of dynamo action in rapidly rotating suns with the 3-D MHD anelastic spherical harmonic (ASH) code to explore the complex coupling between rotation, convection and magnetism. Here we study dynamo action realized in the bulk of the convection zone for two systems, rotating at three and five times the current solar rate. We find that substantial organized global-scale magnetic fields are achieved by dynamo action in these systems. Striking wreathes of magnetism are built in the midst of the convection zone, coexisting with the turbulent convection. This is a great surprise, for many solar dynamo theories have suggested that a tachocline of penetration and shear at the base of the convection zone is a crucial ingredient for organized dynamo action, whereas these simulations do not include such tachoclines. Some dynamos achieved in these rapidly rotating states build persistent global-scale fields which maintain amplitude and polarity for thousands of days. In the case at five times the solar rate, the dynamo can undergo cycles of activity, with fields varying in strength and even changing polarity. As the magnetic fields wax and wane in strength, the primary response in the convective flows involves the axisymmetric differential rotation, which begins to vary on similar time scales. Bands of relatively fast and slow fluid propagate toward the poles on time scales of roughly 500 days. In the Sun, similar patterns are observed in the poleward branch of the torsional oscillations, and these may represent a response to poleward propagating magnetic field deep below the solar surface.

Motivation & Objective

• To investigate the generation of large-scale magnetic fields in rapidly rotating stars lacking a tachocline, challenging the conventional view that such a shear layer is essential for organized dynamo action.
• To explore how rotation rate influences the structure and stability of global-scale magnetic fields in stellar convection zones.
• To determine whether persistent, cyclic magnetic activity can emerge in the absence of a tachocline, particularly in stars rotating at 3–5 times the current solar rate.
• To examine the coupling between magnetic fields and differential rotation, especially the role of axisymmetric flows in responding to magnetic field variations.

Proposed method

• Simulations are conducted using the 3-D anelastic spherical harmonic (ASH) code to model convection and magnetic dynamo action in rotating spherical shells.
• The model includes a stably stratified interior and a convective zone, with rotation rates set at 3× and 5× the current solar rotation rate.
• Axisymmetric magnetic fields are analyzed via azimuthal averaging, and the poloidal vector potential is used to reconstruct large-scale magnetic structures.
• The induction equation is decomposed into mean and fluctuating components to isolate electromotive forces (EMFs) from mean and turbulent flows.
• Time-dependent evolution of magnetic and velocity fields is tracked to identify cyclic behavior and propagation of differential rotation bands.
• The role of magnetic buoyancy and field storage is assessed through analysis of field topology and energy transfer mechanisms.

Experimental results

Research questions

• RQ1 Can large-scale, organized magnetic fields form in the absence of a tachocline in rapidly rotating stars?
• RQ2 How does increasing rotation rate (to 3× and 5× solar) affect the stability and morphology of global-scale magnetic fields?
• RQ3 Do the simulated dynamo systems exhibit cyclic magnetic activity, and if so, what drives the cycle period?
• RQ4 How do convective flows, particularly axisymmetric differential rotation, respond to changes in magnetic field strength and polarity?
• RQ5 To what extent do poleward-propagating bands of fast and slow fluid flow correlate with deep magnetic field dynamics?

Key findings

• Striking, coherent wreath-like structures of magnetic field form in the convection zone of rapidly rotating stars, even without a tachocline.
• At 5× solar rotation rate, the dynamo produces persistent global-scale dipolar fields that vary in strength and polarity over cycles lasting approximately 1,000 days.
• Magnetic field variations are closely linked to changes in axisymmetric differential rotation, which exhibit poleward-propagating bands of fast and slow flow on timescales of ~500 days.
• The simulations demonstrate that large-scale dynamo action can be sustained purely within the convection zone, challenging the necessity of a tachocline for organized field generation.
• The observed poleward propagation of differential rotation bands closely resembles the torsional oscillations seen in the Sun, suggesting a deep magnetic origin for these phenomena.
• Electromotive forces from mean flows and turbulent fluctuations are found to be key drivers of the dynamo, with significant contributions from the $E_{\mathrm{FI}}$ term involving fluctuating velocity and magnetic fields.