On the origin of the solar hemispherical helicity rules: Simulations of the rise of magnetic flux concentrations in a background field

Abstract

Solar active regions and sunspots are believed to be formed by the emergence of strong toroidal magnetic flux from the solar interior. Modeling of such events has focused on the dynamics of compact magnetic entities, colloquially known as "flux tubes", often considered to be isolated magnetic structures embedded in an otherwise field-free environment. In this paper, we show that relaxing such idealized assumptions can lead to surprisingly different dynamics. We consider the rise of tube-like flux concentrations embedded in a large-scale volume-filling horizontal field in an initially quiescent adiabatically-stratified compressible fluid. In a previous letter, we revealed the unexpected major result that concentrations that have their twist aligned with the background field at the bottom of the tube are more likely to rise than the opposite orientation (for certain values of the parameters). This bias leads to a selection rule which, when applied to solar dynamics, is in agreement with the observations known as the solar hemispheric helicity rule(s) (SHHR). Here, we examine this selection mechanism in more detail than was possible in the earlier letter. We explore the dependence on the parameters via simulations, delineating the Selective Rise Regime (SRR), where the bias operates. We provide a theoretical model to predict and explain the simulation dynamics. Furthermore, we create synthetic helicity maps from Monte Carlo simulations to mimic the SHHR observations and demonstrate that our mechanism explains the observed scatter in the rule and its variation over the solar cycle.

arXiv comment38 pages, 19 figures, accepted for publication in ApJ

References in corpus (17)

• Persistent Magnetic Wreaths in a Rapidly Rotating Sun
• A New Dynamical Mean-Field Dynamo Theory and Closure Approach
• The physics of twisted magnetic tubes rising in a stratified medium: two dimensional results
• Buoyant Magnetic Loops Generated by Global Convective Dynamo Action
• Buoyant Magnetic Loops in a Global Dynamo Simulation of a Young Sun
• Emergence of magnetic flux generated in a solar convective dynamo. I: Formation of Sunspots and Active regions, and Origin of Their Asymmetries
• On the Connection between Mean Field Dynamo Theory and Flux Tubes
• Hemispheric Helicity Trend for Solar Cycle 24
• Dynamo action and magnetic buoyancy in convection simulations with vertical shear
• Alpha effect due to buoyancy instability of a magnetic layer
• Development of twist in an emerging magnetic flux tube by poloidal field accretion
• The Nonlinear Evolution of Instabilities Driven by Magnetic Buoyancy: A New Mechanism for the Formation of Coherent Magnetic Structures
• Bihelical spectrum of solar magnetic helicity and its evolution
• Spectral magnetic helicity of solar active regions between 2006 and 2017
• The Rise of a Magnetic Flux Tube in a Background Field: Solar Helicity Selection Rules
• Generation of twist on magnetic flux tubes at the base of the solar convection zone
• Generating buoyant magnetic flux ropes in solar-like convective dynamos

Cited by in corpus (3)

• Impact of subsurface convective flows on the formation of sunspot magnetic field and energy build-up
• The Rise of Buoyant Magnetic Structures through Convection with a Background Magnetic Field
• Theoretical and Observational Evidence for Coriolis Effects in Coronal Magnetic Fields Via Direct Current Driven Flaring Events