[Paper Review] Shear Flows Driven by the Lorentz Force: An Energy Source for Coronal Mass Ejections and Flares

W. B. Manchester|ArXiv.org|Dec 18, 2008

Solar and Space Plasma Dynamics2 references3 citations

TL;DR

This paper proposes that shear flows in solar active regions—key drivers of coronal mass ejections (CMEs) and flares—are physically driven by the Lorentz force arising from emerging, expanding magnetic flux in a gravitationally stratified atmosphere. Simulations demonstrate that these self-organized shear flows transport axial magnetic flux and energy from the solar interior to the corona, inducing loss of equilibrium and triggering fast CME-like eruptions, providing a unified physical mechanism for eruptive solar activity.

ABSTRACT

Shear flows have been prescribed in numerical models of coronal mass ejections and flares for decades as a way of energizing magnetic fields to erupt. While such shear flows have long been observed in the solar atmosphere, until recently, there was no compelling physical explanation for them. This paper will discuss the discovery that such shear flows are readily explained as a response to the Lorentz force that naturally occurs as bipolar magnetic fields emerge and expand in a gravitationally stratified atmosphere. It will be shown that shearing motions transport axial flux, and magnetic energy from the submerged portion of the field to the expanding portion, strongly coupling the solar interior to the corona. This physical process explains active region shear flows and why the magnetic field is found to be nearly parallel to photospheric polarity inversion lines where prominences form. Finally, shear flows driven by the Lorentz force are shown to produce a loss of equilibrium and eruption in magnetic arcades and flux ropes offering a convincing explanation for CMEs and flares.

Motivation & Objective

• To resolve the long-standing mystery of the physical origin of large-scale shear flows observed in solar active regions.
• To establish a self-consistent mechanism linking magnetic flux emergence in the solar interior to coronal energy buildup and eruption.
• To demonstrate that shear flows driven by the Lorentz force can produce loss of equilibrium and drive CMEs and flares without artificial forcing.
• To unify observational evidence of magnetic shear, prominence formation, and eruptive activity under a single physical framework.

Proposed method

• Simulate axisymmetric magnetic flux emergence into the corona using a 3D MHD model with gravity and stratified plasma.
• Track the evolution of horizontal shear velocity (Ux) and magnetic field lines to visualize shearing motions at the polarity inversion line.
• Model the Lorentz force arising from non-uniform expansion of emerging flux, particularly the imbalance in magnetic tension forces.
• Use 3D simulations of coronal arcades and flux ropes to study eruption dynamics under shear flows driven by the Lorentz force.
• Compare simulation results with observations of Hα fibrils, EUV loops, X-ray sigmoids, and TRACE two-ribbon flares.
• Analyze scaling of eruption speed and size with system size to assess feasibility of CME-scale eruptions.

Experimental results

Research questions

• RQ1 What physical mechanism generates the large-scale shear flows observed in solar active regions prior to CMEs and flares?
• RQ2 How does the Lorentz force from emerging magnetic flux lead to the formation of sheared magnetic fields near the polarity inversion line?
• RQ3 Can shear flows driven by the Lorentz force alone produce loss of equilibrium and drive fast CME-like eruptions?
• RQ4 Why is magnetic shear consistently found to correlate with CME and flare productivity?
• RQ5 How is magnetic energy and flux transported from the solar interior to the corona via this mechanism?

Key findings

• Shear flows are self-organized by the Lorentz force during flux emergence, explaining their spontaneous occurrence in active regions.
• The shearing motion results from horizontal flows driven by magnetic tension forces, forming large-amplitude shear Alfvén waves.
• Magnetic field lines near the polarity inversion line evolve to become nearly parallel to the line, matching observations of filaments and fibrils.
• Shear flows transport axial magnetic flux and energy from the submerged portion of the field to the expanding corona, coupling the convection zone to the corona.
• Simulations show that shear flows driven by the Lorentz force induce loss of equilibrium and trigger violent eruptions, with peak speeds reaching 150 km/s and shock formation.
• Eruption velocity scales favorably with system size, suggesting that larger-scale simulations could produce CMEs of observed magnitude.