The Source of Magnetic Shear in CME Source Regions

We present a two and three-dimensional numerical magnetohydrodynamic simulations of magnetic flux emergence in three geometries including horizontal layers, arcades and flux ropes. In all cases, the magnetic structures are initially embedded in gravitationally stratified plasma in non-force-free states in which plasma pressure confines the magnetic fields. As the magnetic fields buoyantly rise, they greatly expand in the reduced pressure of the upper atmosphere. In all cases, we find that the legs of ascending bipole loop structures move in opposite horizontal directions drawing the magnetic field parallel to the neutral line. The shearing motions naturally occur as the magnetic field expands in the stratified atmosphere which produces a gradient in the axial field. This gradient results in a horizontal Lorentz force that drives the legs of the bipole loop in opposite directions. The shearing motions transport axial flux and energy from the submerged portion of the field to the expanding portion, which may cause it to violently erupt. This shearing process is very robust and explains the highly sheared state of the magnetic field associated with prominences, flares and coronal mass ejections.