Date:
Tue, 31/12/201312:30-13:30
Magnetic Instabilities and Resulting Energy Conversion in Astrophysics
Because the universe is primarily composed of plasma, the interaction of plasmas and magnetic fields is of great importance for astrophysics. In this talk, I discuss our recent work investigating two such instabilities and examining their possible effects on astrophysical objects. First, we investigate magnetic reconnection and particle acceleration in moderately magnetized relativistic pair plasmas with three-dimensional particle-in-cell simulations of a kinetic-scale current sheet. We find that in three dimensions the tearing instability produces a network of interconnected and interacting magnetic flux ropes. In its nonlinear evolution, the current sheet evolves toward a three-dimensional, disordered state in which the resulting flux rope segments contain magnetic substructure on kinetic scales and sites of temporally and spatially intermittent dissipation.
We find that reconnection produces significant particle acceleration, primarily due to the electric field in the X-line regions between flux ropes; the resulting particle energy spectrum can extend to high Lorentz factors. We find that the highest energy particles are moderately beamed within $\sim30^\circ-40^\circ$ of the direction of acceleration. Second we derive a dispersion relation and calculate growth rates for triply-diffusive nonaxisymmetric instabilities including the magnetorotational instability (MRI) throughout the Sun, accounting for the effects of both shear and convective buoyancy. The overall instability has unstable modes throughout the convection zone and at colatitudes $\theta<53 \degrees$ in the tachocline. The instability contains three classes of modes: large-scale hydrodynamic convective modes, large-scale hydrodynamic shear modes, and small-scale MRI shear modes. While large-scale convective modes are the fastest-growing modes in most of the convective zone, MRI modes are important in both stably stratified and convectively unstable locations near the tachocline at colatitudes $\theta<53 \degrees$. We find that nonaxisymmetric MRI modes typically grow faster than axisymmetric MRI modes. We consider the saturation of magnetic fields produced by the MRI, finding that they may be comparable to those produced by a convective dynamo.
Because the universe is primarily composed of plasma, the interaction of plasmas and magnetic fields is of great importance for astrophysics. In this talk, I discuss our recent work investigating two such instabilities and examining their possible effects on astrophysical objects. First, we investigate magnetic reconnection and particle acceleration in moderately magnetized relativistic pair plasmas with three-dimensional particle-in-cell simulations of a kinetic-scale current sheet. We find that in three dimensions the tearing instability produces a network of interconnected and interacting magnetic flux ropes. In its nonlinear evolution, the current sheet evolves toward a three-dimensional, disordered state in which the resulting flux rope segments contain magnetic substructure on kinetic scales and sites of temporally and spatially intermittent dissipation.
We find that reconnection produces significant particle acceleration, primarily due to the electric field in the X-line regions between flux ropes; the resulting particle energy spectrum can extend to high Lorentz factors. We find that the highest energy particles are moderately beamed within $\sim30^\circ-40^\circ$ of the direction of acceleration. Second we derive a dispersion relation and calculate growth rates for triply-diffusive nonaxisymmetric instabilities including the magnetorotational instability (MRI) throughout the Sun, accounting for the effects of both shear and convective buoyancy. The overall instability has unstable modes throughout the convection zone and at colatitudes $\theta<53 \degrees$ in the tachocline. The instability contains three classes of modes: large-scale hydrodynamic convective modes, large-scale hydrodynamic shear modes, and small-scale MRI shear modes. While large-scale convective modes are the fastest-growing modes in most of the convective zone, MRI modes are important in both stably stratified and convectively unstable locations near the tachocline at colatitudes $\theta<53 \degrees$. We find that nonaxisymmetric MRI modes typically grow faster than axisymmetric MRI modes. We consider the saturation of magnetic fields produced by the MRI, finding that they may be comparable to those produced by a convective dynamo.