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Proposed Study of Dynamo Activity Associated with Astrophysical Jets

Proposed Study of Dynamo Activity Associated with Astrophysical Jets. Carl Sovinec, Univ. of WI, Engineering Physics study suggested by Stirling Colgate and Hui Li, Los Alamos National Lab. Meeting of the Center for Magnetic Self-Organization Madison, Wisconsin August 4-6, 2004.

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Proposed Study of Dynamo Activity Associated with Astrophysical Jets

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  1. Proposed Study of Dynamo Activity Associated with Astrophysical Jets Carl Sovinec, Univ. of WI, Engineering Physics study suggested by Stirling Colgate and Hui Li, Los Alamos National Lab. Meeting of the Center for Magnetic Self-Organization Madison, Wisconsin August 4-6, 2004

  2. Magnetic configurations surrounding active galactic nuclei and in laboratory spheromaks undergo similar ‘current drive.’ Artist’s sketch of magnetic fields driven by differential rotation of an accretion disk; return path is not shown. [from Hubble website] SSPX (LLNL) hardware and poloidal flux distribution. Electrostatic voltage across electrodes is analogous to disk VB.

  3. In DC-driven spheromaks, MHD instability of the pinched current column leads to flux conversion. Initial (vacuum) poloidal flux distribution for a 3D MHD simulation of SSPX. Saturated driven state with poloidal flux converted from toroidal flux. Conversion is attributed to MHD dynamo activity. • See presentations available from http://www.cptc.wisc.edu/sovinec_research for additional information.

  4. In DC-driven spheromaks, MHD instability of the pinched current column leads to flux conversion.

  5. Astrophysical jets may exhibit similar current-driven MHD activity. • Distortions at large distances from AGN are suggestive of a kink mode. • Colgate & Li: conversion may have an important role in generating magnetic flux in astrophysical systems. • Physics topics include collimation, stability, and nonlinear 3D evolution. Images of M87 from VLA (radio), Hubble (visible), and VLBA (radio).

  6. To date, most numerical studies of jets have been two-dimensional, addressing collimation. • Time-dependent and stationary (Grad-Shafranov) computations • Hydromagnetic and Poynting regimes • Hydro: If Bpol is at a large angle with respect to the axial direction, the centrifugal force along field lines overcomes gravity and accelerates matter away from the disk. [Blanford & Payne, Bell, Ustyugova] (young stellar objects) • Poynting: At small angles, energy and momentum are carried by electromagnetic fields. [Lovelace, Blanford, Romanova] (extragalactic jets and microquasars) • Stationary solutions with a slight (10-11) external pressure show a collimated region threading a force-free region. [Hi Li, et al., Ap. J. 561, 2001]

  7. Applying a 3D fusion code (NIMROD) will help address MHD instability and nonlinear evolution. Poloidal Mesh (Fourier in azimuthal direction) Initial Dipole Flux ‘Force-Free’ Expansion from Differential Rotation • Simulation Issues • Nonlinear computations require dissipation—allows slippage. • Relativistic effects may be important. • Alternative computational path—just evolve deviations from a 2D equilibrium. • Tasks • Port initialization to current NIMROD. • Implement gravity. • Benchmark 2D (axisymmetric) behavior. • Devise a logarithmic mesh [Li].

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