Magnetorotational turbulence draws its energy from gravity and ultimately releases it via dissipation. However, the quantitative details of this energy flow have not been assessed for global disc models. In this work we examine the energetics of a well-resolved, three-dimensional, global magnetohydrodynamic accretion disc simulation by evaluating statistically averaged mean-field equations for magnetic, kinetic and internal energy using simulation data. The results reveal that turbulent magnetic (kinetic) energy is primarily injected by the correlation between Maxwell (Reynolds) stresses and shear in the (almost Keplerian) mean flow, and removed by dissipation. This finding differs from previous work using local (shearing box) models, which indicated that turbulent kinetic energy was primarily sourced from the magnetic energy reservoir. Lorentz forces provide the bridge between the magnetic and kinetic energy reservoirs, converting ∼1/5 of the total turbulent magnetic power input into turbulent kinetic energy. The turbulent energies (both magnetic and kinetic) are mainly driven by terms associated with the turbulent fields, with only a minor influence from mean magnetic fields. The interaction between mean and turbulent fields is most evident in the induction equation, with the mean radial magnetic field being strongly influenced by the turbulent electromotive force. During the quasi-steady turbulent state roughly 2/3 of the Poynting flux travels into the corona, with the remainder transporting magnetic energy in the radial direction. In contrast to previous studies, the stress-related part of the Poynting flux is found to dominate, which may have important implications for `reflection' models of Seyfert galaxy coronae that typically invoke a picture of buoyant rising of magnetic flux tubes via advection.
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- March 2014
- accretion discs;
- Astrophysics - High Energy Astrophysical Phenomena
- 18 pages, 14 figures. Accepted for publication in MNRAS