On the energetics of advection-dominated accretion flows

Using mean field MHD, we discuss the energetics of optically thin, two-temperature, advection-dominated accretion hows (ADAFs). If the magnetic field is tangled and roughly isotropic, flux freezing is insufficient to maintain the field in equipartition with the gas. In this case we expect a fraction of the energy generated by shear in the how to be used to build up the magnetic field strength as the gas flows in; the remaining energy heats the particles. We argue that strictly equipartition magnetic fields are incompatible with a priori reasonable levels of particle heating; instead, the plasma beta in ADAFs (= gas pressure divided by magnetic/turbulent pressure) is likely to be greater than or similar to 5; correspondingly, the viscosity parameter alpha is likely to be less than or similar to 0.2.