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Nobody has seen a black hole up till 2014. Nevertheless, schematic diagram such as the one shown in figure 23 invariably presents a system composed with a central object (the black hole), an accretion disk, and a pair of jets moving along twisted magnetic field lines. This picture of the black hole is based theoretically on the combination of three branches of physics - Fluid Dynamics, Electromagnetism, and Gravitation. The effect of gravity from the black hole is characterized by the escape velocity v _{esc}(r) = (2GM/r)^{1/2}, where M is the mass of the black hole, and r = (R^{2} + Z^{2})^{1/2} provides a link between the spherical and cylindrical coordinates (see upper left corner insert in Figure 23).With the assumptions of infinite conductivity (for the plasma in the system), isotropic pressure, local charge neutrality, non-relativistic inter-particle speeds), the suite of MHD equations are : | |

## Figure 23 Black Hole Schematic [view large image] |

These set of equations can be solved only through numerical computation. In general, the magnetic field has only the z component B

- A dense accretion disk (shown in dark) orbits a compact object of mass M.
- Acceleration takes place mostly at R
_{0}~ 3.6 R_{s}, where R_{s}= 2GM/c^{2}is the Schwartzschild radius. - The dense disk is sandwiched by two layers of tenuous corona (shown stippled) whose temperature is hotter than the disk but still colder than that in the halo. The coronal density is assumed to be 10 times the asymptotic halo density, and the disk density is essentially infinite in the simulations.
- Open magnetic field lines, making an angle
_{}= 30^{o}with the rotation axis, protrude from the disk and through the corona initially. The lines are dragged along by rotation creating a B_{}component. - The corona is replenished continually from the disk. The coronal flows are decidedly jet-like. The flow speed is a strong function of the ratio
_{}= v_{A}/v_{esc}. The jet speed is low for_{}< 1, i.e., for weak magnetic field (relative to gravity). The speed rises sharply when_{}> 1. The ratio_{}is therefore referred to as "magnetic switch" to turn on a pair of jets from a state with no jet. - Figure 25 shows the jet speed as a function of coronal magnetic field strength for both stellar disk (lower and left axes) and black hole's (upper and right axes). The v
_{c}for the proto-star is the velocity at r_{c}= 2x10^{12}cm ~ 30 R_{sun},

## Figure 24 MHD Computation |

## Figure 25 Jet Switch [view large image] |
while = (1 - v^{2}/c^{2})^{-1/2} is the Lorentz factor and _{} = 11(v_{A}/c)^{2} for the black hole. The two dash lines are taken from the "Relativistic Wind Theory" which excludes the effect of gravity. |

By comparing other studies on the same subject but taken into account the relativistic effects, the above-mentioned scenario is still valid with some modifications such as restricting v

N.B. MHD and specifically the Alfven velocity v

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