Contents List of Figures

6.7 Formation of Micro–Jets 307
VLBI jets are extreme plasma pipes, where all communication occurs
over the speed of light – the Alfven speed and the fast magnetosonic speed
are relativistic. The plasma in these pipes is also exotic: it is permanently heated
to high temperatures of the order of 10 12 K by means of internal shocks. The ions
are probably still non–relativistic, while the electrons always assume relativistic
temperatures with < γe > 100, which are then subject to acceleration to even
higher energies by various mechanisms.
6.7.2 The Collimation Zone
I am still convinced that jets in quasars are launched by complete MHD processes
and that this will be modelled some day within General Relativistic MHD. The
following elements should be included
• a dipolar magnetosphere that closes radially towards the horizon;
• hot plasma is launched from the innermost part (probably within the ergosphere
region) in radial direction;
• Along the axis, higly relativsitic plasma is injected from a polar gap. This
plasma is energetically not important, but it carries the closure currents.
• The gap is feeded by a pair plasma created from photoproduction in the ion
torus near the horizon.
• The toroidal field amplified by the frame–dragging effect leads to additional
J × B–forces near the ergosphere.
Structure of the magnetosphere: The calculation of the magnetosphere in the
stationary approach is very cumbersome (Beskin 1996; Camenzind [6]; Fendt &
Greiner [7]), since the current distribution has to be found self–consistently (this
exercise is in fact better done within a time–dependent approach). The transition
from the highly diffusive disk towards the ideal outflow conditions has however
hampered down all modelling so far. What is urgently needed for this process is a
realisation of the GR MHD equations on the background of compact objects. The
form of the magnetosphere shown in Fig. 146 is a first guess.
The Plasma Outflow: It has been shown that the stationary axisymmetric and
polytropic plasma flow along an axisymmetric magnetic flux tube Ψ(r, θ) can be
described by means of an algebraic wind equation (Camenzind [6]; Fendt & Greiner
[7]). The essential quantity which determines the asymptotic plasma flow is the