Zusammenfassung
In a standard, steady, thin accretion disc, the radial distribution of the
dissipation of the accretion energy is determined simply by energy
considerations. Here we draw attention to the fact that while the
(quasi-)steady discs in dwarf novae in outburst are in agreement with the
expected emission distribution, the steady discs in the nova-like variables are
not. We note that essentially the only difference between these two sets of
discs is the time for which they have been in the high viscosity, high
accretion rate state. In such discs, the major process by which angular
momentum is transported outwards is MHD turbulence. We speculate that such
turbulence gives rise to corona-like structures (here called magnetically
controlled zones, or MCZs) which are also able to provide non-negligible
angular momentum transport, the magnitude of which depends on the spatial scale
$L$ of the magnetic field structures in such zones. For short-lived, high
accretion rate discs (such as those in dwarf novae) we expect $L H$ and
the MCZ to have little effect. But, with time (such as in the nova-like
variables) an inverse cascade in the MHD turbulence enables $L$, and the net
effect of the MCZ, to grow. We present a simple toy model which demonstrates
that such ideas can provide an explanation for the difference between the dwarf
novae and the nova-like variable discs.
Nutzer