373 research outputs found
Narrow Moving Fe K-alpha lines from magnetic flares in AGN
We point out that luminous magnetic flares, thought to occur in standard AGN
accretion disks, cannot be located much higher than few pressure scale heights
above the disk. Using this fact, we estimate the fraction of the disk surface
illuminated by a typical flare. This fraction turns out to be very small for
geometrically thin disks, which implies that the instantaneous Fe K-alpha
emission line from a specific magnetic flare is narrow. The line is red- or
blue-shifted depending on the position of the observer relative to the flare
and sweeps across the line band with time. We present several examples of
theoretical time-resolved line profiles from such flares for a non-rotating
black hole. The observations of such moving features with future X-ray
telescopes will present a powerful test of the accretion disk geometry and may
also test General Relativity in the strong field limit.Comment: Revised; no major changes in conclusion
MHD consistent cellular automata (CA) models II. Applications to solar flares
In Isliker et al. (2000b), an extended cellular automaton (X-CA) model for
solar flares was introduced. In this model, the interpretation of the model's
grid-variable is specified, and the magnetic field, the current, and an
approximation to the electric field are yielded, all in a way that is
consistent with Maxwell's and the MHD equations. Here, we reveal which relevant
plasma physical processes are implemented by the X-CA model and in what form,
and what global physical set-up is assumed by this model when it is in its
natural state (SOC). The basic results are: (1) On large-scales, all variables
show characteristic quasi-symmetries. (2) The global magnetic topology forms
either (i) closed magnetic field lines, or (ii) an arcade of field lines above
the bottom plane line, if the model is slightly modified. (3) In case of the
magnetic topology (ii), loading can be interpreted as if there were a plasma
which flows predominantly upwards, whereas in case of the magnetic topology
(i), as if there were a plasma flow expanding from the neutral line. (4) The
small-scale physics in the bursting phase represent localized diffusive
processes. (5) The local diffusivity usually has a value which is effectively
zero, and it turns locally to an anomalous value if a threshold is exceeded,
whereby diffusion dominates the quiet evolution (loading). (6) Flares
(avalanches) are accompanied by the appearance of localized, intense electric
fields. (7) In a variant on the X-CA model, the magnitude of the current is
used directly in the instability criterion. First results indicate that the SOC
state persists. (8) The current-dissipation during flares is spatially
fragmented into a large number of dissipative current-surfaces of varying
sizes, which show a highly dynamic temporal evolution.Comment: 13 pages, 12 figures; in press at Astronomy and Astrophysics (2001
A self-organized criticality model for ion temperature gradient (ITG) mode driven turbulence in confined plasma
A new Self-Organized Criticality (SOC) model is introduced in the form of a
Cellular Automaton (CA) for ion temperature gradient (ITG) mode driven
turbulence in fusion plasmas. Main characteristics of the model are that it is
constructed in terms of the actual physical variable, the ion temperature, and
that the temporal evolution of the CA, which necessarily is in the form of
rules, mimics actual physical processes as they are considered to be active in
the system, i.e. a heating process and a local diffusive process that sets on
if a threshold in the normalized ion temperature gradient R/L_T is exceeded.
The model reaches the SOC state and yields ion temperature profiles of
exponential shape, which exhibit very high stiffness, in that they basically
are independent of the loading pattern applied. This implies that there is
anomalous heat transport present in the system, despite the fact that diffusion
at the local level is imposed to be of a normal kind. The distributions of the
heat fluxes in the system and of the heat out-fluxes are of power-law shape.
The basic properties of the model are in good qualitative agreement with
experimental results.Comment: In press at Physics of Plasmas, July 2010; 11 pages, 5 figure
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