Astrophysical 3-D Dynamics and Visualization
The most recent papers are about:
Magnetohydrodynamic Turbulence and Dynamo Action
Together with Axel Brandenburg
(NORDITA) and
Bob Stein (MSU),
I am trying to understand how astrophysical dynamos work, by
analyzing and visualizing numerical simulations of 3D MHD
turbulence. We recently published a
summary paper, where
we discuss among other things whether or not dynamo action
is suppressed at high magnetic Reynolds numbers (we believe that
is is not).
Supersonic MHD-Turbulence and Star Formation
Together with
Bernard Jones (TAC),
I supervise Paolo Padoan's PhD project,
which is about highly supersonic turbulence in molecular clouds. Here
are some color viewgraphs
(PostScript--use landscape format in viewer) prepared for a recent
conference in Mexico.
Large Scale Structure of the Universe
Together with Örnólfur
Einar Rögnvaldsson (TAC and NORDITA) and
Bernard Jones (TAC),
I am working on a two-fluid (hot gas
+ CDM) MHD code for studying the evolution of large scale structure
and magnetic fields in the Universe. The CDM component is
described by a particle-in-mesh code, coupled to the MHD code with
PVM.
Accretion Discs
This is a joint project with
Axel Brandenburg
(NORDITA),
Bob Stein (MSU) and Ulf Torkelson.
The main results are in a
paper in the June 20 issue of ApJ,
with additional results in a
Letter to the ApJ.
There is also a
paper for the
proceedings of the November 1995 conference in Kyoto.
The Solar Dynamo
Bertil Dorch's
Masters thesis
is about the buoyant rise of magnetic flux tubes in the Sun.
His PhD project
aims at a qualitative understanding of processes involved in the
solar dynamo;
in particular the storage of the
strong toroidal field and the diffusion and drift of the surface
magnetic field.
This is the PhD project of
Klaus Galsgaard,
who is now
with
Eric Priest
in St. Andrews.
Klaus studies
the dissipation of magnetic energy in a low beta plasma stressed from
two opposing boundaries. This is a mock-up of the solar corona,
which is being stressed by the turbulent motions in the solar
photosphere.
The thesis consists of a summary and five papers.
In the first,
scaling relations are derived that allow a precise estimate
of the heating of the solar corona
by solar convective motions
(.ps.Z version)
-- unpacks to 8 Mb.
The work turns out to be consistent with the required coronal heating.
This paper has nice figures, some of which are color, but print OK
in grey scale.
The second paper is about the
kink instability
(.ps.Z version)
-- unpacks to 15 Mb,
and its evolution
past the saturation of the kink. The kink loop turns into a region
with a messy topology, where the magnetic energy keeps growing
steadily, if the boundary rotation is continued after the kink
instability has saturated.
The third paper looks at the
activity in a region containing magnetic
null points
(.ps.Z version)
-- ungpacks to 22 Mb.
A lot of activity takes place when such a
region is stressed by boundary motions, and nice arcade filament
systems eventually forms along the boundaries. This paper contains
color postscript, but is also available in
grey scale.
The fourth paper is about
self-organised criticality in
a simplified model of the coronal field, along the lines of Lu and
Hamilton. We are ready to bet a small amount of money on that
the coronal field is in a self-organized critical state, similar
to a sand pile, where the distribution of solar flares correspond
to the avalanches in a sand pile. Since this cannot yet be proven,
you have a fair chance with a counter bet, but sooner or later you
may loose.
The fifth paper is about the
numerical methods.
There is also a
thesis summary,
with lots of color illustrations, but this expands to an 80 Mb
PostScript file, so be careful with your WWW browsers use of
temporary space (you can set this from Netscape preferences),
and use a symbolic link print method ("lpr -s file.ps" or "lp file.ps"),
to avoid overflowing the print spool directory. Also, unless you
have a fast color printer, you may prefer to print this in black
and white (even though it is a pity on some of the illustrations).
Some of the viewgraphs from the PhD lecture are available as
postscript files in a
directory,
and as a compressed
SGI ShowCase file.
We (this is work done together with
Bob Stein, Michigan State University)
are able to match the observed properties of solar granulation
(the surface manifestation of turbulent heat convection on the Sun)
quantitatively, using numerical simulations with detailed equations
of state and radiative energy transfer. With this result in hand, we
can demonstrate that the structure of convection below the solar surface
is quite different than has hitherto been assumed; convection occurs
in the form of plumes of cold surface fluid, descending in a nearly
isentropic background.
Here is one viewgraph that explain the
background and one that shows a
comparison of numerical results with observations from La Palma
(prepared for the TAC inauguration in October 1994).
More illustrations are included in a
talk
given at the Liege Colloquium 1995.
A reprint is available, with
color
or grey scale images.
There is also an
invited talk from the Bombay meeting
in October 1995 (some misprints are corrected relative to the
printed version).
Sample data is in this directory.
There are also some movies.
A review paper by Henk Spruit
explains the new paradigm that has come out of the numerical
simulations.
Solar P-mode Oscillations
Together with
Bob Stein, Michigan State University at East Lancing,
I am working on understanding the excitation and damping of solar
oscillations.
Eta-Boo P-mode Oscillations and Convection
A group in Århus has discover p-mode oscillations in the
nearby sub-giant eta-Boo. The discovery was reported in
this paper.
Jørgen Christensen Dalsgaard
and friends have submitted a
paper
where they discuss models that fit the p-mode spectrum quite well,
except for a phase factor which may have to do with the detailed
structure near the surface.
In collaboration with
Regner Trampedach and
Colin Rosenthal in
Århus I am making 3D models of eta-Boo, to study the structure
of the surface layers.
We have
a contribution at a meeting in Cape Town,
and two posters at a GONG meeting; one poster on
"Seismology of the Solar Surface Regions"
and another one on
"Convective Perturbations To Solar Oscillations: The f Mode".
3-D non-LTE spectral line formation
Together with Dan Kiselman (Stockholm Observatory and La Palma), I am
working on understanding 3-D effects in spectral line formation. We
have a paper nearly ready for submission to A & A.
The paper is available as
html
(possibly broken -- problems with latex2html) and
ps.gz).
Popularization
Some of the projects mentioned above are discussed in a popular
article, with
Jørgen Christensen Dalsgaard,
in Naturens Verden (in danish).
For exact references to papers, see my
BibTeX file.
Last updated 02-02-95 / [email protected].