March 2015
RESEARCH PROFILE OF DR. SVEN WEDEMEYER
RESEARCH FRAMEWORK
My primary research fields are solar and stellar physics with additional interests
in extra-solar planets.
I worked on a large number of complementary topics relevant to these fields. In
particular, I have focused on the structure, dynamics, and energy balance of the
solar atmosphere, including the structure, evolution, and generation of magnetic
fields, which are of fundamental importance for stellar activity and the processes
behind it. An example is my work on so-called magnetic tornadoes on the Sun.
The discovery of their coronal signature and an explanation of the physical
process behind was published in the journal Nature in June 2012 and highlighted
on the cover page.
Further topics range from the effects of non-equilibrium ionization and molecule
formation in the Sun towards 3-D numerical atmosphere models of cool dwarf
stars, which is one of my current research foci. A short description of my major
research topics is given below.
Due to the complexity of these topics, I pursue two complementary approaches:
(i) Realistic numerical simulations in comparison with space-borne and groundbased high-resolution observations.
(ii) Numerical experiments and simplified simulations for individual physical
processes.
My scientific skills include the development of numerical simulation codes, conducting complex
simulations, interpretation and visualization of the resulting data towards the determination of instrumental
properties of a space-borne telescope and even leading observation campaigns (see p.5 for a detailed list).
(References in squared brackets refer to the publication list on pages 5ff.)
CURRENT RESEARCH PROJECTS
I am currently the PI of the following projects:
•
"Vortex flows and magnetic tornadoes on the Sun and cool stars"
Funded by the Research Council of Norway in the highly competetive FRIPRO/FRINATEK
program for the period 2013-2016.
Official project page: http://www.mn.uio.no/astro/english/research/projects/vortex
•
"Magnetic Activity of the Atmospheres of M-type Dwarf Stars "
Funded by the Research Council of Norway in the Space Science for the period 2011-2014.
Official project page: http://www.mn.uio.no/astro/english/research/projects/dwarfstars
•
"Magnetic Activity of M-type Dwarf Stars and the Influence on Habitable Extra-solar
Planets"
International Team funded by the International Space Science Institute (ISSI) in Bern, Switzerland
(since 2012).
Official team home page at ISSI, Bern: http://www.issibern.ch/teams/mdwarfstar/index.html
Dr. Sven Wedemeyer • Institute of Theoretical Astrophysics, University of Oslo, Norway
CURRENT RESEARCH TOPICS
Magnetic tornadoes – Rotating magnetic
fields in stellar atmospheres
Magnetic tornadoes are thought to be abundant on our
Sun. They are generated by vortex flows, which form
due to the bathtub effect at the solar surface and force
the footpoints of magnetic field concentrations to
rotate. The magnetic fields extend through the
atmospheric layers and thus mediate the rotation
upwards, resulting in a net energy transport into the
upper layers. There, the energy is dissipated by yet
unknown physical processes and may contribute to
the heating of the solar corona to temperatures in
excess of a million degree Kelvin.
Based on the results that I published in Nature in
2012 (Vol. 486, 505 – 508), I now lead a project
funded by the Research Council of Norway, which
Fig. 2: 3-D visualization of a numerical model of the
addresses many fundamental and yet unknown
solar atmosphere showing a close-up region with a
aspects of this novel phenomenon through a
magnetic tornado.
combination of high-resolution observations with
world-leading facilities like the ground-based
Swedish 1-m Solar Telescope (SST) and the space-borne observatories Solar Dynamics Observatory and
advanced numerical simulations with state-of-the-art 3-D radiative magnetohydrodynamics computer codes.
Similar 3-D simulations for red dwarf stars (which constitute about 75 % of all stars in our galaxy) will
reveal if the tornado phenomenon is of general importance for cool stars.
Magnetic Activity of M-type dwarf stars
Red dwarf stars of spectral type M, also called
M-dwarfs, constitute 75% of all stars in the solar
neighbourhood and in our galaxy. They have masses
of less than half a solar mass, effective (surface)
temperatures of 2000 - 4000 K and low luminosities
(L < 0.02 L⊙ ). From observations, we know that they
can have strong magnetic fields and can thus be
magnetically very active, which includes strong flares.
Their large number makes them interesting for our
understanding of stars in general and their role as host
stars of extra-solar planets. And yet many details about
the nature of these stars are unclear.
A grant by the Research Council of Norway enabled
me to extend my research towards M-type dwarf stars
(since 2011). A first set of numerical models, which
clearly exceed the state-of-the-art in the field, has been
produced and is currently analysed [50]. It will be
valuable for the interpretation of observations, which
so far had to rely on static 1-D models. Furthermore, it
will shed light on the physical processes at work in the
atmospheres of cool stars.
Fig. 1: 3-D visualization of a new atmosphere model
of a M-dwarf representative of AD Leonis. The
magnetic field lines (red) are rooted in the footpoints
at the visible surface (grey) and funnel out in the
atmosphere above.
Dr. Sven Wedemeyer • Institute of Theoretical Astrophysics, University of Oslo, Norway
RECENT RESEARCH TOPICS
Revised magnetic field topology of quiet Sun regions
Based on 3-D radiation magnetohydrodynamic simulations and high-resolution observations, I reviewed the
atmospheric structure of quiet Sun regions, which is more complex and dynamic than previously anticipated.
See the review article [19] for details. The most important ingredients are illustrated in Fig. 3. This picture
includes the newly discovered magnetic “small-scale canopies” and the pronounced dynamics of the
chromospheric layer, which have far reaching implications for the structure and heating of stellar
atmospheres. Other important aspects concern the generation of magnetic field (possibly via local smallscale dynamo action) and the interaction of shock waves with the atmospheric magnetic field.
Fig. 3: Revised atmospheric structure of quiet Sun regions.
Realistic Simulations of Stellar Atmospheres
Numerical simulations, which were produced with the
radiation magnetohydrodynamics code CO5BOLD, were
studied with respect to a large number of topics, for instance
surface convection and wave phenomena [e.g., 11, 27, 38,
48]. An important finding was the discovery of a complex
dynamic small-scale pattern at chromospheric heights [2, 3,
4, 27]. This pattern is produced by interaction of propagating
shock waves, which are self-consistently excited by the
convective motions in the lower part of the models. The
resulting rapidly changing, mesh-like shock pattern in the
atmosphere consists of hot threads and enclosed cool postshock regions. I introduced the term “fluctosphere” for this
Fig. 4: Illustration of a local 3-D model of
shock-dominated domain (see Fig. 3) in order to avoid the
the Sun extending from the upper convection
common confusion concerning the term chromosphere, which
zone into the chromosphere/fluctosphere.
should be reserved for the magnetic field dominated
“canopy” domain above [39].
The dynamic and intermittent nature of the fluctosphere turned out to be the key to solve the controversy
about the temperature stratification of the solar atmosphere, confirming the 1D finding by Carlsson & Stein
(1994). While the shocks can explain the measured chromospheric UV emission, the cool post-shock regions
allow for the observed existence of molecules (see below).
Dr. Sven Wedemeyer • Institute of Theoretical Astrophysics, University of Oslo, Norway
Also the magnetic field on small spatial turned out to be very
dynamic. Simulations with CO5BOLD with an initially weak
magnetic field representative of a quiet Sun region feature a
chromospheric layer, where the magnetic field is continuously
rearranged on timescales of less than 1 min. Rapidly moving,
transient magnetic filaments form in the compression zone
downstream and along propagating shock fronts. The surface
of plasma-β = 1 (on average at a height of 1000 km) separates
the layer of highly dynamic magnetic fields from the more
slowly evolving field below [30, 35, 36].
In the photosphere, the magnetic field gets almost completely
expelled from the granule interiors due to the convective Fig. 4: VAPOR visualisation of the top layers of
flows, resulting in a horizontally directed but continuously a 3-D MHD model with granulation and
changing “small-scale canopy” field, which overlays these magnetic field lines.
magnetic “voids”. The resulting “horizontal internetwork
fields” (HIFs) have been observed recently and are currently
debated [15, 40, 42].
Advanced modelling of non-equilibrium processes
A number of important processes deviate from equilibrium
conditions in stellar chromospheres, which makes a detailed
numerical treatment mandatory. Implementing such a realistic
numerical description as part of complex atmosphere models
is a very challenging task.
An important example concerns the ionization of hydrogen,
which is the major constituent of the atmospheric gas.
Numerical simulations with a time-dependent nonequilibrium treatment of hydrogen ionization (Fig. 6) proofed
that the deviations from the equilibrium state have
fundamental influence on the plasma properties (e.g., the
density of free electrons) [8, 13, 34, 37]. The implementation
Fig.6: Simulation of the hydrogen ionization
of hydrogen ionisation was only a first step and will be
fraction in the solar atmosphere with equilibrium
(LTE, top) and time-dependent non-equilibrium followed by a similar description for other atomic species. For
instance, I also investigated the ionization of singly ionized
treatment (NLTE, bottom).
calcium (Ca II) [21].
My revision of the role of carbon monoxide (CO) in the solar
atmosphere is another important example. CO was previously
considered a potentially important cooling agent, which would
be capable of inducing a thermal bifurcation of the
atmospheric gas with cool regions embedded in hotter
material. I produced time-dependent 2-D/3-D non-equilibrium
simulations of the Sun with a detailed chemical reaction
network [30, 34]. The resulting data showed that CO is mainly
concentrated in the cool regions of the reversed granulation
pattern in the middle photosphere but binds a very large
fraction of all carbon atoms throughout the layers above except
for the hot shock waves in the chromosphere [6]. Moreover, I
was able to conclude that CO cannot induce a thermal
bifurcation of the solar atmosphere because the relevant Fig. 5: Numerical model of the carbon monoxide
cooling timescales are too long compared to the hydrodynamic concentration in the solar atmosphere.
ones. Rather, the co-existence of hot and cool gas is produced
by the interaction of propagating shock waves. This finding
settled a controversy that lasted over three decades [10].
Dr. Sven Wedemeyer • Institute of Theoretical Astrophysics, University of Oslo, Norway
High-resolution observations of the Sun
During my various projects, I analysed data from different
observatories, e.g. the Transition Region and Coronal Explorer
(TRACE) spacecraft, the Solar Optical Telescope onboard the Hinode
satellite, and the Dutch Open Telescope (La Palma, Spain). I was PI of
an observation campaign at the German Vacuum Tower Telescope
(Tenerife, Spain), Co-I at the Dunn Solar Telescope (Sunspot, USA),
and participated repeatedly in observations with the Swedish Solar
Telescope (La Palma, Spain). The obtained high-resolution data
provided insight in the small-scale structure and dynamics of the lower
to middle solar atmosphere [e.g., 12, 14, 32], including observational
support for the predicted fluctospheric shock pattern [7, 9].
Attempts were made to directly measure the line-of-sight component of
the magnetic field in the chromosphere from an infrared triplet line of
singly ionized calcium [43, 44].
In 2008, I discovered small-scale swirl events in the solar
Fig. 8: The turret of the Swedish chromosphere. These swirls seem to comprise compact regions
Solar Telescope and an intensity map, (diameters of ~1500 km) of rapidly rotating gas in connection with
exhibiting a chromospheric swirl magnetic flux structures. This process may have fundamental
event (dark ring).
implications for the heating of the upper solar atmosphere [20, 47].
Radiative Transfer and Spectrum
Synthesis
Synthetic spectra and intensity images are
needed for a direct comparison with
observations. In the course of various
projects, I calculated image sequences from
magnetohydrodynamical models at different
wavelengths from the ultraviolet to the
millimetre regime, using different radiative
transfer codes. Applications for the Sun
include, e.g., spectral lines of iron [31, 41],
carbon monoxide (Fig. 9) [10] and singly
ionized calcium, and the continua at (sub)millimetre wavelengths.
The latter were used to predict what the
Atacama Large Millimeter Array (ALMA)
could observe in the near future. For that
purpose, I calculated intensity image
sequences for the accessible wavelength
range at different positions on the Sun from
its disk-centre to its limb. The analysis
produced many results that will be valuable
for the future planning and interpretation of
solar observations with ALMA, defining
constraints on required temporal and spatial
resolution and scientific objectives. It was
found that the formation height range
increases with wavelength and also varies
with inclination angle so that a combination
of instantaneous observations could serve as
tomography, revealing the three-dimensional
thermal structure of the solar chromosphere
[13, 28].
Fig. 9: Synthetic spectra near λ = 4.7 µm with carbon monoxide
lines in comparison to ATMOS3 data.
Fig. 10: Artist’s impression of the Atacama Large Millimeter
Array (Credit ESO) and a synthesized intensity map at a
wavelength of 1mm.
Dr. Sven Wedemeyer • Institute of Theoretical Astrophysics, University of Oslo, Norway
Detailed comparisons between numerical models and observations
Comparisons of solar observations with numerical models are important for:
(i)
(ii)
(iii)
(iv)
Testing the reliability of numerical models and the applied methods.
In-depth analysis of the relevant physical processes and determination of physical quantities.
Pointing out promising observational targets and yet undiscovered phenomena for future missions.
Optimising observational techniques.
For instance, synthetic spectra were used to determine
the chemical abundance of silicon in the Sun and other
stars [1]. Synthetic intensity images based on
magnetohydrodynamic
simulations
have
been
compared to solar observations in different wavelength
regimes for both continua and various spectral lines.
The comparison of observations in the wing of the Ca
II H spectral line, carried out with the Dutch Open
Telescope, confirmed that the reversed granulation
pattern in the middle photosphere is modelled already
realistically [5]. Synthetic maps for the line core of the
Ca II infrared line at 854.2 nm, which is formed at
chromospheric
heights,
were
compared
to
corresponding observations with the Dunn Solar
Telescope [18]. It confirmed that a fluctospheric
pattern exists in quiet Sun regions as part of a
compound of atmospheric regions, which are
dynamically coupled [39].
Fig. 11: Emergent continuum intensity synthesized for a
MHD model of the Sun at a wavelength of λ = 500nm.
Fig. 10: Artist's impression of the Hinode space observatory
(Credit JAXA) and an illustration of the point spread function
for the Solar Optical Telescope onboard (red surface).
A particularly successful example concerns
the contrast and centre-to-limb variation of
the
continuum
intensity
at
visible
wavelengths. For many years, there were
large discrepancies between values derived
from models and observations. This
fundamental problem was resolved by
properly taking into account the detailed
instrumental properties of the employed
telescope. For this purpose, the instrumental
image degradation of the Solar Optical
Telescope onboard the Hinode satellite was
determined in form of a point spread function
with scattered light contributions (see Fig. 7)
[16]. With this, the models now reproduce the
observations very well, which demonstrates
that state-of-the-art numerical simulations
provide a realistic description of solar surface
convection [17, 45].
Dr. Sven Wedemeyer • Institute of Theoretical Astrophysics, University of Oslo, Norway
SCIENTIFIC SKILLS
My experience includes a large range of complementary tasks in theoretical, observational, and
computational astrophysics. A summary with selected examples is provided below.
I
Technical development of numerical codes / methods
•
•
•
•
•
II
Running complex numerical simulations
•
•
•
III
•
•
•
•
Examples: Solar Optical Telescope (SOT) onboard Hinode, Transition Region and Coronal
Explorer (TRACE), Solar Dynamics Observatory (SDO)
Detailed comparison of synthetic and observational data
•
IX
PI at the German Vacuum Tower Telescope (VTT)
Participation in campaigns at VTT and SST (co-I of DST campaigns)
Co-ordinated campaigns with Hinode
Analysing space-borne observations
•
VIII
Compilation of chemical reaction data
Compilation and calculation of atomic data, construction of model atoms
Spectral line parameters
Carrying out and analysing ground-based observations
•
•
•
VII
Synthesising spectra and intensity image sequences from simulation data at different wavelengths
from ultraviolet to the millimetre regime
Using different radiative transfer codes, e.g., LINFOR3D, MULTI_3D
NLTE calculations
Abundance analysis
Example: predictions for solar observations with ALMA
Atomic and chemical reaction data
•
•
•
VI
Programming the IDL-based analysis tool CAT
Visualisation of complex 3D structures, incl. VAPOR
Radiative transfer calculations
•
V
Time-dependent 1D / 2D / 3D realistic simulations of stellar surface convection and atmospheres
Models for the Sun and M-dwarf stars
Carbon monoxide formation, hydrogen ionisation, magnetic fields, wave propagation
Interpretation and visualisation of large-volume data
•
•
IV
Contributions to the radiation magnetohydrodynamics code CO5BOLD
Adaptation of the code to new computer architectures, optimisation
Implementations (examples)
• Time-dependent treatment of chemical reaction networks (with I. Kamp)
• Non-equilibrium hydrogen ionisation (with J. Leenaarts)
• Carbon monoxide radiative cooling (with M. Steffen)
Extension to magnetohydrodynamics (with W. Schaffenberger, O. Steiner, B. Freytag)
Contributions to the radiative transfer code LINFOR3D (with M. Steffen, H.-G. Ludwig)
Synthetic intensity images and spectra compared to solar observations in different wavelength
regimes, e.g.: Ca II H (DOT), Ca II K (VTT), Ca II 854nm (SST), 160 nm band (TRACE) etc.
Simulating instrumental effects
•
•
Effect of spatial resolution on power spectral density for TRACE
New forward modelling method for the determination of detailed non-ideal point spread functions
with stray-light contributions, applied to SOT
Dr. Sven Wedemeyer • Institute of Theoretical Astrophysics, University of Oslo, Norway
PUBLICATION LIST
A complete publication list can be found on the SAO/NASA ADS.
Click here for a direct link:
http://adsabs.harvard.edu/cgi-bin/nphabs_connect?db_key=AST&db_key=PHY&db_key=PRE&qform=AST&sim_query=YES&aut_xct=NO&aut_logic=O
R&obj_logic=OR&author=wedemeyerböhm,+s.&object=&start_mon=&start_year=&end_mon=&end_year=&ttl_logic=OR&title=&txt_logih
COVER ILLUSTRATIONS:
A&A Vol. 460 No. 1 (Dec. II 2006)
A&A Vol. 462 No. 3 (Feb. II 2007)
Nature Vol. 486 (June 28th, 2012)
A&A HIGHLIGHT:
A&A 462-3: Radiative cooling by CO lines
Dr. Sven Wedemeyer • Institute of Theoretical Astrophysics, University of Oslo, Norway