2015 research profile of dr. sven wedemeyer - …folk.uio.no/svenwe/swb_researchprofile.pdf · dr....

Dr. Sven Wedemeyer • Institute of Theoretical Astrophysics, University of Oslo, Norway

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 ground-

based 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

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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 addresses many fundamental and yet unknown aspects of this novel phenomenon through a 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. 2: 3-D visualization of a numerical model of the solar atmosphere showing a close-up region with a magnetic tornado.

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.


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 small-scale dynamo action) and the interaction of shock waves with the atmospheric magnetic field.

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 post-shock regions. I introduced the term “fluctosphere” for this shock-dominated domain (see Fig. 3) in order to avoid the common confusion concerning the term chromosphere, which 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).

Fig. 3: Revised atmospheric structure of quiet Sun regions.

Fig. 4: Illustration of a local 3-D model of the Sun extending from the upper convection zone into the chromosphere/fluctosphere.


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 flows, resulting in a horizontally directed but continuously changing “small-scale canopy” field, which overlays these 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 non-equilibrium 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 of hydrogen ionisation was only a first step and will be followed by a similar description for other atomic species. For instance, I also investigated the ionization of singly ionized 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 cooling timescales are too long compared to the hydrodynamic 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].

Fig. 5: Numerical model of the carbon monoxide concentration in the solar atmosphere.

Fig.6: Simulation of the hydrogen ionization fraction in the solar atmosphere with equilibrium (LTE, top) and time-dependent non-equilibrium treatment (NLTE, bottom).

Fig. 4: VAPOR visualisation of the top layers of a 3-D MHD model with granulation and magnetic field lines.


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 chromosphere. These swirls seem to comprise compact regions (diameters of ~1500 km) of rapidly rotating gas in connection with magnetic flux structures. This process may have fundamental 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. 8: The turret of the Swedish Solar Telescope and an intensity map, exhibiting a chromospheric swirl event (dark ring).

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.


Detailed comparisons between numerical models and observations Comparisons of solar observations with numerical models are important for:

(i) Testing the reliability of numerical models and the applied methods.

(ii) In-depth analysis of the relevant physical processes and determination of physical quantities.

(iii) Pointing out promising observational targets and yet undiscovered phenomena for future missions.

(iv) 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].

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].

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).

Fig. 11: Emergent continuum intensity synthesized for a MHD model of the Sun at a wavelength of λ = 500nm.


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

• 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)

II Running complex numerical simulations

• 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

III Interpretation and visualisation of large-volume data

• Programming the IDL-based analysis tool CAT • Visualisation of complex 3D structures, incl. VAPOR

IV Radiative transfer calculations

• 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

V Atomic and chemical reaction data

• Compilation of chemical reaction data • Compilation and calculation of atomic data, construction of model atoms • Spectral line parameters

VI Carrying out and analysing ground-based observations

• 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

VII Analysing space-borne observations

• Examples: Solar Optical Telescope (SOT) onboard Hinode, Transition Region and Coronal Explorer (TRACE), Solar Dynamics Observatory (SDO)

VIII Detailed comparison of synthetic and observational data

• 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.

IX 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


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/nph-abs_connect?db_key=AST&db_key=PHY&db_key=PRE&qform=AST&sim_query=YES&aut_xct=NO&aut_logic=OR&obj_logic=OR&author=wedemeyer-böhm,+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

2015 research profile of dr. sven wedemeyer - …folk.uio.no/svenwe/swb_researchprofile.pdf · dr....

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