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| 2. |
- Ludwig, Hans-Günter, et al.
(författare)
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Convection and Small-scale Magnetic Fields in M-type Atmospheres
- 2003
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Ingår i: Short Contributions of the Annual Scientific Meeting of the Astronomische Gesellschaft in Freiburg ( Astronomische Nachrichten). - : Wiley. - 0004-6337 .- 1521-3994. ; 324:Suppl. Iss. 3, s. 65-65
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Konferensbidrag (refereegranskat)
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| 3. |
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| 4. |
- Wedemeyer, S., et al.
(författare)
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3-D hydrodynamic simulations of the solar chromosphere
- 2003
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Ingår i: Astronomical Notes - Astronomische Nachrichten. - : Wiley. - 0004-6337 .- 1521-3994. ; 324:4, s. 410-411
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Tidskriftsartikel (refereegranskat)abstract
- We present first results of three-dimensional numerical simulations ofthe non-magnetic solar chromosphere, computed with the radiationhydrodynamics code CO5BOLD. Acoustic waves which are excitedat the top of the convection zone propagate upwards into thechromosphere where the waves steepen into shocks. The interaction of thewaves leads to the formation of complex structures which evolve on shorttime scales. Consequently, the model chromosphere is highly dynamical,inhomogeneous, and thermally bifurcated.
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| 5. |
- Dorch, S B F, et al.
(författare)
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Small-scale magnetic fields on late-type M-dwarfs
- 2002
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Ingår i: Astronomical Notes - Astronomische Nachrichten. - 0004-6337. ; 323:3-4, s. 402-406
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Tidskriftsartikel (refereegranskat)abstract
- We performed kinematic studies of the evolution of small-scale magneticfields in the surface layers of M-dwarfs. We solved the inductionequation for a prescribed velocity field, magnetic Reynolds number ReM,and boundary conditions in a Cartesian box, representing a volumecomprising the optically thin stellar atmosphere and the uppermost partof the optically thick convective envelope. The velocity field isspatially and temporally variable, and stems from detailedradiation-hydrodynamics simulations of convective flows in aproto-typical late-type M-dwarf (Teff =2800pun {K}, logg =5.0, solarchemical composition, spectral type ~M6). We find dynamo action for ReM>= 400. Growth time scales of the magnetic field are comparable tothe convective turn-over time scale (~ 150pun {sec}). The convectivevelocity field concentrates the magnetic field in sheets and tubularstructures in the inter-granular downflows. Scaling from solarconditions suggests that field strengths as high as 20pun{kG} might bereached locally. Perhaps surprisingly, ReM is of order unity in thesurface layers of cooler M-dwarfs, rendering the dynamo inoperative. Inall studied cases we find a rather low spatial filling factor of themagnetic field.
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| 6. |
- Fendt, Ch, et al.
(författare)
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Magnetic deformation of the white dwarf surface structure
- 2000
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Ingår i: Astronomical Notes - Astronomische Nachrichten. - 0004-6337. ; 321:3, s. 193-206
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Tidskriftsartikel (refereegranskat)abstract
- The influence of strong, large-scale magnetic fields on the structure and temperature distribution in white dwarf atmospheres is investigated. Magnetic fields may provide an additional component of pressure support, thus possibly inflating the atmosphere compared to the non-magnetic case. Since the magnetic forces are not isotropic, atmospheric properties may significantly deviate from spherical symmetry. In this paper the magnetohydrostatic equilibrium is calculated numerically in the radial direction for either for small deviations from different assumptions for the poloidal current distribution. We generally find indication that the scale height of the magnetic white dwarf atmosphere enlarges with magnetic field strength and/or poloidal current strength. This is in qualitative agreement with recent spectropolarimetric observations of Grw+10°8247. Quantitatively, we find for e.g. a mean surface poloidal field strength of 100 MG and a toroidal field strength of 2-10 MG an increase of scale height by a factor of 10. This is indicating that already a small deviation from the initial force-free dipolar magnetic field may lead to observable effects. We further propose the method of finite elements for the solution of the two-dimensional magnetohydrostatic equilibrium including radiation transport in the diffusive approximation. We present and discuss preliminary solutions, again indicating on an expansion of the magnetized atmosphere.
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| 7. |
- Lagerkvist, C.-I., Karlsson, O., Hahn, G., Mottola, S., Doppler, A., Gnädig, A.
(författare)
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The Uppsala-DLR Trojan Survey of L4 the Preceding Lagrangian Cloud of Jupiter
- 2002
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Ingår i: Astron. Nachr.. - 0004-6337. ; 323, s. 475-483
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Tidskriftsartikel (refereegranskat)
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| 8. |
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