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Sökning: hsv:(NATURVETENSKAP) hsv:(Fysik) hsv:(Astronomi astrofysik och kosmologi)

  • Resultat 6981-6990 av 11065
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6981.
  • Käpylä, P. J., et al. (författare)
  • Turbulent viscosity and magnetic Prandtl number from simulations of isotropically forced turbulence
  • 2020
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 636
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Turbulent diffusion of large-scale flows and magnetic fields plays a major role in many astrophysical systems, such as stellar convection zones and accretion discs. Aims. Our goal is to compute turbulent viscosity and magnetic diffusivity which are relevant for diffusing large-scale flows and magnetic fields, respectively. We also aim to compute their ratio, which is the turbulent magnetic Prandtl number, Pmt, for isotropically forced homogeneous turbulence. Methods. We used simulations of forced turbulence in fully periodic cubes composed of isothermal gas with an imposed large-scale sinusoidal shear flow. Turbulent viscosity was computed either from the resulting Reynolds stress or from the decay rate of the large-scale flow. Turbulent magnetic diffusivity was computed using the test-field method for a microphysical magnetic Prandtl number of unity. The scale dependence of the coefficients was studied by varying the wavenumber of the imposed sinusoidal shear and test fields. Results. We find that turbulent viscosity and magnetic diffusivity are in general of the same order of magnitude. Furthermore, the turbulent viscosity depends on the fluid Reynolds number (Re) and scale separation ratio of turbulence. The scale dependence of the turbulent viscosity is found to be well approximated by a Lorentzian. These results are similar to those obtained earlier for the turbulent magnetic diffusivity. The results for the turbulent transport coefficients appear to converge at sufficiently high values of Re and the scale separation ratio. However, a weak trend is found even at the largest values of Re, suggesting that the turbulence is not in the fully developed regime. The turbulent magnetic Prandtl number converges to a value that is slightly below unity for large Re. For small Re we find values between 0.5 and 0.6 but the data are insufficient to draw conclusions regarding asymptotics. We demonstrate that our results are independent of the correlation time of the forcing function. Conclusions. The turbulent magnetic diffusivity is, in general, consistently higher than the turbulent viscosity, which is in qualitative agreement with analytic theories. However, the actual value of Pmt found from the simulations (≈0.9-0.95) at large Re and large scale separation ratio is higher than any of the analytic predictions (0.4-0.8).
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6982.
  • Käpylä, Petri, et al. (författare)
  • Magnetic flux concentrations from turbulent stratified convection
  • 2016
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 588
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. The formation of magnetic flux concentrations within the solar convection zone leading to sunspot formation is unexplained. Aims. We study the self-organization of initially uniform sub-equipartition magnetic fields by highly stratified turbulent convection. Methods. We perform simulations of magnetoconvection in Cartesian domains representing the uppermost 8 : 5 24 Mm of the solar convection zone with the horizontal size of the domain varying between 34 and 96 Mm. The density contrast in the 24 Mm deep models is more than 3 x 10(3) or eight density scale heights, corresponding to a little over 12 pressure scale heights. We impose either a vertical or a horizontal uniform magnetic field in a convection-driven turbulent flow in set-ups where no small-scale dynamos are present. In the most highly stratified cases we employ the reduced sound speed method to relax the time step constraint arising from the high sound speed in the deep layers. We model radiation via the diffusion approximation and neglect detailed radiative transfer in order to concentrate on purely magnetohydrodynamic effects. Results. We find that super-equipartition magnetic flux concentrations are formed near the surface in cases with moderate and high density stratification, corresponding to domain depths of 12 : 5 and 24 Mm. The size of the concentrations increases as the box size increases and the largest structures (20 Mm horizontally near the surface) are obtained in the models that are 24 Mm deep. The field strength in the concentrations is in the range of 3-5 kG, almost independent of the magnitude of the imposed field. The amplitude of the concentrations grows approximately linearly in time. The effective magnetic pressure measured in the simulations is positive near the surface and negative in the bulk of the convection zone. Its derivative with respect to the mean magnetic field, however, is positive in most of the domain, which is unfavourable for the operation of the negative effective magnetic pressure instability (NEMPI). Simulations in which a passive vector field is evolved do not show a noticeable difference from magnetohydrodynamic runs in terms of the growth of the structures. Furthermore, we find that magnetic flux is concentrated in regions of converging flow corresponding to large-scale supergranulation convection pattern. Conclusions. The linear growth of large-scale flux concentrations implies that their dominant formation process is a tangling of the large-scale field rather than an instability. One plausible mechanism that can explain both the linear growth and the concentration of the flux in the regions of converging flow pattern is flux expulsion. A possible reason for the absence of NEMPI is that the derivative of the effective magnetic pressure with respect to the mean magnetic field has an unfavourable sign. Furthermore, there may not be sufficient scale separation, which is required for NEMPI to work.
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6983.
  • Kärnfelt, Johan, 1963 (författare)
  • Av galaxer är du kommen
  • 2017
  • Ingår i: Populär astronomi (webbupplagan).
  • Annan publikation (övrigt vetenskapligt/konstnärligt)
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6984.
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6985.
  • Kärnfelt, Johan, 1963 (författare)
  • Historien om van de Kamps planeter
  • 2009
  • Ingår i: Populär astronomi. - 1650-7177. ; 10:3
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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6986.
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6987.
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6988.
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6989.
  • Kärnfelt, Johan, 1963 (författare)
  • Reception and dissemination of American amateur telescope making in Sweden
  • 2017
  • Ingår i: Journal of Astronomical History and Heritage. - 1440-2807. ; 20:1, s. 126-135
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper discusses the appropriation of the American Amateur Telescope Making (ATM) movement in Sweden in the 1940s and 1950s. A key player was the Swedish Astronomical Society, which in 1943, and inspired by the American example, launched a campaign to raise interest in ATM and disseminate the necessary knowledge amongst potential amateur astronomers. The campaign was successful and in just a few years it quadrupled the number of amateurs with access to telescopes. Swedish amateurs kept on building telescopes through the 1950s, but the activities then stalled with the introduction of cheap mass-market telescopes. The appropriation of ATM in Sweden is an important example of how technical innovations have shaped the course of amateur astronomy.
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6990.
  • Kärnfelt, Johan, 1963 (författare)
  • Solförmörkelselogistik
  • 2017
  • Ingår i: Populär astronomi (webbupplagan).
  • Annan publikation (övrigt vetenskapligt/konstnärligt)
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