Solar and stellar magnetohydrodynamics

Solar and stellar magnetohydrodynamics

Magnetic fields are responsible for the restless activity of the Sun and other cool stars, i.e. the emergence and disappearance of dark spots, mass ejections and bursts of radiation connected with the so-called "flares". Solar activity can severly affect terrestrial infrastructure (e.g., breakdown of power grids, interruption of radio communication, radiation load on airplane passengers, disturbance or damage of satellite systems). It is therefore important to understand the physical processes underlying the generation of magnetic fields and their interaction with the plasma (electrically conducting gas) in the in the atmospheres of the Sun and other stars. Magnetic fields originate from induction processes driven by the convective motions which carry the energy generated by nuclar fusion towards the stellar surface. Our research addresses a variety of topics connected with solar and stellar magnetism, including

  • interaction of magnetic fields with radiative convection in the near-surface layers,
  • formation and structure of magnetized vortex flows,
  • generation of magnetic fields by a large-scale and small-scale self-excited dynamo processes,
  • structure and dynamics of sunspots and smaller concentrations of magnetic flux,
  • surface emergence of magnetic flux and formation of bipolar magnetic regions,
  • evolution of the solar surface field as a result of the emergence of magnetic flux and its transport by horizontal flow fields (meridional circulation, differential rotation, large-scale convection): reversals and build-up of polar fields,
  • large-scale convective patterns in the deep solar convection zone, their surface manifestations and effects on the magnetic field.
Snapshot from a simulation of solar convective flows. The green volume rendering indicates swirling flows near the optical solar surface, which is color-coded with vertical flow velocity (downflows in red and upflows in blue). The size of the box shown is 4800 km × 4800 km horizontally and 1400 km in depth. The optical surface is hidden in the lower right quadrant, uncovering the swirling structure in the subsurface layers.

Our research mainly utilizes numerical simulations based on the equations of magnetohydrodynamics and radiative transfer. The simulation results are also used to calculate observable quantities (e.g., brightness maps, spectral line profiles, polarization signatures) in order to permit a direct comparison to observations. In this way, the simulations can be validated and, in turn, be used as a tool for the interpretation of observational results. While observations mostly provide information only from a thin layer in the photosphere, the simulations reveal the full three-dimensional structure of the underlying physical processes.

Formation of a current sheet (a very thin layer with strong electrical current) at the sharp interface between oppositely directed magnetic fields in the upper solar photosphere. The images show various quantities in a horizontal cut at a height of about 400~km above the optical solar surface. Left: vertical magnetic field (black: about 400 Gauss downward directed, white: about 300 Gauss upward directed); middle: temperature (varyingbetween about 4000 K in the background and 8500 K in the bright current sheet); left: vertical flow velocity (blue: upflow, red: downflow, reaching speeds of about 15 km/s in the current sheet, which are driven by reconnection of magnetic field lines in the current sheet.)

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