Werner Schaffenberger

THE HORIZONTAL INTERNETWORK MAGNETIC FIELD: NUMERICAL SIMULATIONS IN COMPARISON TO OBSERVATIONS WITH HINODE

Observations with the Hinode space observatory led to the discovery of predominantly horizontal magnetic fields in the photosphere of the quiet internetwork region. Here we investigate realistic numerical simulations of the surface layers of the Sun with respect to horizontal magnetic fields and compute the corresponding polarimetric response in the Fe i 630 nm line pair. We find a local maximum in the mean strength of the horizontal field component at a height of around 500 km in the photosphere, where, depending on the initial state or the boundary condition, it surpasses the vertical component by a factor of 2.0 or 5.6. From the synthesized Stokes profiles, we derive a mean horizontal field component that is 1.6 or 4.3 times stronger than the vertical component, depending on the initial state or the boundary condition. This is a consequence of both the intrinsically stronger flux density of and the larger area occupied by the horizontal fields. We find that convective overshooting expels horizontal fields to the upper photosphere, making the Poynting flux positive in the photosphere, whereas the Poynting flux is negative in the convectively unstable layer below it.

First local helioseismic experiments with CO5BOLD

With numerical experiments we explore the feasibility of using high frequency waves for probing the magnetic fields in the photosphere and the chromosphere of the Sun. We track a plane-parallel, monochromatic wave that propagates through a non-stationary, realistic atmosphere, from the convection-zone through the photosphere into the magnetically dominated chromosphere, where it gets refracted and reflected. We compare the wave travel time between two fixed geometrical height levels in the atmosphere (representing the formation height of two spectral lines) with the topography of the surface of equal magnetic and thermal energy density (the magnetic canopy or β =1 contour) and find good correspondence between the two. We conclude that high frequency waves indeed bear information on the topography of the ‘magnetic canopy’.

Tutorials in Introductory Physics: The Pain and the Gain

In an introductory physics sequence with a large enrollment of premedical students, traditional recitation sessions were replaced by Tutorials in Introductory Physics, developed by the Physics Education Group at the University of Washington. Initially, summative test scores (as well as FCI scores) dramatically increased, but so did student complaints and workload. Both effects decreased over time. The paper discusses issues that instructors should consider when contemplating implementation of the tutorials.

Analysis of mirror modes convected from the bow shock to the magnetopause

Abstract Spacecraft observations confirm the existence of mirror fluctuations in the magnetosheath. The mirror instability occurs in an anisotropic magnetized plasma when the difference between perpendicular and parallel (with respect to the magnetic field) plasma pressure exceeds a threshold depending on the perpendicular plasma beta. The anisotropy of the plasma pressure increases from the shock to the magnetopause as a result of magnetic field line stretching. This gives rise to plasma fluctuations which in turn lead to a relaxation between parallel and perpendicular temperatures. Mirror perturbations do not propagate and are convected with plasma flow along the streamlines. Using an anisotropic steady-state MHD flow model, we calculate the growth of mirror fluctuations from the bow shock to the magnetopause along the subsolar streamline. For the anisotropic MHD model, we use the empirical closure equation suitable for the AMPTE/IRM observations. The amplitudes of mirror fluctuations, which are obtained as a function of distance from the magnetopause, are directly compared with AMPTE/IRM observations on October 24, 1985. With regard to both the amplification of the magnetic field and the plasma density oscillations, as well as the location of maximum amplitudes, model calculations are in good agreement with values obtained from the AMPTE/IRM data.

Cellular Automata Models for Convection

We present here three models for convection. The models make use of the concept of cellular automata (CA). CA are discrete systems. The advantages of CA are their simple and parallel structure. The simplest of the presented models simulates two-dimensional Boussinesq convection. The two other models are extensions to compressible fluids and three-dimensional convection, respectively. We derive the model equations for the simplest model and present some of our results.

On the Influence of Supernova Shockfronts on the Stability of the Solar System

We studied the influence of a supernova shock wave on the orbits of celestial bodies of our solar system. The influence of the shock waves for planets is negligible. We found considerable effects only for smaller objects like comets and hypothetical objects in the Kuiper belt.