Issue 1, 2014
On 31 May 2013, Jörn Warnecke, the last of four PhD students supported by the ERC AstroDyn project, successfully defended his dissertation on "Combining Models of Coronal Mass Ejections and Solar Dynamos".
In his thesis consisting of 6 refereed papers, Jörn produced the first dynamo models coupled self-consistently to an outer corona-like layer. His first models consisted of a Cartesian domain in which plasmoid ejections were produced above the surface as a consequence of the time variability of the dynamo beneath the surface. He then extended his work to spherical wedge-like domains which produced ejections that strongly resemble coronal mass ejections in the Sun. His models were based on convection-driven dynamo action. Unlike earlier models without outer coronal layer, he found more realistic differential rotation contours with more nearly spoke-like contours of constant angular velocity, as opposed to cylindrical contours.
Professor Juri Toomre from JILA acted as opponent. The examination committee consisted of Drs. Henrik Lundstedt from the IRF in Lund, Anita Kullen from the Alfvén Lab at KTH, and Dan Kiselman from the astronomy department in Stockholm.
On 29 May, Atefeh Barekat presented her Master's thesis on "Hydrodynamic simulations with a radiative surface".
Atefeh and Jörn on the formal and informal receptions after their thesis defences.
In her work, she began bridging the gap between more idealized simulations of solar convection using fixed radiative conductivity profiles in the optically thick approximation on the one hand and realistic simulations with table lookups for opacity and the equation of state on the other. Using a power law dependence of opacity on density and temperature, and allowing for the transition to the optically thin regime, she showed that one recovers a family of polytropic solutions in the optically thick lower part of the domain, together with an isothermal part in the upper optically thin part. The polytropic index depends in a simple manner on the power law exponents of the opacity. She also showed that the Peclet number (a measure of the degree of nonlinearity) can be controlled by changing the prefactor of the opacity. For numerical reasons, it cannot be too large. In fact, simulations without ad-hoc numerical diffusivities, must use prefactors that are lower than in the Sun by approximately five orders of magnitude.
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