HAO 2010 PROFILES IN SCIENCE: Dr. Matthias Rempel
Area of expertise: Sun and Upper Atmosphere
Specialties: computer modeling of sunspots and other solar features
Dr. Matthias Rempel is a Scientist III in the High Altitude Observatory of the National Center for Atmospheric Research. He received his PhD in Astronomy and Astrophysics in 2001 from the University of Göttingen, Germany. His initial work with HAO began in 2002 as an ASP Fellow. He focuses his research on modeling of MHD processes in the solar interior, coupled models of the differential rotation, meridional flow, and large-scale dynamo, addressing non-kinematic effects and cycle variations of the solar differential rotation (torsional oscillations). Rempel uses computer models to study key features of the Sun, including regions that cannot be directly observed. He led an international team of scientists to create the first-ever comprehensive model of sunspots, the dark patches on the Sun's surface that are associated with the 11-year solar cycle and solar storms. His work, which also focuses on an area of the Sun known as the overshoot region that is beneath the visible solar surface, can help scientists better understand the Sun's magnetic fields and the transport of energy from the Sun's interior into the solar system.
RT/MHD modeling of the solar surface layers
The solar photosphere is a transition region in which the primary energy transport mechanism switches from convection to radiative transfer. At the same time the plasma becomes partially ionized due to the lower temperatures, requiring a more complicated equation of state. Also, the role of the magnetic field is changing: while the interior of the sun is dominated by the gas pressure, in or above the photosphere the magnetic pressure becomes the dominant contribution. Due to the rather short density scale height the photosphere is a highly stratified medium in which convective motions easily steepen up to supersonic flows and shock waves. The combination of all these conditions make numerical modeling of the photosphere challenging, but also extremely interesting due to the strong interaction between convection, magnetic field and radiation and the possibility for in depth comparison with high-resolution observations.
After recent success comprehensive numerical simulations of complete sunspots, the research focused on two direction: 1) High-resolution simulations of sunspot fine structure to evaluate the robustness of numerical results with respect to resolution. 2) Simulations at moderate resolution that focus on large domain sizes and cover a time span of up to 48 hours. To evaluate the robustness of numerical results a convergence study was performed with resolution ranging from 96x96x32 to 16x16x12 km, the highest resolution case is presented in the figure. We found that the magneto-convection process that has been identified previously is robust and the flow velocity of the Evershed outflow is well captured starting from a resolution of about 48x24 km. Using different boundary conditions allowed us to simulate an individual sunspot (instead of a pair as previously done), resulting in a more realistic outer penumbra. We conducted a series of simulations in up to 16 Mm deep domains to study the influence of the bottom boundary condition on the structure and stability of sunspots. While the bottom boundary has significant influence in shallow domains used for the high-resolution studies mentioned above, its influence is substantially diminished in a 16 Mm deep domain. Simulations with time durations of 24 to 48 hours were used to model self-consistently the wave propagation through sunspots with the goal of testing and improving helioseismic inversion methods. The data was analyzed in collaboration with scientists from CoRA/NWRA as part of the SDO Science Center project. In collaboration with the Lockheed Martin Solar and Astrophysics Laboratory (LMSAL) the numerical sunspot model was also used to perform a flux emergence simulations with radiative transfer on the scale of solar active regions. Previous work on flux emergence is currently extended to a scale of 150x75x16 Mm, which allows for the first time to form sunspots with more then 1022 Mx flux.
In the near future research will focus on high-resolution simulations of sunspot fine structure including non-grey radiative transfer to allow for forward modeling of spectral signatures and their comparison with high-resolution observations. For this project we are in close collaboration with the Max Planck Institute for Solar System Research (Germany), the Kiepenheuer Institute for Solar Physics (Germany) and the National Solar Observatory (NSO). At the same time we will continue with our focus on large domain sizes to address the flux emergence and sunspot formation process with greater realism.
Team: Matthias Rempel (HAO/NCAR), scientists from CoRA/NWRA, Lockheed Martin Solar and Astrophysics Laboratory, Max Planck Institute for Solar System Research (Germany), the Kiepenheuer Institute for Solar Physics (Germany) and the National Solar Observatory (NSO).