The bulk of radiation of a star originates from the photosphere, its visible surface. The temperature surprisingly increases outwards through the stellar atmosphere until, in case of the Sun, million of degrees are reached in the corona. While it has been shown that the required heating in the corona is due to magnetic fields, it is yet not clear for the layer in between, the so-called chromosphere.
The chromosphere is hard to observe and difficult to model so that, despite large progress during the last de cades, the thermal structure of stellar chromospheres, including the one of the Sun, and the related heating processes are still poorly understood and controversially debated. The heating, that is attained by pure mechanical heating via shock waves and/or processes connected to magnetic fields, must provide sufficient energy to counterbalance the radiative emission derived from observations of chromospheric diagnostics like the spectral lines of calcium and magnesium. This emission varies strongly between different stars, suggesting a different coverage with magnetic fields, but is always larger than the so-called basal flux. Recent simulations and also high-resolution observations suggest that the layer is highly structured and very dynamic. A time-dependent and spatially resolved numerical simulation is thus mandatory for a realistic description.
The project proposed here aims at the development and implementation of new methods to realistically describe the energy balance of stellar chromospheres, including simple model atoms for the most important agents calcium and magnesium and the resulting coupling between radiation field and chromospheric gas. The final goal is a set of three-dimensional self-consistent magnetohydrodynamics simulations with realistic chromospheric radiative transfer. Detailed comparisons with observations will be the ultimate key to the understanding of structure and heating of the chromosphere of the Sun and other stars.
Fields of science
Call for proposal
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