R. Hammer

The solar atmosphere

As a typical star, and the only one that can be spatially resolved by direct means, the study of the Sun has provided an insight into many of the fundamental processes taking place in stellar atmospheres, often at small scales. A prime example is magneto-convection or the formation of coronae and the consequent emission of copious amounts of X-rays. In addition, the Sun’s apparent brightness allows measurements with unprecedented accuracy. Thus the Sun is the standard against which cosmic abundances are compared. Its high apparent brightness also means that the Sun is a strong source at almost all wavelengths and thus detectable with simple, not particularly sensitive equipment such as the early instruments flown in space. Thus for many wavelengths the Sun was the first (or one of the first) cosmic source(s) detected.

On the Intrinsic Difficulty of Producing Stellar Coronae With Acoustic Waves

We discuss theoretical limits on the potential of acoustic waves to produce coronae around inactive solar-like stars. For a star of solar mass and radius, short-period shock waves (which are produced most efficiently in the convection zone) could generate a corona only for base pressures and energy fluxes that are smaller than the observed values by several orders of magnitude. The energy budget of solar coronal holes could be supplied only by shock waves of extremely long periods (≈ 2Pa), which are difficult to produce in solar-like stars, and which suffer from propagation restrictions in the photosphere and chromosphere. This fundamental difficulty to produce coronae with acoustic waves is even aggravated towards stars of smaller gravity.

Chromospheric Dynamics and Line Formation

The solar chromosphere is very dynamic, due to the presence of large amplitude hydrodynamic waves. Their propagation is affected by NLTE radiative transport in strong spectral lines, which can in turn be used to diagnose the dynamics of the chromosphere. We give a basic introduction into the equations of NLTE radiation hydrodynamics and describe how they are solved in current numerical simulations. The comparison with observation shows that one‐dimensional codes can describe strong brightenings quite well, but the overall chromospheric dynamics appears to be governed by three‐dimensional shock propagation.

Minimum Coronal Energy Requirements: Constraints for Heating Mechanisms

New calculations of the minimum energy requirements of magnetically open coronal regions on cool stars are presented. Several of the simplifying assumptions underlying previous calculations have been omitted; thus the present results are more accurate and can be extended to extreme regions of the parameter space. The models can be scaled from one star to another. For low gravity stars, the waviness of the emissivity as a function of temperature leads to multiple local minima of the coronal energy loss.