From numerical simulations of highest spatial resolution and high resolution observations, we may guess the science questions to be tackled with future large solar telescopes. Selected examples are discussed, trying to determine the instrument requirements for answering questions regarding polarimetric accuracy or spatial and temporal resolution. Besides such foreseeable science questions, however, it is worthwhile to recall that the best discoveries come unexpectedly and to wonder about what instrument capabilities may be most likely conducive to the unexpected.

René Salhab

The rms of the relative fluctuation of the continuum intensity over the solar surface is sometimes taken as a measure for the quality of seeing and the telescope optics. Given space conditions, it increases with increasing telescope aperture, hence with increasing spatial resolution. Not so for the simulations, which show a fairly constant high contrast, independent of spatial resolution. This apparent paradox is also reflected in the intensity histogram. Instead of the bimodal histograms known from observations and simulations with two peaks corresponding to the granular and the intergranular intensities, we predict the disappearance of this bimodality at high spatial resolution.

René Salhab

From observations of the Sun, we know of the existence of a small-scale magnetism in the form of photospheric magnetic flux concentrations of granular and sub-granular scales. This small-scale magnetic structur has consequences for global quantities such as the total solar irradiance or the convective blueshift. Here we have a look at properties of the small-scale magnetism of cool stellar atmospheres, in particular at effects on the radiative intensity and flux from the stellar surface. Lack of direct observations, these properties are derived from three-dimensional radiation magnetohydrodynamic simulations, which start with either, a homogeneous vertical magnetic field, or, for comparison, without a magnetic field. The two settings are thought to represent states of high and low small-scale magnetic activity corresponding to maximum and minimum of a stellar magnetic cycle. Besides radiative, we also discuss interesting magnetic properties of the stellar small-scale magnetism.

An important discovery achieved with Hinode’s Solar Optical Telescope/Spectro-Polarimeter (SOT/SP) was that in the mean, the magnetic field over wide areas of the quiet Sun photosphere is predominantly horizontal, i.e., parallel to the solar surface. This result, however, remained not undisputed because the transversal Zeeman component is much more affected by photon noise than the longitudinal component. This noise may be the source for a gross overestimate of the horizontal field. The present contribution briefly reviews the status of the debate and proposes a bias-free measurement method. Furthermore, the polarimetric accuracy required for a definitive answer of the quest of the horizontal magnetic field is discussed. A new method for the removal of systematic errors, practiced at IRSOL, is presented.