The solar perspective on radiative transfer and interferometry

Interferometry with large telescopes such as the VLTI is at present not a realistic option in solar physics, nor will it be feasible in the near future. Its proximity, however, allows us to view the Sun in far more detail than we can reasonably expect for other stars even with the next generations of interferometric instruments. For that reason the Sun imposes extremely high requirements on the radiative transfer computations that serve to explain the observations. In the last few decades we have seen the picture of the solar photosphere evolve to a point where we think that we have "understood" most of the basic structures and their physics. The chromosphere, however, still presents a challenge for (magneto) hydrodynamic and radiative transfer modeling. Time-independent gray or multiband radiative transfer assuming local thermodynamic equilibrium (LTE), which for obvious reasons work well in the photosphere, are not valid in the chromosphere. In principle all equilibrium assumptions and time independence should be dropped. The theoretical formulation of that problem is straightforward, but it leads to a numerical problem whose solution requires far more computational resources than currently available. For that reason modelers will have no other option but to continue reducing the full problem to a number of tractable simpler problems with different assumptions and trying to figure out which assumptions are valid.