The emergence of three-dimensional magneto-hydrodynamic (MHD) simulations of stellar atmospheres has sparked a need for efficient radiative transfer codes to calculate detailed synthetic spectra. We present RH 1.5D, a massively parallel code based on
the RH code and capable of performing Zeeman polarised multi-level non-local thermodynamical equilibrium (NLTE) calculations with partial frequency redistribution for an arbitrary amount of chemical species. The code calculates spectra from 3D, 2D or 1D atmospheric models on a column-by-column basis (or 1.5D). While the 1.5D approximation breaks down in the cores of very strong lines in an inhomogeneous environment, it is nevertheless suitable for a large range of scenarios and allows for faster convergence with finer control over the iteration of each simulation column. The code scales well to at least tens of thousands of CPU cores, and is publicly available. In the present work we briefly describe its inner workings, strategies for convergence optimisation, its parallelism, and some possible applications.
We revisit an old question: what are the effects of observing stratified atmospheres on scales below a photon mean free path? The mean free path of photons emerging from the solar photosphere and chromosphere is near 100 km. Using current 1m-class te
lescopes, the mean free path is on the order of the angular resolution. But the Daniel K. Inoue Solar Telescope will have a diffraction limit of 0.020 near the atmospheric cutoff at 310nm, corresponding to 14 km at the solar surface. Even a small amount of scattering in the source function leads to physical smearing due to this solar fog, with effects similar to a degradation of the telescope PSF. We discuss a unified picture that depends simply on the nature and amount of scattering in the source function. Scalings are derived from which the scattering in the solar atmosphere can be transcribed into an effective Strehl ratio, a quantity useful to observers. Observations in both permitted (e.g., Fe I 630.2 nm) and forbidden (Fe I 525.0 nm) lines will shed light on both instrumental performance as well as on small scale structures in the solar atmosphere.
Measurements from the Solar Irradiance Monitor (SIM) onboard the SORCE mission indicate that solar spectral irradiance at Visible and IR wavelengths varies in counter phase with the solar activity cycle. The sign of these variations is not reproduced
by most of the irradiance reconstruction techniques based on variations of surface magnetism employed so far, and it is not clear yet whether SIM calibration procedures need to be improved, or if instead new physical mechanisms must be invoked to explain such variations. We employ three-dimensional magneto hydrodynamic simulations of the solar photosphere to investigate the dependence of solar radiance in SIM Visible and IR spectral ranges on variations of the filling factor of surface magnetic fields. We find that the contribution of magnetic features to solar radiance is strongly dependent on the location on the disk of the features, being negative close to disk center and positive toward the limb. If features are homogeneously distributed over a region around the equator (activity belt) then their contribution to irradiance is positive with respect to the contribution of HD snapshots, but decreases with the increase of their magnetic flux for average magnetic flux larger than 50 G in at least two of the Visible and IR spectral bands monitored by SIM. Under the assumption that the 50 G snapshots are representative of quiet Sun regions we find thus that the Spectral Irradiance can be in counter-phase with the solar magnetic activity cycle.
