Photon mean free paths, scattering, and ever-increasing telescope resolution

Abstract

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 telescopes, 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.

Solar Physics, in press

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Cited by in corpus (7)

• Partial redistribution in 3D non-LTE radiative transfer in solar atmosphere models
• A Statistical Comparison between Photospheric Vector Magnetograms Obtained by SDO/HMI and Hinode/SP
• Numerical non-LTE 3D radiative transfer using a multigrid method
• New light on an old problem of the cores of solar resonance lines
• From Polarimetry to Helicity: Studies of Solar Magnetic Fields at the Huairou Solar Observing Station
• Three-dimensional non-LTE radiative transfer effects in Fe I lines IV. Line formation at high spatial resolution
• Accelerating 3D Non-LTE Synthesis with Graph Neural Networks