Photospheric fine structure: An observational challenge

An analysis of radiative transfer effects on the visibility of small-scale structures

Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany

Corresponding author: J. Bruls, bruls@kis.uni-freiburg.de, http://www.kis.uni-freiburg.de/~bruls

Received: 14 August 2000
Accepted: 26 October 2000

Abstract

The requirement on spatial resolution of a large solar telescope is frequently based on the assumption that the smallest observable features in the solar photosphere could not be much different from a photon mean free path due to smoothing by radiative transfer. Although hydrodynamic simulations show that much smaller structures should exist, a resolution of the order of 100 km is typically considered sufficient for observations in the photosphere. We performed 2D non-LTE radiative transfer computations for thin flux sheets with widths ranging from 10 to 160 km in the solar photosphere. We demonstrate that such small structures -should they exist -could be observed as small scale variations of intensity and polarization. We conclude that the size limit below which photospheric structure cannot be observed due to smoothing radiative transfer effects must lie well below 10 km. A spatial resolution limit for telescopes based on photon mean free path arguments should therefore be abandoned.