Some developments in the theory of magnetic flux concentrations in the solar atmosphere

Abstract

Most of the magnetic flux in the solar photosphere is concentrated in small-scale structures of large field strength, called magnetic elements. We discuss briefly the observationally determined properties of magnetic elements and the theoretical concepts for the origin of magnetic flux filamentation and concentration. New results of model calculations for 2D magnetic flux sheets on the basis of numerical simulation of the compressible MHD equations including a full (grey) radiative transfer are presented. Synthetic Stokes profiles of spectral lines and continuum intensity distributions serve to compare the theoretical models with observational data. Among the key results are: (1) The upper layers of the magnetic structure become hotter than the environment due to radiative illumination effects; (2) a strong convective flow evolves with horizontal velocity of 2 km/s towards the flux sheet and a narrow “downflow jet” with velocity up to 6 km/s adjacent to the magnetic structure; (3) both flux sheet and non-magnetic environment oscillate with a period around 5 minutes. Comparison with observed properties of solar magnetic elements reveals: (4) Calculated and semi-empirical temperature profiles as function of height in the photosphere are in reasonable agreement; (5) the calculated velocity field around flux concentrations explains the area asymmetry of the observed Stokes V-profiles including their center-limb variation; (6) the calculated continuum intensity of a flux sheet model is compatible with the values inferred from high spatial resolution observations of bright points at solar disk center; (7) the observed center-limb variation of facular contrast at low or medium spatial resolution is reproduced by arranging calculated flux sheets in arrays. We stress the importance of MHD simulation models for the analysis and interpretation of data from future facilities for high spatial resolution observations like OSL and LEST.