Intensity contrast of solar plage as a function of magnetic flux at high spatial resolution

Abstract

Magnetic elements have an intensity contrast that depends on the type of region they are located in (for example quiet Sun, or active region plage). Observed values also depend on the spatial resolution of the data. Here we investigate the contrast-magnetic field dependence in active region plage observed near disk center with SUNRISE during its second flight in 2013. The wavelengths under study range from the visible at 525 nm to the near ultraviolet (NUV) at 300 nm and 397 nm. We use quasi-simultaneous spectropolarimetric and photometric data from the Imaging Magnetograph eXperiment (IMaX) and the Sunrise Filter Imager (SuFI), respectively. We find that in all wavelength bands, the contrast exhibits a qualitatively similar dependence on the line-of-sight magnetic field, BLOS, as found in the quiet Sun, with the exception of the continuum at 525 nm. There, the contrast of plage magnetic elements peaks for intermediate values of BLOS and decreases at higher field strengths. By comparison, the contrast of magnetic elements in the quiet Sun saturates at its maximum value at large BLOS. We find that the explanation of the turnover in contrast in terms of the effect of finite spatial resolution of the data is incorrect with the evidence provided by the high-spatial resolution SUNRISE data, as the plage magnetic elements are larger than the quiet Sun magnetic elements and are well-resolved. The turnover comes from the fact that the core pixels of these larger magnetic elements are darker than the quiet Sun. We find that plages reach lower contrast than the quiet Sun at disk center at wavelength bands formed deep in the photosphere, such as the visible continuum and the 300 nm band. This difference decreases with formation height and disappears in the Ca II H core, in agreement with empirical models of magnetic element atmospheres.