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Early magnetographic observations indicated that magnetic field in the solarphotosphere has unresolved small-scale structure. Near-infrared and opticaldata with extremely high spatial resolution show that these structures havescales of few tens of kilometres, which are not resolved in the majority ofsolar observations. The goal of this study is to establish the effect ofunresolved photospheric magnetic field structure on Stokes profiles observedwith relatively low spatial resolution. Ultimately, we aim to develop methodsfor fast estimation of the photospheric magnetic filling factor andline-of-sight gradient of the photospheric magnetic field, which can be appliedto large observational data sets. We exploit 3D MHD models ofmagneto-convection developed using MURAM code. Corresponding profiles of Fe I6301.5 and 6302.5 $\mathrm{\AA}$ spectral lines are calculated using NICOLEradiative transfer code. The resulting I and V Stokes [x,y,$\lambda$] cubeswith reduced spatial resolution of 150 km are used to calculate magnetic fieldvalues as they would be obtained in observations with Hinode/SOT or SDO/HMI.Three different methods of the magnetic filling factor estimation areconsidered: the magnetic line ratio method, Stokes V width method and a simplestatistical method. We find that the statistical method and the Stokes V widthmethod are sufficiently reliable for fast filling factor estimations.Furthermore, we find that Stokes $I\pm V$ bisector splitting gradient can beused for fast estimation of line-of-sight gradient of the photospheric magneticfield.

Analysis of unresolved photospheric magnetic field structure using Fe I 6301 and 6302 lines