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Magnetic flux is continuously being removed and replenished on the solarsurface. To understand the removal process we carried out 3D radiative MHDsimulations of the evolution of patches of photospheric magnetic field withequal amounts of positive and negative flux. We find that the flux is removedat a rate corresponding to an effective turbulent diffusivity, of 100--340km^2/s, depending on the boundary conditions. For average unsigned fluxdensities above about 70 Gauss, the percentage of surface magnetic energycoming from different field strengths is almost invariant. The overall processis then one where magnetic elements are advected by the horizontal granularmotions and occasionally come into contact with opposite-polarity elements.These reconnect above the photosphere on a comparatively short time scale afterwhich the U loops produced rapidly escape through the upper surface while thedownward retraction of inverse-U loops is significantly slower, because of thehigher inertia and lower plasma beta in the deeper layers.

Decay of a simulated mixed-polarity magnetic field in the solar surface layers