THE EFFECT OF SEEING ON SOLAR MAGNETIC FLUX MEASUREMENTS

We investigate the influence of seeing upon measurement of magnetic flux of photospheric fields. For this purpose we quantify seeing variation in one day's observation at Big Bear Solar Observatory in terms of the Fried function, a Modulation Transfer Function for the atmospheric seeing. The temporal variation of seeing quality is compared with that of magnetic flux measured in a quiet region with size 5 \times 4 near the solar disk center. A good correlation is found between the seeing change and apparent evolution of magnetic flux values, implying, as a possibility, that magnetic flux measurement might have been modulated by seeing. Based on a simple model of ensembles of Gaussian magnetic elements we argue that even the net flux as well as the total flux can change due to seeing variation if the magnetograph has a finite detection threshold and if the intrinsic fluxes in one and the other polarities are unbalanced.