Rotation measure (RM) grids of extragalactic radio sources have been widelyused for studying cosmic magnetism. But their potential for exploring theintergalactic magnetic field (IGMF) in filaments of galaxies is unclear, sinceother Faraday-rotation media such as the radio source itself, interveninggalaxies, and the interstellar medium of our Galaxy are all significantcontributors. We study statistical techniques for discriminating the Faradayrotation of filaments from other sources of Faraday rotation in futurelarge-scale surveys of radio polarization. We consider a 30 degree times 30degree field-of-view toward the south Galactic pole, while varying the numberof sources detected in both present and future observations. We select sourceslocated at high redshifts and toward which depolarization and opticalabsorption systems are not observed, so as to reduce the RM contributions fromthe sources and intervening galaxies. It is found that a high-pass filter cansatisfactorily reduce the RM contribution from the Galaxy, since the angularscale of this component toward high Galactic latitudes would be much largerthan that expected for the IGMF. Present observations do not yet provide asufficient source density to be able to estimate the RM of filaments. However,from the proposed approach with forthcoming surveys, we predict significantresiduals of RM that should be ascribable to filaments. The predicted structureof the IGMF down to scales of 0.1 degree should be observable with data fromthe SKA, if we achieve selections of sources toward which sight lines do notcontain intervening galaxies and RM errors are less than a few rad m-2.
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