Correlated magnetic noise from Schumann resonances threatens to contaminate the observation of a stochastic gravitational-wave background in interferometric detectors. In previous work, we reported on the first effort to eliminate global correlated noise from the Schumann resonances using Wiener filtering, demonstrating as much as a factor of two reduction in the coherence between magnetometers on different continents. In this work, we present results from dedicated magnetometer measurements at the Virgo and KAGRA sites, which are the first results for subtraction using data from gravitational-wave detector sites. We compare these measurements to a growing network of permanent magnetometer stations, including at the LIGO sites. We show the effect of mutual magnetometer attraction, arguing that magnetometers should be placed at least one meter from one another. In addition, for the first time, we show how dedicated measurements by magnetometers near to the interferometers can reduce coherence to a level consistent with uncorrelated noise, making a potential detection of a stochastic gravitational-wave background possible.
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The authors would like to thank Dr. Brian O’Reilly for a careful reading of an earlier version of the manuscript. The authors gratefully acknowledge the LIGO Observatories, European Gravitational Observatory (EGO), and KAGRA for technical support. The magnetic field measurement in Kamioka was supported by the Joint Usage/Research Center program of Earthquake Research Institute, the University of Tokyo. The authors also would like to thank Christina Daniel, Margarita Vidreo, and Julia Kruk, who helped bury the LHO site magnetometers, both during the site selection process and the final installation. M. C. was supported by the David and Ellen Lee Postdoctoral Fellowship at the California Institute of Technology. A. C. is supported by the INFN Doctoral Fellowship at the University of Genova. N. C.’s work is supported by NSF Grant No. PHY-1505373. E. T. is supported through ARC FT150100281. J. K., A. K., and J. M. were supported by the National Science Center, Poland, under Grant No. 2012/04/M/ST10/00565.
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