  One of the most ambitious goals of gravitational-wave astronomy is to observe
the stochastic gravitational-wave background. Correlated noise in two or more
detectors can introduce a systematic error, which limits the sensitivity of
stochastic searches. We report on measurements of correlated magnetic noise
from Schumann resonances at the widely separated LIGO and Virgo detectors. We
investigate the effect of this noise on a global network of interferometers and
derive a constraint on the allowable coupling of environmental magnetic fields
to test mass motion in gravitational-wave detectors. We find that while
correlated noise from global electromagnetic fields could be safely ignored for
initial LIGO stochastic searches, it could severely impact Advanced LIGO and
third-generation detectors.
