  It has been recognized that the magnetic fields from the Schumann resonances
could affect the search for a stochastic gravitational-wave background by LIGO
and Virgo. Presented here are the observations of short duration magnetic field
transients that are coincident in the magnetometers at the LIGO and Virgo
sites. Data from low-noise magnetometers in Poland and Colorado, USA, are also
used and show short duration magnetic transients of global extent. We measure
at least 2.3 coincident (between Poland and Colorado) magnetic transient events
per day where one of the pulses exceeds 200 pT. Given the recently measured
values of the magnetic coupling to differential arm motion for Advanced LIGO,
there would be a few events per day that would appear simultaneously at the
gravitational-wave detector sites and could move the test masses of order
$10^{-18}$ m. We confirm that in the advanced detector era short duration
transient gravitational-wave searches must account for correlated magnetic
field noise in the global detector network.
