  We propose to control light trapping in a large ensemble of cold atoms by an
external, static magnetic field. For an appropriate choice of frequency and
polarization of the exciting pulse, the field is expected to speed up the
fluorescence of a dilute atomic system. In a dense ensemble, the field does not
affect the early-time superradiant signal but amplifies intensity fluctuations
at intermediate times and induces a very slow, nonexponential long-time decay.
The slowing down of fluorescence is due to the excitation of spatially
localized collective atomic states that appear only under a strong magnetic
field and have exponentially long lifetimes. Our results therefore pave a way
towards experimental observation of the disorder-induced localization of light
in cold atomic systems.
