A thoron field is a field comprised of thoron particles. One can use a thoron field to distort and jam the sensor readings of threat forces.
In a confrontation with two Cardassian warships shortly after the discovery of the wormhole, Major Kira Nerys at starbase Deep Space 9 used a combination of thoron fields and duranium shadows to give the appearance that the space station was bristling with armaments. (DS9: "Emissary")
In the Klingon confrontation at DS9 in 2372, General Martok charged that the station was using these same two tactics to give the appearance that the station was bristling with armaments; unfortunately for him, that hypothesis proved to be quite false. (DS9: "The Way of the Warrior")