A thoron field was a field comprised of thoron particles. A thoron field could distort and jam sensor readings.
In a confrontation with three Cardassian warships shortly after the discovery of the Bajoran wormhole in 2369, Major Kira Nerys of space station Deep Space 9 ordered a high-energy throron field established to prevent the warships from scanning the station's defenses. When the Cardassians finally penetrated the field, the station used duranium shadows to create the illusion of massive armaments. (DS9: "Emissary")
Later that year, Miles O'Brien launched a class 4 probe into a subspace rupture. The probe penetrated the thoron field of the anomaly. (DS9: "If Wishes Were Horses")
Prior to a Klingon attack in 2372, General Martok charged that the station was using thoron fields and duranium shadows to give the appearance that the station was bristling with armaments; however, by this time the station's defenses had indeed been significantly upgraded. (DS9: "The Way of the Warrior")