Magnetic field measurements in flux transfer events (FTEs) at the Earth's magnetopause often reveal the existence of a strong core magnetic field. We describe a possible mechanism for the origin of such strong core fields in the framework of a recently proposed model for FTEs. In this model bursty reconnection at a single X line, which extends over a large longitude segment, leads to loop like magnetic fields. As these loops move due to the Maxwell stress along the magnetopause, the magnetopause magnetic field directed perpendicular to the loops is swept up and correspondingly enhanced. When the downward pulling force due to the stress of the swept up magnetopause field equals the upward pulling force due to the loops an electromagnetic force equilibrium is reached. The core field strength determined by the force balance is proportional to the longitude extent of the reconnection line. Events with a large core field are therefore expected to have a higher detection probability than events with a small core field. ¿ American Geophysical Union 1988