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A mechanism for the formation of polar cap patches is proposed based on data collected by the Sondrestrom incoherent scatter radar, magnetometers located on the west and east coasts of Greenland and Digisondes, digital ionospheric sounders, operated at Qaanaaq and Sondrestrom. This instrumentation has served to identify the formation of patchlike structures and follow their temporal evolution and entry into the polar cap. Prior to the onset of the event, the background ionospheric plasma was moving in a poleward direction and formed part of a tongue of ionization directed toward and into the polar cap. The event starts with the appearance of a fast plasma jet containing eastward directed velocities in excess of 2 km s−1. This plasma jet consists of a channel extending 300 km in width, where the F region ion temperature reaches values in excess of 4000 K and the E layer Ti is enhanced above 5000 K. The existence of a region containing high electron temperatures associated with soft precipitation (likely the cusp) and located adjacent and equatorward of the fast plasma jet is very suggestive of the jet being the result of a tension force acting upon recently reconnected field lines. This view is supported by the fact that the fast plasma jet was seen in the prenoon sector under IMF By negative conditions. The elevated Ti values inside the plasma jet are exactly collocated with depleted F region densities. We suggest that the recombination loss of O+ is increased by a factor >10 due to the dependence of the O+ + N2 reaction on the ion temperature. The eroding action of the fast plasma jet proceeds until the poleward moving tongue of ionization is divided into regions containing high and low densities. Magnetic field perturbations associated with the fast plasma jet were observed by the different magnetometer stations. The large negative bays recorded at most of the sites have been used to trace the poleward motion of the jet. At the time that the fast plasma jet reaches Qaanaaq, the Digisonde there measured low ƒ0F2 values. A few minutes before and after this minimum, high ƒ0F2 values were observed. The ratio of the enhanced ƒ0F2 values on either side of the minimum, to the minimum ƒ0F2 is about 2 (or a density ratio of 4). This is the commonly accepted signature of a polar cap patch. The series of events leading to the formation of the patchlike density structure has been designated a density breakoff event.