On January 15, 1991, the Sondrestrom incoherent scatter radar probed the midday high-latitude ionosphere to gather evidence for the formation and entry of polar cap patches. During the experiment the interplanetary magnetic field (IMF) Bz was positive and steady for few hours until 1548 UT when a short negative excursion of Bz occurred. Prior to the Bz excursion, and when this parameter was directed northward, the Sondrestrom radar detected a quasi-stationary density feature situated near the poleward boundary of the oval. Convergent electric fields and slightly elevated Te values were seen accompanying the F region density feature. A numerical model of the high-latitude ionosphere that uses a steady north Bz Heppner and Maynard convection pattern suggests that under these IMF conditions a tongue of ionization (TOI) can be formed near the midday sector, but it is confined to the poleward boundary of the auroral oval. It does not traverse into the polar cap. This simulated Bz northward TOI resembles the density feature seen in the radar data prior to the Bz negative excursion. When the Bz value was oriented southward, the radar detected the density feature moving poleward and then disappearing to the north of the radar field of view. At this time of Bz negative the radar data also displayed elevated Ti values and a new pattern of line-of-sight velocities. Nearly 34 min after the density feature departed from the field of view of the Sondrestrom radar, the Qaanaaq digisonde measured a factor of 2 increase in the f0F2 values. Similar enhancements are typically attributed to the passage of a patch. We also conducted a numerical simulation of the transit of the density feature from its initial location near the polar cap boundary up to its passage through the Qaanaaq station. The time that the density feature reaches Qaanaaq in our simulations is in good agreement with the actual time that the enhanced patch-like number density was observed at Qaanaaq. The Bz switching mechanism does not dispute the validity of other patch formation mechanisms; it merely suggests that a Bz northward TOI can end up as a polar cap patch if a timely reversal of Bz occurs. ¿ 1998 American Geophysical Union