Simultaneous CUTLASS and EISCAT Svalbard radar (ESR) observations on February 1, 1998, are used to study the generation of plasma density suppression, which may ultimately result in polar cap patch formation, occurring around the cusp region under IMF Bz negative and By positive conditions. The CUTLASS HF radars in Iceland and Finland observed F region plasma drifts in a wide area including the ESR field of view while the ESR monitored electron density, electron temperature, ion temperature, and ion motion along the geomagnetic field. We focus on two events for which the density suppressions (30--60%) formed in harmony with strong plasma drifts (>1500 m s-1) lasting for 5--12 min. In one event, the suppression is mainly caused by enhanced chemical reactions in the F region due to the intensified convection flows which also raise ion temperature by 1500--2000 K through frictional heating. This process can chop preexisting high-density region produced by energetic particle precipitation, maybe giving rise to polar patches. In the other event, the density suppression is related to an appearance of eastward directed high-speed plasma jets in a limited region with a latitudinal width of >100 km and a longitudinal extension of >500 km. The ESR data, however, show no ion temperature increase, suggesting that the suppression may not be caused by enhanced chemical reactions. We tentatively propose that the eastward plasma jets transported less dense plasma from earlier local times over the ESR. Rapid change of IMF By polarity is another candidate for producing the density suppression. A role of HF radar wave refraction in explaining this event is discussed. ¿ 2001 American Geophysical Union