Plasma density suppression process around the cusp revealed by simultaneous CUTLASS and EISCAT Svalbard radar observations

Simultaneous CUTLASS and EISCAT Svalbard radar (ESR) observations on Februa ry 1, 1998, are used to study the generation of plasma density suppression, which may ultimately result in polar cap patch formation, occurring: aroun d the cusp region under IMF B, negative and By positive conditions. The CUT LASS 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 ge omagnetic field. We focus on two events for which the density suppressions (30-60%) formed in harmony with strong plasma drifts (> 1500 ms(-1)) lastin g 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 energeti c particle precipitation, maybe giving rise to polar patches. In the other event, the density suppression is related to an appearance of eastward dire cted 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 b e caused by enhanced chemical reactions. We tentatively propose that the ea stward plasma jets transported less dense plasma from earlier local times o ver the ESR. Rapid change of IMF By polarity is another candidate for produ cing the density suppression. A role of HF radar wave refraction in explain ing this event is discussed.