Simultaneous measurement of duskside subauroral irregularities from the CUTLASS Finland radar and EISCAT UHF system

Dusk scatter event (DUSE) (first reported by Ruohoniemi et al. [1988] ) is one of the most reproducible features among the SuperDARN radar backscatter within the subauroral ionosphere. Hosokawa et al. [2001] analyzed the scattering occurrence percentage of the Northern Hemisphere SuperDARN radars in a statistical fashion and pointed out that the region where the DUSE appears has a close relationship with the duskside end of the midlatitude trough in longitudinal direction. They proposed a model explaining the generation of the DUSE which employs a Sunward density gradient at the duskside edge of the trough and an ambient poleward electric field. In order to confirm the model proposed by Hosokawa et al. [2001] , we have investigated two DUSE events which had been observed by the CUTLASS Finland radar and the EISCAT UHF system simultaneously. Consequently, when the CUTLASS Finland radar observed the DUSE, a Sunward directed density gradient was observed by the EISCAT in the vicinity of the DUSE. After the passage of the DUSE the EISCAT observed an ion temperature enhancement which suggested that the EISCAT entered the trough through its duskside edge. These observational facts suggest that the geometry of the parameters around the DUSE is quite consistent with the model proposed by Hosokawa et al. [2001] . In addition, the EISCAT observed an enhancement of the poleward electric field around the DUSE, and a signature of the substorm was identified by the ground magnetometer on the nightside. We suggest generation mechanisms of the trough responsible for the DUSE during substorm conditions in terms of the role of the enhanced subauroral electric field and discuss a relationship between the DUSE and the other substorm‐related phenomena.