Plasma irregularities in the duskside subauroral ionosphere as observed with midlatitude SuperDARN radar in Hokkaido, Japan

We revealed the spatial distribution of HF radar echoes in the subauroral region by conducting a statistical analysis of scattering occurrence from the midlatitude Super Dual Auroral Radar Network (SuperDARN) radar in Hokkaido, Japan. Consequently, Dusk Scatter Event (DUSE) was identified as a most prominent backscatter target in these latitudes. Past studies have intended to associate an appearance of DUSE with the density gradient at the poleward or sunward edges of the midlatitude trough. However, exact spatial collocation between the source region of DUSE and the midlatitude trough has not been revealed because the SuperDARN radars in the auroral region could not observe the whole part of DUSE due to a limitation of the field-of-view coverage. Thus, it has been unclear which of the density gradients associated with the midlatitude trough is responsible for generating DUSE. The data from the Hokkaido radar enabled us to estimate the lower-latitude boundary of DUSE as around 59° in AACGM magnetic latitude. In addition, by adding the data from the King Salmon radar in Alaska we derived a complete statistical distribution of DUSE. The latitudinal extent of DUSE is about 9° from 59° to 68°, which is approximately 1000 km. The statistical distribution of DUSE was compared with the model of the midlatitude trough. As a result, the source region of DUSE is closely colocated with the minimum of the trough, which suggests that the electron density gradient at the sunward edge of the trough is responsible for DUSE. This means that the location of the duskside sunward edge (i.e., local time extent) of the midlatitude trough can be monitored by using an appearance of DUSE as a proxy. The current statistical analysis also suggests that we can derive a spatial distribution of HF radar echoes from the midlatitudes to high latitudes by combining the data from the Hokkaido and King Salmon radars located in the Far Eastern area, which would be a very powerful diagnostic tool for investigating the global distribution of plasma irregularities.