Spatial Distribution of Irregularity Occurrence Rate in the Subauroral F Region as Observed by the SuperDARN Radars

Keisuke HOSOKAWA, Toshihiko IYEMORI (2) Akira Sessai YUKIMATU, Natsuo SATO (1)
(1) Kyoto University, Japan
(2) National Institute of Polar Research, Japan

We have investigated the scattering occurrence rate of northern hemisphere SuperDARN radars statistically and clarified the distribution of the irregularity appearance in the subauroral F region. Most significant result from the statistical analysis is that the distribution of scattering occurrence rate (which is almost identical to the irregularity occurrence rate) peaks in the duskside subauroral region around the terminator as mapped into geographical coordinate system. These peak regions around terminator are closely corresponds to the area of higher longitudinal density gradient. On the contrary, weak peak was obtained around the dawn terminator only in winter and no detectable peak was found in the other seasons. This irregularity enhanced region is located in dusk sector subauroral region whose invariant latitude is slightly lower than the equatorward edge of the auroral oval in geomagnetic coordinate system. The area coincides with the plasma density depleted region known as the mid-latitude trough. In short, enhancement of the subauroral F region irregularities is associated with both the terminator and the mid-latitude trough. Main feature of the scattering occurrence peak region in our statistical analysis is quite similar to the characteristics of the dusk scatter event reported in Ruohoniemi et al. [1988], which indicates that enhanced scattering occurrence around the dusk terminator is mainly composed by the dusk scatter event. The dependence of the peak region on the level of geomagnetic disturbance as estimated by Kp index was also examined. In the seasons except for winter, scattering occurrence rate does not peak around the dusk terminator. In general, mid-latitude trough shifts equatorward as the auroral oval expands, and trough is out of the SuperDARN observation area in spring, summer and fall. This is the reason for the absence of the peak region around the terminator in the season except for winter. In addition, it is supposed that the asymmetry of irregularity occurrence between dawn and dusk is due to the equatorward shift of the mid-latitude trough in the dawn meridians.

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