Source of field-aligned irregularities in the subauroral F region as observed by the SuperDARN radars

Abstract

The HF radars of the Super Dual Auroral Radar Network (SuperDARN) observe the E x B convective drift of ionospheric plasma when suitable small-scale field-aligned irregularities (FAI) from which to scatter are present. In order to estimate the distribution of FAIs in the subauroral F region, we have investigated the scattering occurrence percentage using data from six Northern Hemisphere radars during 39 months between July 1995 and September 1998. We have identified a morphological feature known as the dusk scatter event (DUSE; first reported by Ruohoniemi et al. [1988]) in detail and clarified its relation to the boundaries of the plasma density within the subauroral F region. In all months, the DUSE appears within a few hours local time on the eveningside of sunset, where the magnetic latitude is slightly lower than the equatorward edge of the auroral oval, which corresponds to the density-depleted structure known as the midlatitude trough. There exists a clear UT effect in the characteristics of DUSE, such that DUSE is more pronounced for UTs corresponding to dusk on the prime magnetic meridian (0° geomagnetic longitude). Furthermore, an enhancement of scattering occurrence around sunrise within the trough, which is termed dawn scatter event (DASE), was newly identified. The region where the DUSE or DASE occurs has a close relationship with the duskside and dawnside ends of the midlatitude trough in the longitudinal direction (which are termed the sunward edges of the trough in this paper), where sunward directed steep plasma density gradients exist. The Kp dependence of the scattering occurrence was also examined. In disturbed conditions, the DUSE appears at earlier local times compared with that in quiet conditions, which is consistent with the Kp dependence of the sunward edge of the trough. Three models based on the gradient drift instability were discussed. It was found that the model which is based on the sunward plasma density gradient at the sunward edges of the midlatitude trough and ambient electric field is most favorable for the growth of FAIs in this region and can consistently explain the statistical features of DUSE and DASE. These facts indicate that we can estimate the longitudinal extent of the midlatitude trough from the appearance of DUSE and DASE observed by SuperDARN. Copyright 2001 by the American Geophysical Union.