Source of field-aligned irregularities in the subauroralFregion as observed by the SuperDARN radars

摘要: The HF radars of the Super Dual Auroral Radar Network (SuperDARN) observe E x B convective drift, ionospheric plasma when suitable small-scale field-Migned irregularities (FAI) from which to scatter are present. In order estimate distribution FAIs in subauroral F region, we have investigated scattering occurrence percentage using data six Northern Hemisphere during 39 months between July 1995 and September 1998. We ident, ified a morphological tbature known as dusk event (DUSE; first reported by Ruohoniemi et al. (1988)) detail clarified its relation boundaries density within submroral region. all mont, hs, DUSE appears fbw hours local time on eveningside sunset, where magnetic latitude is slightly lower than equatorward edge mroral oval, corresponds density-depleted structure midlatitude trough. There exists clear UT effect characteristics DUSE, such that more pronounced tbr UTs corresponding prime meridian (0 o geomagnetic longitude). Furthermore, an enhancement around sunrise t, he trough, termed dawn event, (DASE), was newly identified. region or DASE occurs has close relationship with duskside dawnside ends trough longitudinal direction (which sunward edges this paper), directed steep gradients exist. Kp dependence also examined. disturbed conditions, appee(rs at, earlier times compared quiet consistent Three models based gradient instability were discussed. It, tbund model densit, y at ambient electric field most favorable growth can consistently explain statistical tbat, ures DASE. These facts indicate est, imat, e extent fi'om appearance observed SuperDARN.