RATES OF SCATTERING OCCURRENCE IN ROUTINE HF RADAR OBSERVATIONS DURING SOLAR-CYCLE MAXIMUM

The HF radars of the Super Dual Auroral Radar Network (SuperDARN) prov ide continuous monitoring of the high-latitude ionosphere over large a reas in both the northern and southern hemispheres. The HF technique g enerates estimates of the E X B convective drift of ionospheric plasma when suitable small-scale (similar to 10 m) irregularities are presen t and viewed under favorable magnetic aspect conditions (radar k vecto r perpendicular to to B). In this paper we report on the statistics of HF scattering occurrence. The study period encompasses 5.5 years (198 8 to mid-1993) of routine observations with the HF radar located at Go ose Bay, Labrador, Canada, and corresponds to the most recent period o f solar cycle maximum. The data were carefully filtered for those inst ances when high-confidence F region velocity measurements were obtaine d within 1 degrees X 3 degrees magnetic latitude/longitude regions and 12-min UT intervals. For average conditions the rates at which scatte r occurred at particular values of invariant latitude ranged from <10% to >40% with MLT. The probability of making velocity measurements som ewhere in the radar field of view varied from similar to 80% on the ni ghtside to similar to 45% on the noon meridian. When scatter was detec ted, the average latitudinal coverage varied from a minimum of 4 degre es Lambda on the dayside to over 6 degrees Lambda on the nightside. Th ere were significant dependencies on Kp and season. The highest occurr ence rates (>60%) were obtained on the nightside for quiet conditions and in the afternoon for disturbed conditions. Winter was the most act ive season. HF scattering was limited by the equatorward boundary of t he Feldstein oval but extended poleward of the oval into the polar cap . Scattering in the noon sector was associated with the aggregate cusp /mantle/low-latitude boundary layer region. The scattering activity wa s notably suppressed in the morning sector under disturbed conditions and in the noon and afternoon sectors in summer. We discuss the geophy sical factors that influence the likelihood of generating HF scatter. These results have value for the design of experiments involving the S uperDARN HF radars.