Occurrence of F region echoes for the polar cap SuperDARN radars

Observations by six Super Dual Auroral Radar Network (SuperDARN) polar cap radars, three in the northern hemisphere and three in the southern hemispheres, are considered to assess F region echo occurrence rates over solar, season, and day cycles and to establish relationship between the echo occurrence rate and the background electron density and plasma flow velocity magnitude. The echo occurrence rate is shown to increase toward the solar cycle maximum, more distinctly on the nightside, consistent with a general trend of the background electron density. Over the last 5 years, the echo occurrence rates decline at a rate of 5–10% per year. The pattern of seasonal and diurnal variations in echo occurrence is found to be consistent with previous SuperDARN publications. Minor dips in echo occurrence rate are observed in winter solstices, and these are related to an overall decrease in the electron density. In most of the time sectors, the echo occurrence rate increases with the electron density but only up to a certain threshold value after which the dependence saturates. The level of the saturation depends on season, local time, and average plasma flow velocity magnitude. For the summer daytime observations, the echo occurrence rate correlates with variations of both electron density and plasma flow velocity magnitude.[graphic not available: see fulltext]