Fig. 4

Data on the polar cap SuperDARN echo occurrence and Sun’s activity between January 2008 and December 2017. a The solar 10.7-cm radio flux. b The electron density at the F layer peak in the NH and SH and the E × B flow magnitude in the area of radar observations in the NH, both parameters inferred from SuperDARN measurements. c Solid lines are radar echo occurrence rates in percent (daily afternoon values, 15 ± 3 h of local time) smoothed by applying a running 1-year boxcar filter for the NH polar cap radars at CLY, RKN, and INV. The decline in the echo occurrence rate in 2014–2017 is assessed by the linear fit lines and their slopes are reported in Table 2. Dashed lines show the occurrence rate for the INV and CLY radars in the midnight sector (0 ± 3 h). d The same as c but for the DCE, MCM, and SPS radars in the southern hemisphere