Fig. 7

a Year-to-year variations in the number of possible days for conducting artificial aurora experiments from 2000 to 2017, in the case of 2.7-MHz frequency. The red bars indicate possible days, the gray bars indicate days in which conducting the experiments is not possible, and the black bars indicate days when there are no data. b Same as a, but in the case of 4-MHz frequency (Tsuda et al. 2018). c Year-to-year variation in the occurrence rate of possible days for conducting artificial aurora experiments from 2000 to 2017. The red indicates results in the case of 2.7-MHz frequency, and the black indicates results in the case of 4-MHz frequency. d Year-to-year variation in 1-year-averaged \(F_{10.7}\) from 2000 to 2017