Fig. 2

Temporal variations of a E_s parameters (f_oE_s, f_bE_s, and ∆f = f_oE_s − f_bE_s) observed by ionosonde at Kokubunji and b maximum Ca⁺ density at 90–120 km over Tachikawa. Red dots indicate the maximum density during ± 5 min (10 min) every 15 min. The gray hatched area indicates data obtained after local sunrise at 100 km altitude. c Is a correlation diagram of maximum Ca⁺ density shown by red dots in (b) and maximum electron density estimated from f_oE_s