Fig. 3

a Wave-like structures observed at 91–97 km from 13:45 UT to 15:00 UT. Ca⁺ layers, which had a distance of approximately 1.2 km, fluctuated vertically with a period of ~ 15 min. They moved upward with a speed of ~ 0.5 m/s. Temporal variations of average Ca⁺ density at b 95–97 km, c 93.5–95 km, and d 91–93.5 km. Vertical dashed lines indicate the time when the densities became the maximum and horizontal dashed lines indicate e⁻¹ values of the maximum densities. Black arrows indicate the time when average densities became smaller than e⁻¹ values