Ionospheric effects of the 23–24 April 2023 geospace storm captured by the multifrequency multiple path software-defined radio system at oblique incidence over the People's Republic of China

We present the analysis of variations in the ionosphere based on the HF radio wave characteristics measured on radio wave paths of different lengths and orientations during a major two-step geospace storm of 23–24 April 2023, which was caused by the coronal mass ejection. The extreme magnetospheric storm was accompanied by severe magnetic storms and severe and strong ionospheric storms. The ionospheric storms were both negative, with negative ionospheric indexes I_NIS of 6.6 and 4.0. Significant perturbations were observed in radio wave characteristics in the 5- to 10-MHz frequency range and in ionospheric parameters between ∼130- and 260-km altitude via the multifrequency multiple path software-defined radio system for oblique incidence sounding. Not only did the parameters of a regular ionosphere were significantly disturbed (the electron density decreased by a factor of 2.5–4.6 times), but wave activity in the magnetic field, thermosphere, and the ionosphere on 22 and April 23, 2023 also considerably increased, especially in the 100–120-min period range. The slow quasi-sinusoidal processes in the ionosphere (characteristic times of ∼100–120 min) acted to shift the radiowave reflection height by 30–130 km at an average speed of ∼10–60 m/s, depending on the propagation path. Short-term reflection height shifts of 30–70 km occurred at an average speed of 50–100 m/s over a span of ∼10 min. The amplitude of perturbations in the electron density with 8–15-min to 40-min periods varied from 1.6–3.5 % to 29–39 %, respectively.