[P-EM12] Coupling Processes in the Atmosphere-Ionosphere System

[PEM12-P28] Oscillation in the D-region ionosphere induced by X-class solar flares in May 2024 using OCTAVE/AVON LF transmitter signals

*Kubota Akane¹, Hiroyo Ohya¹, Fuminori Tsuchiya², Hiroyuki Nakata¹ (1.Graduate School of Science and Engineering, Chiba University, 2.Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University)

When solar flares occur, X-rays and EUV radiations increase and the electron density in the ionosphere (60-1000 km altitude) increases. This can cause problems with satellite attitude control, satellite communication problems, and reduced GPS positioning accuracy. To validate these effects, it is necessary to quantify the increase in ionospheric plasma density caused by solar flares, although this has not yet been precisely clarified. Previous studies on VLF (very low frequency, 3-30 kHz)/LF (low frequency, 30-300 kHz) transmitter waves have shown that the amount of reflected height decrease in the D-region ionosphere tends to differ with respect to the solar zenith angle at the midpoint of the path depending on the class of solar flares, and that electron density variation due to solar flares is estimated to be smaller for north-south and east-to-west propagation than west-to-east propagation (Nakayama et al., 2024). On the other hand, due to the limited number of observed cases, the response of the D-region ionosphere to more intense solar flares remains unclear. In this study, we investigated D-region ionospheric variations using LF transmitter signals for a solar flare of the X5.89- class that occurred at 01:10 UT on May 11, 2024. This was the largest solar flare among the consecutive solar flares that occurred in May 2024. The LF transmitter signals used in this study were OCTAVE/AVON data operated by this research group around the world. The LF waves were transmitted from JJY60 (60.0 kHz) in Japan and BPC (68.5 kHz) in China, and received at Pontianak (PTK) , Indonesia and Rikubetsu (RKB), Japan. Normally, when a solar flare occurs, the phase of standard radio waves exhibits time variations similar to those of X-ray flux, but during the X5.89-class solar flare, oscillations with different phase and amplitude from the usual variations were observed. Based on wavelet analysis of these oscillations, the periods were 549 s for the phase of the JJY60-PTK path and 349 s for the phase of the BPC-PTK path. However, such oscillations were not observed during the JJY60-Rikubetsu (RKB) path over Japan, so the oscillation in the LF amplitude and phase could be a phenomenon specific to low latitudes. Since oscillations with a similar phase period were observed for the Lyman α-ray at the same time, further detailed analysis is needed. No oscillations similar to those observed in the phase data were found in the Lyman β or the particle precipitation, such as solar protons and energetic electrons. In this session, detailed results will be reported.