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

[PEM12-40] D-region ionosphere response to fireballs and satellite reentries using OCTAVE VLF/LF transmitter signals

*大矢 浩代¹、古谷 凌汰¹、土屋 史紀²、山本 真行³、鷲見 貴生⁴、中田 裕之¹、渡邉 堯⁵、小林 美樹⁶ (1.千葉大学大学院工学研究院、2.東北大学大学院理学研究科惑星プラズマ・大気研究センター、3.高知工科大学、4.国立天文台、5.茨城大学、6.日本流星研究会)

Fireballs are exceptionally bright meteors of magnitude of >-4 that enter Earth’s atmosphere and burn up due to the aerodynamic heating. Fireballs occur when larger meteoroids or small asteroids enter the atmosphere at high speeds, heating up and glowing intensely. On the other hand, satellite reentries refer to the process when artificial satellites or other human-made objects in orbit around Earth reenter the atmosphere. This can happen in a controlled or uncontrolled manner. During reentries, these objects experience also the same heating process, which causes it to burn up partially or completely. This process can ionize the surrounding neutral atmosphere (meteor tails), which affects the D-region ionosphere (60-90 km altitude), and can be detected using VLF(very low frequency, 3-30 kHz)/LF (low frequency, 30-300 kHz) transmitter signals. Chernogor (2015) and Ohya et al. (2024) have shown that acoustic and atmospheric gravity waves (AGWs) are generated when fireballs fall, which causes fluctuations in the D-region ionosphere. However, studies of variations in the D-region ionosphere caused by fireballs and satellite reentries are limited, and the detailed mechanism remains unclear. Thus, this study aims to quantitatively elucidate lower ionospheric variations when atmospheric entry objects are observed, using OCTAVE VLF/LF transmitter signals, the wideband seismic observation network F-net by the National Research Institute for Earth Science and Disaster Resilience, and infrasound data operated by Kochi University of Technology and the National Astronomical Observatory of Japan. The OCTAVE is an observation network of VLF/LF transmitter signals for monitoring the D-region ionosphere that we installed in the world. We focus on two events: a fireball observed at 14:33 UT on April 23, 2023, and the satellite reentry of Starlink satellite (Starlink Group 6-32 debris 31119) observed at 12:38 UT on December 26, 2023. The transmitters are JJI (22.2 kHz, Miyazaki, Japan), JJY40 (40.0 kHz, Fukushima, Japan), JJY60 (60.0 kHz, Saga, Japan), and BPC (68.5 kHz, China). The receivers were located at RKB (Rikubetsu, Japan) and KAG (Kagoshima, Japan). For the Hokuriku fireball, we analyzed the amplitude and phase of transmitter signals on the JJI-RKB, JJY40-RKB, JJY60-RKB, and BPC-RKB paths. Variations with periods of 200-500 s were observed in the amplitude and phase of the JJY60-RKB path. Additionally, the JJY40-RKB path exhibited distinct variations (18 dB and 200 degrees) during the fireball, which differed from the other three paths. This was the only path among the four paths where the fireball passed overhead, indicating a possibility that the observed variations were induced by strong ionization due to the fireball trail. Furthermore, variations with periods of 300-400 and 200-400 s were detected in the vertical seismic velocity component of the seismometer (KZK) and the infrasound (MZM), respectively. Coherence analysis between the transmitter signals, seismometer, and infrasound revealed significant peaks at periods between 200-400 s. For the artificial satellite reentry, variations with periods of 200-300 s were observed in the amplitude and phase of the JJY40-KAG transmitter signals, and similar variations were seen in the infrasound data (TTM) at the arrival time of the acoustic waves generated by the reentry reached the ground. Coherence analysis between the transmitter signals and infrasound showed a peak at 256.0 s. These results suggest that acoustic waves excited by the fireball and satellite reentries propagated upward, causing disturbances in the D-region ionosphere and the Earth’s surface. In the session, we will report the detailed results.