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

[PEM12-18] Acoustic resonance in the D-region ionosphere due to Pekeris wave after the Hunga Tonga-Hunga Ha'apai volcanic eruption using AVON VLF/LF transmitter signals

*Hiroyo Ohya¹, Fuminori Tsuchiya², Tamio Takamura³, Hiroyuki Shinagawa⁴, Yukihiro Takahashi⁵, Alfred B. Chen⁶ (1.Graduate School of Engineering, Chiba University, 2.Graduate School of Science, Tohoku University, 3.Center for Environmental Remote Sensing, Chiba University, 4.Kyushu University, 5.Hokkaido University, 6.National Cheng Kung University)

The Hunga Tonga-Hunga Ha'apai volcano in Tonga (in southern Pacific, 20.54S, 175.38W) explosively erupted around 04:10 UT on 15 January, 2022, and large pressure variations occurred from the volcano. Large and medium scale traveling ionospheric disturbances (LSTID and MSTID) due the eruptions were observed (Themens, 2022), which were caused by Lamb wave excited by the eruptions. In addition to the Lamb wave, Pekeris waves were generated by the eruption (Watanabe et al., 2022). The Lamb waves are a kind of acoustic one, and propagate horizontally with phase velocity of ~310 m/s. On the other hand, Pekeris waves are internal resonance mode that propagate horizontally with phase velocity of ~240 m/s. The Pekeris waves have anti-phase between upper and lower stratopause, while the Lamb waves are in-phase vertically. The energy of Pekeris waves is enclosed between the stratopause and mesopause, within two atmospheric temperature minima, and the amplitudes of Pekeris waves are higher at the upper stratopause (>45 km). However, variations in the D-region ionosphere due to the Lamb and Pekeris waves associated with the eruptions have not been revealed at all. In this study, we investigate variations in VLF/LF transmitter signals and atmospheric electric field (or potential gradient) to understand coupling between the D-region ionosphere and atmosphere associated with Tonga volcanic eruptions of 15 January, 2022. The VLF/LF transmitters used in this study were JJY(60 kHz, Japan), JJI(22.2 kHz, Japan), and BPC(68.5 kHz, China). The receivers were Tainan (TNN, 23.07N, 120.12E) in Taiwan, where is one of Asia VLF observation network (AVON). We used 0.1-s sampling amplitude data. Unfortunately, there were no phase data for all paths on that day. The minimum distances of the JJI-TNN, JJY60kHz-TNN, and BPC-TNN propagation paths from the Tonga volcano were 8167.7 km, 8311.6 km, and 8499.9 km, respectively. The atmospheric electric field has been observed in Chiba University (CHB), (35.63N, 140.10E), Japan. The distances of CHB from the Tonga volcano were 7789.5 km. At arrival times of Lamb (~307 m/s) and Pekeris waves (~235 m/s), both variations in VLF/LF amplitudes were observed. Here we show observed oscillations of 100–200 s in VLF/LF transmitter signals and the magnetic and atmospheric electric fields, which were caused by Pekeris wave. The period of the variations in the D-region ionosphere due to Pekeris wave was similar to that due to Lamb wave, although the amplitudes of the variations for Pekeris wave were larger than those for Lamb wave. On the other hand, no corresponding changes with the period of 100–200 s in atmospheric pressure due to Pekeris waves were observed on the ground. A simulation of neutral wind revealed Pekeris waves oscillating near the mesopause, suggesting resonance. Therefore, the oscillation in atmospheric electric field is interpreted that the resonance in the lower ionosphere was projected onto the Earth's surface via a global electric circuit. In this presentation, we will discuss the mechanism of the phenomena in detail.