Modeling Atmosphere-Ionosphere Coupling on Gravity Waves during 2019 Typhoon 15 and 19 using FDTD Method

Atmospheric Gravity Waves (AGWs) are important phenomena that propagate heat to wider areas in the atmosphere through heat convection. It is especially occurred during typhoons, propagating radially from the typhoon’s eye. UEC’s VLF network has two parallel paths from the VLF transmitter NPM in Hawaii to receiving stations Moshiri (MSR) and Yamaguchi (YMG) in Japan to monitor the ionospheric perturbation due to powerful typhoons. These paths are used to observe the propagation of the gravity waves during Typhoons 15 and 19 in 2019 which occurred in the Pacific Ocean. The spectrum intensity of VLF signals on NPM-YMG and NPM-MSR showed that there is the possibility of AGWs occurrence on September $4 ^{th}$ and $5 ^{th}$ during Typhoon 15 and October $7 ^{th}$ and $8 ^{th}$ during Typhoon 19. The spatial dependence of ionospheric perturbation is modeled using a 2D Finite Difference Time Domain (FDTD). The perturbation height variations on NPM-YMG and NPM-MSR from the simulation are then compared to the fluctuation of the observation VLF signal during Typhoons. The ionospheric perturbation variation over time shows a similar fluctuation pattern on September $4 ^{th}$ and $5 ^{th}$ during Typhoon 15 in NPM-YMG and NPM-MSR with a delay of 2.5 hours and 2 hours respectively. It fluctuated in the ionospheric height between 88 km to 83 km over 2 hours period. This fluctuation indicates the amplitude of the gravity wave propagation. It is then calculated that the gravity waves on Typhoon 15 have a phase velocity of 94.2 m/s and 117.8 m/s on September $4 ^{th}$ and $5 ^{th}$ respectively.