Ionospheric disturbances detected during a typhoon based on GNSS phase observations: A case study for typhoon Mangkhut over Hong Kong

Southern China is the region often affected by typhoons which have serious impacts on human beings. In this study, ionospheric disturbances possibly excited by typhoon Mangkhut in September 2018 have been investigated for the first time, by applying the Slant Total Electron Content (STEC) sequences derived from phase observations collected from Continuously Operating Reference GPS Receiver Stations located in Hong Kong (HK-CORS). Savitzky-Golay smoothing filter is employed to de-trend STEC observations, and the S transform is used to convert time domain to frequency domain for ionospheric disturbances detection. Since most significant ionospheric disturbances occurred at day time in autumn/winter and at nighttime in spring/summer, we selected nighttime GPS observations to demonstrate that the ionospheric disturbances might occur during the period of typhoon Mangkhut. On the other hand, some IGS stations far away from the typhoon were selected to verify whether the ionospheric disturbances were induced by typhoon Mangkhut. The results show that there are obvious ionospheric disturbances with a distance of about 1050 and 230 km away from the typhoon eye before and after typhoon landfall, which is related to the power of the typhoon. Meanwhile, the disturbances detected from the de-trended STEC (dSTEC) sequences for the same satellites’ observations collected from other stations located in Hong Kong occurred nearly simultaneously. The disturbance achieved its maximum with a distance about 1000 km away from the typhoon eye and then decreased. It disappeared with the distance about 2000 km away from the typhoon eye. This indicates that the typhoon can induce ionospheric disturbances after considering the influence of geomagnetic and solar activities.