Ionospheric responses to typhoons in Australia during 2005–2014 using GNSS and FORMOSAT-3/COSMIC measurements

Tropical cyclones are a serious natural disaster concern in Australia due to El Niño and La Niña phenomena. However, little attention has been paid to the ionospheric responses to severe typhoons that have occurred in Australia. In this study, dual-frequency GPS measurements and ionosonde data were used to investigate the variations in ionospheric total electron content (TEC) and the critical frequency of the F2 layer (foF2) during Typhoons Ingrid, George, Pancho and Gillian. The results showed that the variations in the solar-terrestrial environment during all these typhoons were not significant, with the first three typhoons yielding similar TEC and foF2 results (both increased within 2 days of the day when the wind speed reached the peak). However, the results for Typhoon Gillian were different, as its TEC and foF2 values decreased within 2 days of the peak wind speed. To further investigate the spatial variation in the ionospheric electron density (IED) during Typhoon Gillian, the IEDs obtained from the FORMOAST-3/COSMIC ionospheric profiles were constructed for both reference and observation periods. The results showed that the differences between the two IED periods in the region around the storm eye ranged from − 1.0 to − 1.5 × 106 el/cm3, which is equivalent to about − 37.5 to − 50%. These low IED values were mainly focused in the 300–350 km altitude range, especially at the 300 km altitude level. The results also showed that all the ionospheric anomalies were located at the edge of the typhoon rather than the storm eye. This study will be helpful in deepening our understanding of the ionospheric responses to typhoons in Australia.