The ionospheric hole detected by the GRACE Follow-On K-band ranging system after the 2022 Hunga-Tonga Hunga-Ha'apai volcanic eruption on 15 January 2022

We present the observation of the travelling ionospheric disturbance (TID) all around the world after the Hunga-Tonga Hunga-Ha'apai (HTHH) volcanic eruption on 15 January 2022.  This is based on the dual frequency K-band low-low tracking from the GRACE Follow-On satellites as well as the GNSS high-low tracking from a constellation of Spire Global CubeSats.  The latter complements the GRACE Follow-On measurements in terms of spatial sampling.  We discuss the measurements of global ionospheric disturbance and speed of the detected TID (~320 m/s) following the atmospheric Lamb waves initiated by the HTHH volcanic eruption.  Interestingly, we found 2-3 orders of magnitude reduction in electron density (“ionospheric hole”) at ~480 km altitude right after the first passage of the atmospheric Lamb wave and it lasted several hours before being recovered.  This characteristic is being observed consistently by the GRACE Follow-On and CubeSat measurements.  They are brand-new data that can be used to study less known high-altitude (>500 km) ionospheric processes.  We present our on-going analysis and modelling of these measurements.