How Did the Hunga‑Tonga Eruption Shake Japan’s Ionosphere?

On January 15, 2022 the Hunga‑Tonga volcano erupted with enough force to launch pressure waves, known as Lamb waves, into the atmosphere. These waves traveled thousands of kilometres, reaching Japan within a few hours. There, barometric sensors recorded pressure spikes while the GEONET GNSS network showed changes in the ionosphere’s total electron content (TEC).\n\nScientists identified two distinct phases. The first, linked to the Lamb waves, produced mild TEC oscillations and modest gradients, keeping GNSS positioning errors below 10 centimetres. The second phase was more dramatic: equatorial plasma bubbles (EPBs) created intense ionospheric turbulence, generating much stronger TEC gradients. In these conditions, low‑latitude GNSS stations recorded positioning errors up to half a metre, with 95th‑percentile peaks exceeding one metre.\n\nThese findings demonstrate how a volcanic eruption can affect satellite navigation accuracy, especially near the equator. Monitoring ionospheric gradients could become a useful tool to forecast GNSS performance drops, aiding critical activities such as aviation, precision farming, and telecommunications.

The Hunga‑Tonga eruption generated a large‑scale atmospheric disturbance characterized by low‑frequency pressure waves (Lamb waves) that propagate almost without dispersion in the stratosphere. These waves are usually associated with seismic events, but the energy released by a volcanic eruption can produce a comparable impulse. When Lamb waves reach the ionosphere, the compression and expansion of air modify gas density, altering the distribution of electric charges and, consequently, the total electron content (TEC).\n\nThe first observed event was marked by a sinusoidal TEC variation (dTEC) with an amplitude of a few TECU (total electron content units), accompanied by horizontal VTEC gradients below 0.1 TECU/km. These variations caused marginal GNSS positioning errors, as ionospheric correction models can quickly compensate for low‑amplitude fluctuations.\n\nThe second, more intense event was triggered by equatorial plasma bubbles (EPBs). EPBs form under Rayleigh‑Taylor instability conditions, typical of equatorial regions at twilight. The eruption supplied an energy impulse that amplified these instabilities, generating electron density structures ranging from 10 to 100 km. Such structures create TEC gradients exceeding 1 TECU/km, which cannot be corrected with standard models.\n\nThe GNSS consequences are twofold: (i) increased signal scintillation, causing loss of lock and carrier‑phase degradation; (ii) ionospheric correction errors, as global TEC maps lack the resolution to capture local variations. GEONET data showed stations below 30° latitude experienced average horizontal errors of 0.5 m and 95th‑percentile errors above 1 m during the EPB peak. These findings suggest that integrating real‑time ionospheric gradient observations (e.g., dense barometer and GNSS networks) could improve GNSS performance degradation forecasts, providing timely alerts for precision‑critical applications.