Real-time data collected by the Global Differential Global Positioning System network, operated by NASA's Jet Propulsion Laboratory, shows the atmospheric signature of the Hunga Tonga Hunga Ha'apai volcanic eruption in Tonga on Jan. 15, 2022.

The data is a measure of the density of electrons (known as total electron content units, or TECU) in the ionosphere – the outermost layer of the atmosphere, which starts between 50 and 56 miles (80 to 90 kilometers) above Earth's surface. Navigation radio signals, like those received by location sensors on smartphones, are broadcast by global navigation satellite systems (GNSS) and experience delays when passing through the ionosphere. The extent of the delay depends on the density of electrons within the path of the GNSS signal in this atmospheric layer.

When an explosive event such as a volcanic eruption or large earthquake injects energy into the atmosphere, the pressure waves from that event change the electron density in the ionosphere. These perturbations show up as tiny changes to the delays that GNSS radio signals usually experience as they pass through the atmosphere.

The vertical red line in the data plot indicates the time of the eruption. The horizontal squiggles show electron density profiles picked up in the signals of four GNSS constellations, or groups of satellites: GPS, GLONASS, Galileo, and BeiDou. The slanted dashed and dotted lines indicate the velocity of waves.