Coronal mass ejections (CMEs) move from the surface of the Sun towards Earth through space. Top: CME is generated as an outflow of plasma and magnetic fields from the Sun, moving through space to Earth over a matter of hours. Bottom: The magnetic fields of the CME and outflowing solar wind interact with Earth’s magnetic field, which shields it from greater effect. This interaction causes the auroras at the poles./EarthScope, Hayley Bricker

Editor's note: Yellowstone Caldera Chronicles is a weekly feature written by the Yellowstone Volcano Observatory.

For a couple nights in May an extreme solar storm created a beautiful night sky for many people in areas that normally are too far away to see auroras. Some of the GPS stations in Yellowstone are among the tools used to monitor space weather like that solar storm.

On the night of May 10, 2024, the aurora borealis (northern lights) was seen as far south as Florida in the United States—a rare spectacle for residents of much of the USA. The aurora is a result of “space weather” that is usually caused by events originating from the Sun. In the case of this past May, we experienced a "severe to extreme" geomagnetic storm, according to the National Oceanic and Atmospheric Administration (NOAA) Space Weather Prediction Center. This event was caused by a coronal mass ejection (CME).

Within the Sun, extraordinary pressure fuses hydrogen atoms into helium atoms, releasing excess energy in the form of heat. As the heat reaches the Sun’s surface, it creates magnetic fields that can become concentrated, forming sunspots (dark, cooler spots on the Sun’s surface). The magnetic fields can interact in complicated ways and become entangled, and under certain conditions can cause a sudden release of electromagnetic energy and plasma into space. This plasma can travel at speeds of up to 1,900 miles per second, and if ejected towards Earth can reach our planet in 15–18 hours.

These CMEs don’t just blow past Earth unnoticed. Due to a kind of magnetic shield produced by Earth’s magnetic poles, the CME’s magnetic field deflects around the planet and funnels charged particles towards the Northern and Southern poles of Earth. As these particles enter the Earth’s atmosphere, they collide with other atoms and molecules, causing light to be emitted. The beautiful lights observed in the atmosphere as aurorae are determined by the type of atom that released the light—nitrogen generates blue light, oxygen generates greenish-yellow light, and argon generates purple light.

GPS instruments on the Earth’s surface, like those that monitor ground deformation in Yellowstone, can sense space weather events like CMEs and alert us as they develop. This is because GPS stations receive signals from satellites that must first travel through the portion of the atmosphere called the ionosphere. Part of the upper atmosphere, the ionosphere can be a source of error in GPS measurements, even without the presence of an incoming CME. GPS signals get refracted and reflected in the ionosphere, which causes a delay in the signal reaching the antenna on the ground.

Total electron content (TEC) data—a measure of activity in the ionosphere—at three GPS stations in Yellowstone. Each line color is a measurement using a different satellite passing overhead. Note how the data are steady until the evening of May 10, 2024, when the signals start to fluctuate wildly due to the arrival of the Coronal Mass Ejection. A spectacular aurora was visible across large parts of the United States that evening, University of Colorado Boulder, Pin-hsuan Cheng

Geomagnetic storms like the one on May 10 cause an increase in the number of electrons within the ionosphere, which in turn excites the gas atoms and molecules to produce an aurora. But increasing the total electron content of the ionosphere causes more interference for GPS and other communications signals—an important hazard associated with space weather. This interference means that it is harder to pinpoint the location of a station, and applications that rely on high-precision GPS, like monitoring very subtle movement of Earth’s surface, won’t work quite as well. If the interference is sufficiently strong, it may become altogether impossible at that moment to determine a position.

So what does this have to do with volcano and earthquake science in Yellowstone National Park? Geologists use GPS stations track and detect ground deformation caused by the volcanic and tectonic activity, but the stations can also be used to help other scientists track space weather. Because space weather interferes with GPS signals, this interference can actually be used to measure the amount of ionospheric activity during events like the May 10 CME. This helps space weather scientists better characterize events of this type. In fact, one of the GPS stations in the park feeds data into the Space Weather Prediction Center’s total electron content map.

While the GPS stations in Yellowstone serve primarily to help scientists measure what’s happening below the surface, they can also help us track what is happening in the uppermost atmosphere. And in both cases, the data can help us watch for hazards—and also better understand how these Earth systems work.