The Suprathermal Ion Sensor (STIS) onboard NOAA's Space Weather Follow On – Lagrange 1 (SWFO-L1) observatory, began sending particle measurements back to Earth on September 30, 2025.
SWFO-L1 is NOAA’s newest space weather satellite, launched on September 24, 2025, designed to deliver real-time observations of the sun to help improve forecasts of solar events that can impact Earth. These include coronal mass ejections (CMEs) and other plasma structures flying out from the solar atmosphere, as well as the geomagnetic storms resulting from the impacts of CMEs with Earth. These storms, which also generate auroras, have major space weather impacts.
As with other instruments onboard the spacecraft, STIS measures properties of the solar wind, which is the continuous outflow of particles that starts at the sun’s atmosphere and expands to fill out the entire solar system. The instrument, in particular, measures those particles that travel somewhat faster than average and are called suprathermal. Thus, STIS data are valuable because they will enable early warnings for both the slower, average-speed part of the solar wind, but also for extremely fast and damaging particles (the so-called solar energetic particles, or SEPs) that are released later. In that way, suprathermal particle data will continue to provide useful information to forecasters and other users of space weather information.
NOAA forecasters and researchers have developed several space weather applications for both suprathermal ion and electron data which include:
- Suprathermal ion data help estimate the intensity of resulting geomagnetic storms before they form.
- Suprathermal electrons can be an early alert signal before the arrival of more harmful and heavier SEPs, which pose a danger to astronauts and to airline crews on polar routes.
Following the October 21, 2025 CME, the STIS instrument on the SWFO-L1 deep-space mission measured solar-origin suprathermal electrons (first panel) and ions (second). The same event released faster and more damaging SEPs (mainly protons; third panel) observed simultaneously by the SEISS detector on board NOAA’s Geostationary Operational Environmental Satellite 18 (GOES-18) satellite. Image credit: NOAA NCEI and SWPC.
A clear example occurred on October 21-23, 2025. STIS measured particle fluxes shown in the figure above, produced by NOAA’s National Centers for Environmental Information (NCEI) and Space Weather Prediction Center (SWPC). Prior to the event, NOAA’s Compact Coronagraph 1 (CCOR-1), onboard NOAA’s Geostationary Operational Environmental Satellite 19 (GOES-19) satellite, observed an intense CME being released. NASA’s Moon to Mars Space Weather Analysis Office later confirmed based on numerical modeling that the CME traveled towards Venus, almost on the opposite side of the sun from Earth and SWFO-L1’s position. Even so, the powerful storm scattered energetic particles widely throughout the solar system, and the STIS instrument measured intense suprathermal particles traveling in the direction of Earth. The figure shows some of the highest fluxes of such electrons (with energies in the range of 25-300 keV) and ions (25-6,000 keV).
The top panel of the figure shows the suprathermal electrons that traveled past the spacecraft around 21:30 UT on October 21, 2025. Faster, higher-energy particles arrived first (higher energies at top of the panel), while slower, lower-energy particles arrived later with a peak at around 06:00 UT on October 22, 2025. A second peak occurred at 21:00 UT on the same day.
The middle panel shows the suprathermal ions, which travel more slowly than electrons but still faster than the regular solar wind. A strong ion beam arrived around 10:00 UT on October 22, 2025. As with electrons, the higher-energy particles (at around 300 keV) traveled past the spacecraft first, followed by lower-energy particles.
Finally, the bottom panel shows much higher-energy SEPs released during the same event, which were detected by NOAA’s GOES satellites orbiting Earth (here: GOES-18). The graph shows levels beginning to rise after 02:00 UT on October 22, 2025. The several-hour-long gap between the appearance of the suprathermal electrons in the top panel and the SEPs in the bottom panel represents the valuable lead time that forecasters have to issue warnings. Real-time SEP data are available from SWPC’s GOES Proton Flux conditions webpage.
When SWFO-L1 reaches the Lagrange point 1 region in mid-January 2026, it will be renamed as the Space weather Observations at L1 to Advance Readiness – 1 (SOLAR-1) observatory. After completing additional testing and validation, SOLAR-1 will become operational in April 2026, and NOAA's SWPC will begin using the STIS data, along with other data from this and other satellites, for its official space weather watches and warnings. Additional updates and future SWFO-L1 data releases will be shared on the SWFO Data and Science webpage.
Data from SWFO-L1 during the post-launch testing phase should be considered preliminary and non-operational.