SPWC has issued a G1 – Minor Geomagnetic Storm Watch for September 8 as several coronal mass ejections (CMEs) are forecast to reach Earth from early September 8 into 9. According to the center’s latest Forecast Discussion, issued at 12:30 UTC on September 7, modeling of several CMEs observed over the previous 24–36 hours suggests a possible combined arrival during this period.
Geomagnetic conditions are expected to become unsettled to active on September 8 and 9, with isolated G1 – Minor storm periods likely. There’s also a slight chance of reaching G2 – Moderate levels if the anticipated CMEs impact Earth as forecast. However, confidence remains low in both the timing and magnitude of the CME arrivals.
Solar wind conditions were already enhanced on September 7 ahead of the expected CME sequence. SWPC reported that the interplanetary magnetic field strengthened to as much as 18 nT, while its north-south Bz component reached -14 nT and solar-wind speed peaked at 434 km/s.
Forecasters attributed the conditions to a corotating interaction region transitioning into a coronal hole high speed stream, together with effects from a filament CME that left the Sun on September 2.
Further solar wind enhancements are expected on September 8 into 9 as the anticipated CMEs arrive while the coronal hole high speed stream remains influential.
The magnitude of any storm will depend not only on whether the CMEs arrive, but also on their magnetic structure when they reach Earth. Sustained southward Bz can increase coupling between the solar wind and Earth’s magnetosphere and support stronger geomagnetic activity.
At G1 level, weak power-grid fluctuations and minor satellite effects are possible, while aurora may become visible farther from the poles. If G2 conditions develop, disturbances can become more pronounced.
Regarding the energetic particle environment, SWPC reported that the >10 MeV proton flux declined toward background levels on September 7, adding that levels could increase again on September 8–9 if the anticipated CMEs arrive as forecast.
A separate large and fast moving CME observed leaving the Sun’s east limb at approximately 02:00 UTC on September 7 was determined to be a far side event and not expected to affect Earth.
The main uncertainty over the next 24–48 hours is therefore the timing, interaction, and magnetic configuration of the incoming CME sequence.
Solar activity is likely to be at low levels due to occasional C-class flare activity, with a slight chance for M-class flares through September 9.
References:
1 Forecast Discussion – NOAA/SWPC – Issued at 12:30 UTC on September 7, 2026