A geomagnetic storm reached G3 – Strong levels on September 30, after a coronal mass ejection (CME) shock/sheath impacted Earth. According to NOAA Space Weather Prediction Center (SWPC) alerts, the geomagnetic field was most disturbed between 03:00 and 06:00 UTC, when the planetary K-index reached 7.
Subsequent alerts reported a decline in activity, with G2 – Moderate during 06:00–09:00 UTC, followed by G1 – Minor storming in the same synoptic period.
Real-time solar wind measurements from DSCOVR indicated a total magnetic field (Bt) exceeding 15 nT and sustained southward IMF Bz near –10 nT during the storm’s peak. Solar wind density briefly exceeded 20 particles per cm³, while speeds remained modest at 380–430 km/s, confirming the primary driver was magnetic field orientation and strength within the CME sheath, rather than high wind speeds.
The storm was amplified by the seasonal Russell–McPherron effect. Near the equinoxes, Earth’s axial tilt and the geometry of the interplanetary magnetic field increase coupling efficiency, allowing moderate solar wind conditions to drive stronger geomagnetic disturbances.
Impacts reported by SWPC for G3 events include possible voltage irregularities in power systems, surface charging on spacecraft, increased drag on low Earth orbit satellites, intermittent HF radio propagation at high latitudes, and aurora visibility as low as Pennsylvania to Iowa to Oregon.
Forecasts indicate active to unsettled geomagnetic conditions through October 2. A trailing weak CME could produce additional disturbances, although confidence in its arrival is low.
NOAA forecasts note that coronal hole high-speed streams are expected to become geoeffective around October 3, potentially sustaining minor to moderate activity into late week.
Solar activity remained moderate over the past 24 hours, driven by multiple M-class flares from several active regions. The largest event was an M3.7 flare produced by Region 4232 (beta-gamma) at 01:45 UTC on September 29. The region’s leader showed slight decay while its trailer became more symmetric, indicating that activity may decrease as the structure stabilizes.
The former Region 4230 (beta-gamma) has fragmented into three distinct regions. Region 4230 remains in the northeast of the cluster and displayed growth in both its leader and trailer, with an increased spot count. To its south, Region 4238 (beta) formed and showed slight growth, while Region 4239 (alpha) developed to the west and remained stable through the period.
Region 4233 (beta) was active but retained a relatively simple structure with rudimentary penumbra in both its leader and trailer. Region 4236 (beta-gamma) produced numerous C-class flares as well as one M-class flare. Rapid flux emergence in this region over the past 24 hours created a ring-shaped structure, raising its probability for continued flaring.
Coronagraph imagery detected a diffuse CME off the southeast part of the solar disk at 02:24 UTC on September 29. Forecast confidence for Earth impact remains very low, though a weak disturbance from the CME’s trailing edge is possible around October 1.
With several regions continuing to exhibit growth, flare activity is expected to remain at moderate levels, with a slight chance of an X-class flare through October 2.
References:
1 Forecast Discussion – NOAA/SWPC – September 30, 2025 at 00:30 UTC