G3 – Strong geomagnetic storm on September 30

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# G3 – Strong geomagnetic storm on September 30

ByTeo Blašković Tuesday, September 30, 2025Tuesday, September 30, 2025

**Earth’s magnetic field reached G3 – Strong storm levels on September 30, driven by CME impacts and sustained southward IMF. The event was enhanced by seasonal Russell–McPherron geometry near equinox, increasing coupling efficiency. Storm strength has since eased to G2 and G1 as IMF weakened and Bz returned closer to neutral.**

Credit: gmwand

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.

Image credit: SWPC

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 geomagn