A full-halo coronal mass ejection (CME) produced by an Earth-facing filament eruption on April 21, 2023, impacted Earth at 17:37 UTC on April 23. The impact sparked G4 – Severe geomagnetic storm and produced vivid aurora worldwide. Geomagnetic storms of this intensity occur about 100 times per solar cycle.

Solar wind speeds abruptly increased from ~360 km/s to 400 – 500 km/s following the CME impact.

Total magnetic field strength increased from ~9 nT to 25 nT during the shock. The Bz component rotated south and maintained a far south configuration through 20:15 UTC, at which point Bz rapidly rotated northward. Solar wind speeds further increased to above 600 km/s by 19:43 UTC, but low density produced sporadic measurements later.

In response to the arrival of full-halo CME, the geomagnetic field reached G4 – Severe geomagnetic storm conditions at 19:44 UTC.

G4 – Severe geomagnetic storm potential impacts:
The area of impact is primarily poleward of 45 degrees Geomagnetic Latitude. Induced currents – Possible widespread voltage control problems and some protective systems may mistakenly trip out key assets from the power grid. Induced pipeline currents intensify.
Spacecraft – Systems may experience surface charging; increased drag on low earth orbit satellites, and tracking and orientation problems may occur.
Navigation – Satellite navigation (GPS) degraded or inoperable for hours.
Radio – HF (high frequency) radio propagation sporadic or blacked out.
Aurora – Aurora may be seen as low as Alabama and northern California.