Two CMEs strike Earth, producing G3 – Strong geomagnetic storm

Two coronal mass ejections (CMEs) impacted Earth on September 18, 2023, producing G3 – Strong geomagnetic storming.

In 24 hours to 00:30 UTC on September 19, solar wind parameters were indicative of a likely transient arrival and passage early in the day, followed by an interplanetary shock arrival as measured at the DSCOVR spacecraft at 12:57 UTC on September 18. When the shock arrived, the interplanetary magnetic field (IMF) increased from 10 to 22 nT by 12:58 UTC.

The Bz component was slowly rotating from south to north directed with the initial transient passage. There was little change in the Bz component with shock arrival and Bz remained mainly northward until about 16:00 UTC when it turned southward and reached a maximum deviation of -18 nT at 16:30 UTC. The Bz oscillated afterward.

Solar wind speed had initially risen to 484 km/s by 02:07 UTC on September 18 with the first transient arrival before it decreased to ~425 km/s. Speed promptly increased with shock arrival from the second CME and reached about 550 – 560 km/s before it slowly declined and was at 500 km/s by 00:30 UTC on September 19. The phi angle was mostly in a negative sector.

Solar wind enhancements are expected to continue throughout September 19 due to CME influences. It remains uncertain at this time whether the shock arrival at 12:57 UTC was an early arrival of the September 17 CME or a previous event. Current thinking is that this was likely the September 17 CME shock arrival and continuing disturbances are likely throughout the day. Waning CME influences will give way to weak CH HSS effects on September 20 and 21.

The geomagnetic field reached G1 – Minor storm levels in response to the two CME arrivals. A sudden impulse (SI) occurred at 13:30 UTC and reached a deviation of 75 nT as measured at the magnetometer observatory in Wingst, Germany. The SI was in response to the shock arrival at Earth of the second CME arrival detected at the DSCOVR spacecraft at 12:57 UTC. The geomagnetic reactions associated with the SI resulted in a quick escalation to active levels.

Geomagnetic K-index of 7 (G3 -Strong geomagnetic storm) threshold was reached at 05:59 UTC on September 19.

Potential Impacts: Area of impact primarily poleward of 50 degrees Geomagnetic Latitude.

• Induced Currents – Power system voltage irregularities possible, false alarms may be triggered on some protection devices.
• Spacecraft – Systems may experience surface charging; increased drag on low Earth-orbit satellites and orientation problems may occur.
• Navigation – Intermittent satellite navigation (GPS) problems, including loss-of-lock and increased range error may occur.
• Radio – HF (high frequency) radio may be intermittent. Aurora – Aurora may be seen as low as Pennsylvania to Iowa to Oregon.

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References:

¹ Forecast Discussion Issued: 2023 Sep 19 0030 UTC – Prepared by the U.S. Dept. of Commerce, NOAA, Space Weather Prediction Center

Featured image credit: The Watchers

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