G2 – Moderate to G3 – Strong geomagnetic storm conditions forecast for September 19

halo CME filament eruption september 16 2023

An approximate 35-degree solar filament eruption centered near N22W17 began lifting off at 04:34 UTC on September 16, 2023. The event resulted in a halo coronal mass ejection (CME) that was first observed off the W limb at 09:12 UTC. Analysis of the filament eruption and subsequent halo CME determined a likely arrival early on September 19.

Solar wind parameters in 24 hours to 00:30 UTC on September 18 suggested influence from a transient which aligns with the anticipated arrival of a CME that left the Sun on September 14.

An increase from background conditions began around 01:50 UTC on September 17. Total magnetic field strength increased from 5 nT to a peak of 10 nT. The Bz component was mostly oriented far north for the event but briefly reached as far south as -9 nT during the onset. Solar wind speeds reached ~500 km/s near 03:00 UTC on September 17 and steadily declined to ~440 km/s by the end of the UTC day.

Solar wind parameters are expected to trend towards nominal conditions on September 18. An enhancement is anticipated over September 19 in response to influence from the filament eruption on September 16, which is expected to continue into September 20.

The geomagnetic field was quiet to active during the same period due to weak CME influence in the solar wind. A geomagnetic sudden impulse of 24 nT was observed at the Canberra magnetometer site at 02:32 UTC on September 17.

The geomagnetic field is likely to be at quiet to unsettled levels over September 18 under the waning transient influence. An increase to G1 – Minor to G2 – Moderate geomagnetic storm conditions, with a chance for G3 – Strong, is likely on September 19 with the arrival of a CME that left the Sun on September 16. A decrease to active levels is likely on September 20 as solar wind parameters taper back towards background levels.

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References:

1 SWPC forecast discussion – September 16 – 18, 2023

Featured image credit: NASA, ESA SOHO LASCO C2, C3, Helioviewer, The Watchers

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