NOAA Issues G2 Geomagnetic Storm Watch for Oct. 9 After M1.4 Solar Flare

Q: A solar eruption is racing toward Earth today, but what hidden magnetic secret will determine its true impact on October 9?

The deciding factor is the CME’s hidden magnetic orientation — especially whether its embedded magnetic field points strongly southward, known as a southward Bz component. Forecast models now support an Earth-directed impact from the Oct. 6 eruption out of sunspot region AR4549, with arrival around 1 a.m. EDT on Oct. 9 and at least minor geomagnetic storm conditions possible. But forecasters stress that a direct hit alone does not determine how strong the storm will be. What matters most is how the CME’s magnetic field lines line up with Earth’s magnetosphere. If the CME arrives with a sustained southward Bz, it can reconnect efficiently with Earth’s magnetic field, allowing more solar energy to enter the magnetosphere. That would raise the odds of a stronger geomagnetic storm and brighter auroras extending farther south than usual. If the field points northward instead, much of the energy is deflected and the event could be relatively modest. That is why the true impact cannot be known with confidence until the CME is much closer. The key magnetic measurements are typically taken by solar-wind monitors near the Sun-Earth L1 point, which provide only about 15 to 60 minutes of warning before the solar wind reaches Earth. So the “secret” is not simply whether the CME hits, but the orientation and strength of its magnetic field at arrival. A strong southward Bz would be the ingredient that turns an ordinary aurora chance into a more significant Oct. 9 space-weather event.

3 articles · Updated · spaceweather.gov · Oct 6

G1 to G2 geomagnetic storms are likely on Oct. 9 as a coronal mass ejection from the Sun reaches Earth, prompting a moderate storm watch.
The CME was tied to an M1.4 flare that erupted from Active Region 4549 on Oct. 6, the same region now driving the forecast.
Active Region 4549 still carries a high chance of additional M-class flares in the coming days, raising the risk of further space-weather disturbances.
Earlier forecasts had pointed to only minor storm conditions, so the new watch signals a stronger expected impact from the Oct. 6 eruption.