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By Ed Browne
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Earth was buffeted by a huge solar storm late on September 26 through to this morning, and the cause is unclear.
At around 10 p.m. UTC (6 p.m. ET) on September 26, the U.S. National Oceanic and Atmospheric Administration's (NOAA) Space Weather Prediction Center (SWPC) released a space weather alert stating that a geomagnetic K-index of 4 was expected—essentially an indication that Earth's magnetic field was being very mildly affected by some solar activity.
Within a couple of hours the SWPC's alert was updated to warn of a K5 storm, then a K6, and by 2:17 a.m. UTC on September 27, an alert for a K-index of 7 or greater was issued.
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This equates to a strong geomagnetic storm that has the potential to trigger alarms on power system protection devices, cause increased drag and orientation problems for satellites, intermittent GPS navigation issues, and spark auroras in U.S. states from Pennsylvania to Iowa and Oregon, the SWPC said.
The K-index is a physical measure of the strength of geomagnetic storms, which are the result of material from the sun disrupting Earth's magnetic field, causing power grid issues, radio signal disruption, and more. A K-index of less than 4 is typically not worth mentioning, while a K-index of 9 would be a very rare extreme solar event.
The SWPC said the K7 storm warning was valid until 9 a.m. UTC on the morning of September 27.
According to enthusiasts monitoring the events, it isn't clear what exactly is causing the space weather, but it appears to be a significant event.
"Honestly, I'm scratching my head as to how these numbers are so high," wrote one user in the forums of the SpaceWeatherLive solar activity website in the early hours of September 27.
They said the current strength of the interplanetary magnetic field—which is the portion of the sun's magnetic field that flows through the solar system—should indicate that a strong solar explosion known as a coronal mass ejection (CME) had taken place. However, CME trackers didn't seem to have given any indication that such a strong explosion had happened.
"Something is brewing in the near-Earth solar wind," tweeted Erika Palmerio, a scientist at solar research company Predictive Science Inc. "We are likely being impacted by a high-speed stream, unclear [at the moment] whether a sneaky weak CME is hiding ahead of it!"
As of 7:36 a.m. BST, Palmerio said the conditions appeared to have calmed.
Coronal mass ejections (CMEs) are large expulsions of plasma and magnetic field from the sun's atmosphere. Strong ones generally occur when the sun's twisted magnetic field lines suddenly shift and realign, causing a sudden release of energy.
Another potential cause of the recent activity may have been a solar high speed stream, which is a stream of rapid solar wind that escapes from an open area of the sun's magnetic field known as a coronal hole.
Both phenomena can cause geomagnetic storm effects on Earth.
As of Tuesday morning it wasn't clear whether any disruption had been caused by the overnight solar activity, though any effects would likely not have been experienced by most of the general public.
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