On the night of September 26 and the morning of September 27, Earth was hit by a solar storm, the cause of which was not immediately clear.

From late September 26th until this morning, Earth was struck by a massive solar storm, the cause of which is unclear.

Around 10 o’clock. UTC (6 p.m. ET) on September 26, the U.S. National Oceanic and Atmospheric Administration’s (NOAA) Space Weather Prediction Center (SWPC) issued a space weather warning stating that a geomagnetic K-index of 4 is expected— An indication that the Earth is essentially magnetized. Some solar activity had a very mild effect on the field. SWPC’s warning was updated to warn of a K5 storm within hours, then a K6, and at 2:17 UTC on September 27, a warning was issued for a K-index of 7 or higher.

This is similar to a strong geomagnetic storm capable of triggering alarms in power system protection devices, causing increased drag and orientation problems for satellites, intermittent GPS navigation problems, and aurora sparks in the U.S. is reported from Pennsylvania to Iowa and Oregon, SWPC says. The K-index is a physical measure of the strength of geomagnetic storms, material from the Sun disrupting Earth’s magnetic field, causing power grid problems, radio signal interference, and more.

A K-index of less than 4 is generally not worth mentioning, a K-index of 9 would be a very rare extreme solar event. The K7 storm warning is valid until 9 a.m. UTC on September 27, SWPC said. It’s not clear what exactly causes space weather, but it appears to be a significant event, according to enthusiasts who track the events.

“Honestly, I’m scratching my head how these numbers are so high,” wrote one user on the forums of the SpaceWeatherLive solar activity website on the morning of September 27. The current strength of the interplanetary magnetic field, the part of the Sun’s magnetic field that flows through the Solar System, indicates that a powerful solar explosion, known as a coronal mass emission (CME), has occurred.

However, the CME trackers do not seem to have given any indication that such a powerful explosion occurred. “Something is brewing in the near-Earth solar wind,” tweeted Erika Palmerio, a scientist at solar research firm Predictive Science Inc. “We’re likely to be hit by a high-speed jet, unclear [at the moment] whether a weak CME is lurking ahead of it!” As of 7:36 am BST, Palmerio said conditions appeared to have calmed down.

Coronal mass ejections (CMEs) are large ejections of plasma and magnetic field from the Sun’s atmosphere. Strong ones usually 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 could be a solar superfast stream, a stream of fast solar wind that exits an open area in the Sun’s magnetic field called a coronal hole. Both of these phenomena can cause geomagnetic storm effects on Earth. As of Tuesday morning, it was unclear if there had been any disruption due to the overnight solar activity, although most of the general public would not experience any impacts.