INTERPLANETARY SHOCK WAVE: This was not in the forecast. An unexpected interplanetary shock wave hit Earth's magnetic field on Feb. 11th at approximately 1800 UT: data. The impact sparked a G1-class geomagnetic storm and some very colorful auroras inside the Arctic Circle. Aurora alerts: SMS Text.
24 HOURS LATER: A CME hit Earth's magnetic field on Feb. 9th. The geomagnetic storm (category G1) erupted 24 hours later. "Auroras burst into life over the coast of Scotland," reports Alan C. Tough, who sends this picture from the seaside village of Hopeman:
What took so long? The storm waited a whole day while Earth moved deeper into the CME's wake. More than 3 millon kilometers behind the shock front, Earth encountered the kind of intense south-pointing magnetic fields that spark geomagnetic storms. See the data.
This kind of delay is not unusual. Minor CMEs, like the one that struck on Feb. 9th, often fail to produce a storm on first contact; the initial blow is not enough. The CME's wake can be more effective, sparking storms one or two days later.
Magnetometer readings obtained by Stuart Green in Preston UK highlight the delay between the CME impact on Feb. 9th and geomagnetic unrest on Feb. 10th:
The CME itself caused no obvious ripple in Green's recording. Nevertheless, a storm was coming; G1 conditions were detected for more than 6 hours on Feb. 10th. Interestingly, the Starlink Incident of Feb. 4th was caused by just such a delayed storm.
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