Around noon eastern time on Labor Day solar winds increased, the Bz shifted South and reports of Aurora started rolling in. Three hours later, SWPC has issued a G1 storm watch for the next 24 hours and the KP index has reached G3 storming levels with a KP of 7.33. Northern Lights Now is expecting this should be a very good storm for Northern Europe and Iceland, and, if it continues for the next 6 hours, a good storm for at least the North and Eastern parts of North America. Here’s the current notifications graphic from SWPC:
The first visual hint that today’s storm might be terrific was this post from ISS Astronaut Scott Kelly:
— Scott Kelly (@StationCDRKelly) September 7, 2015
This storm is considered a “surprise” storm because it was not forecast in advance. In fact, the highest KP expected for today was one 3-hour block of KP=4 due to a weak coronal hole high speed stream. The NLN 3-day forecast clock gave no hint that this storm was coming. The updated forecast clock (expected to be updated and available here) should show max KP values above 5 once it is published around 8:30EST.
So what caused this stealth storm? The most likely candidate right now is a large Earth-directed filament eruption that happened on Sept 4. It is possible that this storm launched a CME or that it kicked off instability in the Sun’s atmosphere that then caused another eruption on another part of the Sun that launched another CME. Another possibility is shown in this animated GIF from images from taken by SDO. The main eruption is clearly visible at the beginning of the sequence. Just after the main eruption, to the south and left of the main eruption a second filament is visible. It is a little hard to see. Just as it is getting ready to erupt, imagery is lost from SDO due to an eclipse. The eclipse shows as several black frames. When the video continues, the second filament eruption has completed. It is possible that this second filament launched the CME that is arriving at Earth today. None of the models incorporated because the visual data about the eruption is missing. Thus it became a surprise storm. Watch the following GIF closely to see if you can see it:
Either way, this is looking like a great aurora storm for Europe and Iceland, and it may produce Aurora for North America