Aurora hunters, solar weather watchers, and satellite operators experienced a surprise geomagnetic storm that registered KP=7.67 (G3 on the NOAA geomagnetic storm scale) Thursday. The storm arrived just after sunrise on January 7th for the US East coast, but allowed people in mid and western North America to see northern lights just before sunrise. Some lucky people in New Zealand saw faint aurora as dusk was setting in.
This was the strongest geomagnetic disturbance since the arrival of a CME associated with a filament eruption October 1, 2013. In total, there was a period of about 75 minutes where the KP value was above 7. At it’s peak, the ovation model was showing a wide swath of activity above most of Central and Western Canada. (click to see image larger)
The storm was nearly a complete surprise. No watches were posted in advance, and it wasn’t until nearly a day after the CME’s arrival that plausible theories were suggested as a source of the storm. It now seems likely that this “Stealth CME” was launched from an area near a large southern hemisphere coronal hole. We’ve seen CME activity correlated with coronal holes in the past. A discussion on Twitter between Dr. Tamitha Skov (@TamithaSkov) and @haloCME was the first place NLN saw this suggestion:
— Dr. Tamitha Skov (@TamithaSkov) January 8, 2015
— Halo CME (@halocme) January 8, 2015
As the hole expanded, it may have released a CME. NLN edited a larger before/after version of the AIA 211 you can click on to see the coronal hole expansion in detail. Here, we circled the arcade hanging over the upper right portion of the coronal hole in the before image. In the after image from about 12 hours later, the hole is clearly larger, and the arcade is gone:
There are several plausible theories about why the disappearance of the arcade may have caused a CME:
- Did the arcade lift off the sun and become the materiral of the CME as it was propelled into space?
- Did the arcade collapse and launch a CME?
- Was the arcade dissipation correlated but unrelated to a CME that happened as a result of the expanding coronal hole?
Events like this leave more questions than answers and are part of what make understanding space weather exciting. Studying this CME event and others like it will make for excellent doctoral theses and post-doc research projects. These research projects will expand the space weather community’s understanding of our Sun. Maybe next time we’ll predict the arrival of the next “stealth-CME” and the onset of the geomagnetic storm. Are you still looking for a PhD thesis? This might be a good place to start!