Purple Auroras

Purple Auroras

 Over the weekend, the sky above Canada and many northern-tier US states turned purple. It was the aurora borealis, sparked by a CME impact during the late hours of June 7th. "Wonderful purple and blue auroras spanned the sky, peaking between 2 and 2:30 a.m. MDT on June 8th," reports Alan Dyer, who captured the colors outside an old barn in Alberta, Canada:

 

In auroras, purple is a sign of nitrogen. While oxygen atoms produce the green glow in Dyer's image, the purple comes from molecular nitrogen ions at very high altitudes. For some reason, high-altitude nitrogen was unusually excited during this G2-class geomagnetic storm, and many people witnessed its telltale hue.

More purple could be in the offing. A solar wind stream following in the wake of the CME has kept Earth's magnetic field unsettled two full days after the CME's impact. Solar wind speeds are now greater than 500 km/s, prompting NOAA forecasters to boost the odds of a polar geomagnetic storm on June 9th to 50%.