A geomagnetic storm commenced Thursday night as the first of two waves of plasma from the sun – known as coronal mass ejections (CMEs)- bombarded the Earth’s atmosphere. Dramatic aurora filled the high-latitude skies. But this first wave – rated G1 on the 1-5 scale for geomagnetic storms – is expected to be weaker than the blast arriving today. It could peak at the “strong” G3 level.
Even more magnificent and widespread aurora – perhaps as far south as Chicago – are possible tonight with this second wave.
“G3 (Strong) Geomagnetic Storm Watch is still in effect for September 13th due to the combined influence of this [first] CME and the one projected to arrive late on the 12th,” writes NOAA’s Space Weather Prediction Center.
The pair of CMEs originated from back-to-back solar flares Tuesday and Wednesday. Wednesday’s flare was particularly powerful, an X-class flare, the most intense kind.
“[T]he sunspot that unleashed these flares is about 5 times the width of the Earth,” writes Boston meteorologist Eric Fisher.
The University of Alaska expects “high auroral activity” tonight due to the geomagnetic storming. “[A]uroral activity will be … visible low on the horizon from Seattle, Des Moines, Chicago, Cleveland, Boston, and Halifax,” it says.
The K-index – a measure of geomagnetic activity – provides a good guide as to where aurora are viewable. “Kp-indices of 5 or greater indicate storm-level geomagnetic activity,” says NOAA’s Space Weather Prediction Center.
Thursday night, levels peaked at 5, favoring viewing in very northern tier of the U.S. into Canada and over northern Europe (although one photographer in Arizona caught a glimpse, see below). In order for aurora to appear in the Mid-Atlantic, we’d want to see that index peak in the 8 to 9 range, or higher.
A reasonable forecast for tonight’s level, given G3-level storminess, is around a 7.
Link: Track the K Index
The southern extent of aurora is very difficult to forecast and, like any night sky phenomena (aside from clouds), optimal viewing requires clear and dark skies. Your best bet for aurora viewing? Get away from city lights and look north. Check the K-index before heading out, too.
Photos of the aurora Thursday night
The above image was reportedly photographed last night in central Arizona – much farther south than you might expect aurora to have been visible. Is it legit? SpaceWeather.com says yes:
Some readers may be skeptical of this observation, but it is real. Schur is a veteran photographer of auroras; he knows what they look like. Moreover, red is the correct color for auroras at low latitudes.
“This pink glow right under Ursa Major was visible to the naked eye before the moon came up,” says Schur. “These were the first auroras here in Arizona since the previous solar maximum.”