Auroras
INTERPLANETARY SHOCK WAVE: An interplanetary shock wave hit Earth's magnetic field on April 19th around 23:50 UT. When the disturbance arrived, the density of solar wind flowing around our planet abruptly quadrupled and a crack opened in Earth's magnetic field. The resulting G2-class geomagnetic storm sparked unusual "electric blue" auroras.
"I've been flying airplanes
for 20 years and photographing aurora for 10 years, but I've never
seen anything like this before," reports pilot Matt Melnyk who
photographed the display from 39,000 feet:
"Electric blue auroras!" he
says. "This was while on a red eye flight from Edmonton to Toronto
around 4 am over northern Manitoba. Unbelievable sky. I was able to grab
some hasty shots with a cell phone."
Auroras are usually green--a sign of oxygen. Rare blue auroras are caused by nitrogen molecules. Energetic particles striking N2+ at the upper limits of Earth's atmosphere can produce an azure glow during intense geomagnetic storms.
During the storm, Northern
Lights spilled across the Canadian border into the United States as far
south as Indiana. Hongming Zheng, a student at Purdue University, saw
the blue glow just five miles from his dorm:
"I was preparing for bed at
1:32 am on April 20th when I read that there was an Interplanetary
Shockwave," says Zheng. "I immediately started driving north to see the
show. A weak green wisp showed up at 2am and faded, but shortly after
5am a sudden outburst occurred. Purple pillars were easily visible to
the naked eye. It's funny how one minute you are in a humid dorm
struggling to get the laundry door closed, and the next minute you are
chasing one of the most spectacular phenomenon known to man."
What is an interplanetary
shock wave? It is a supersonic disturbance in the gaseous material of
the solar wind. These waves are usually delivered by coronal mass
ejections (CMEs). Indeed, this one might have been a minor CME that left
the sun unrecognized earlier this week.
Alternately, it might have
been an unusually sharp co-rotating interaction region (CIR). CIRs are
transition zones between slow- and fast-moving streams of solar wind.
They contain plasma density gradients and magnetic fields that often do a
good job sparking auroras. Free: Aurora Alerts
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