On July 22, 2012, a massive cloud of solar material erupted off the
sun's right side, zooming out into space and passing one of NASA's twin
Solar Terrestrial Relations Observatory, or STEREO, spacecraft along the
way. Scientists clocked this giant cloud, known as a coronal mass
ejection, or CME, as traveling over 1,800 miles per second as it left
the sun.
Conversations began to buzz and the emails to fly: this was the
fastest CME ever observed by STEREO, which since its launch in 2006 has
helped make CME speed measurements much more precise. Measuring a CME at
this speed, traveling in a direction safely away from Earth,
represented a fantastic opportunity for researchers studying the sun's
effects. Now, a paper in Nature Communications, published on March 18,
2014, describes how a combination of events worked together to create
these incredible speeds.
"The authors believe this extreme event was due to the interaction of
two CMEs separated by only 10 to 15 minutes," said Joe Gurman, project
scientist for STEREO at NASA's Goddard Space Flight Center in Greenbelt,
Md. "Plus the CMEs traveled through a region of space that had been
cleared out by another CME four days earlier."
The researchers describe the July 2012 event as a perfect storm,
referring to the phrase originally coined for the October 1991 Atlantic
Ocean storm to describe an event where a rare combination of
circumstances can drastically aggravate a situation.
Such work helps scientists understand how extreme solar events form
and what their effects might be if aimed toward Earth. At Earth, the
harshest space weather can have strong effects on the magnetic system
surrounding the planet, which in turn can affect satellites and
interrupt GPS and radio communications. At its worst, rapidly changing
magnetic field lines around Earth can induce electric surges in the
power utility grids on the ground. One of the best ways to protect
against such problems, and perhaps learn to predict the onset of one of
these storms, is to make computer models matching the observations of
past events.
In the case of the July 2012 event, three spacecraft offered data on
the CMEs: the two STEREO spacecraft and the joint European Space
Agency/NASA Solar and Heliospheric Observatory, or SOHO. SOHO lies
between Earth and the sun, while the two STEREO spacecraft have orbits
that for most of their journey give views of the sun that cannot be had
from Earth. Each spacecraft observed the CMEs from a different angle,
and together they could help map out a three-dimensional image of what
happened.
The authors suggest it was the successive, one-two punch of the CMEs
that was the key to the high speeds of the event – speeds that would
lead to circling Earth five times in one minute. A CME from four days
earlier had an impact too. First, it swept aside particles in the way,
making it all the easier for the next CMEs to travel. Second, it
altered the normal spiral of the magnetic fields around the sun to a
straighter pattern above the region that was the source for these CMEs,
thus allowing for freer movement.
"A key finding is that it’s not just the initial conditions on the
sun that can produce an extreme space weather storm," said Gurman. "The
interactions between successive coronal mass ejections farther out in
interplanetary space need to be considered as well."
The researchers found that state-of-the-art models that didn't take
the effects of successive CMEs into consideration failed to correctly
simulate the July 2012 event. Such information will be incorporated
into the models being tested by space weather forecasters. This should
lead to better predictions of the worst storms and better protection of
Earth and our technology in space.
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