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 two
Solar Terrestrial Observatory (STEREO) spacecraft along the way.
Scientists clocked this giant cloud, known as a coronal mass ejection,
or CME, as traveling over 3000 kilometers 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.
Dr. Ying Liu of the Chinese Academy of Sciences' State Key Laboratory
of Space Weather and coauthors from the US (including two STEREO
Principal Investigator teams), Austria, Finland, and France believe this
extreme event was due to the interaction of two CMEs separated by only
10 to 15 minutes. The resulting interplanetary CME's remarkable speed
may be due to its traveling through a region 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 by Sebastian Junger 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 effect 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 learn to predict the onset of one of these
storms, is to make computer models based on data gleaned from past
events.
In the case of the July, 2012 event, three spacecraft obtained data
on the CMEs: the two STEREO spacecraft and the joint NASA/European Space
Agency 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 that also allowed for freer movement.
As Dr. Liu describes this work, a key finding is that it's not just
the initial conditions on the Sun that can produce an extreme space
weather storm. The interactions between successive coronal mass
ejections further 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 simulate the
July 2012 event correctly. Such information needs to be incorporated
in future models, and should lead to better predictions of the worst
storms, which will lead to better protection of Earth and our technology
in space.