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The "Perfect" Solar Storm?
录入者:  |  时间:2014-04-01 18:56:35  |  作者:  |  浏览次数:

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.

 

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