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Old 02-06-2009, 04:39 PM   #1
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Default Auroras: What powers the greatest light show on Earth?



Auroras, substorms and more hazardous kinds of space weather all begin with the solar wind - a thin, hot gas of charged particles ejected from the sun, laced with magnetic fields and threaded with electric currents. This magnetic hurricane is blowing over the planet at 1.6 million kilometres per hour, but we don't feel so much as a breeze. That's because most of it is deflected by the Earth's magnetic field, which maintains a zone of relatively calm weather around the planet, called the magnetosphere. As the solar wind blows past the Earth, it pushes and stretches this protective shield out on the night side of the planet, like hair blown in the wind.

Despite this protection, the solar wind buffets and stirs up the magnetosphere, sending high-energy particles showering into the Earth's upper atmosphere. There they light up the gases like a neon tube, creating an aurora that appears as a slowly shifting curtain of green light as the charged particles smash into oxygen atoms. These "quiet auroral arcs" are usually quite faint. "People will often not realise there's an aurora. The sky will look a bit weird maybe, with a diffuse glow," says Eric Donovan of the University of Calgary, Alberta, who monitors the aurora borealis in Canada.

When a substorm rips through the magnetosphere, though, unleashing the energy of a few megatons of TNT, the effects are unmistakable. Magnetic fields whip through space, the electrical currents that circle the magnetosphere thrash wildly, and the aurora is transformed into a much brighter and more dynamic display that sweeps across the sky for 10 to 15 minutes. "It is not uncommon to get a hundred or thousandfold increase in brightness," says Donovan. The aurora also becomes more colourful, as high-energy electrons smash into molecules of the air, exciting red and green light from oxygen and blue from nitrogen.

NASA launched a flotilla of satellites, collectively named THEMIS, in February 2007 to catch substorms as they happen. The five small spacecraft orbit the Earth like juggled balls, each following a different looping path, so when something interesting happens in the magnetosphere there's a good chance that they will be in a suitable arrangement to see it.

Three months after launch, THEMIS encountered the beginnings of a substorm. "We had five spacecraft lined up in a row perpendicular to the outer boundary of Earth's magnetic field, some just inside, some just outside," says Sibeck.

This position turned out to be the perfect spot to answer one of the mission's questions: how the solar wind pumps energy into the magnetosphere to power a substorm.

THEMIS's recordings revealed changes in the Earth's magnetic field as the solar wind connected with the magnetosphere. A bulge of twisted magnetic fields formed and slid along its boundary, towards the night side of the Earth. The team recognised this as a phenomenon called a flux rope, which some researchers had suggested would be linked to substorms.

Flux ropes connect the magnetic fields in the solar wind with those of the magnetosphere and the two become entwined, linking Earth's domain with that of the sun. This allows high-energy particles to stream in, loading the magnetosphere with pent-up energy (see diagram).
http://www.newscientist.com/data/ima...4/26941801.jpg


As the solar wind blows over the Earth, it pulls on its end of the flux rope, dragging the rope and its magnetic fields away from Earth's day side and out into the tail of the magnetosphere.

As more and more flux ropes form and are pulled into the tail, the day side of the Earth loses more and more of its magnetic field. That does not go on forever, of course. "It would completely deplete the day-side magnetic field", says Vassilis Angelopoulos of the University of California in Los Angeles, who heads the THEMIS mission. Earth's shield would disappear, leaving us exposed to carcinogenic cosmic rays. Over geological timescales, the atmosphere might even be stripped away by the solar wind.

Several things happen almost simultaneously: the tail snaps, hurling plasma towards the Earth, and the electric current that girdles the Earth is disrupted. But which of these triggers the substorm and the resulting aurora?
To find out, the THEMIS researchers needed to know which happens first.

There are two competing theories. One school of thought has it that the impetus must come from the powerful electric current that flows around the magnetosphere about 60,000 kilometres up. The motion of magnetic fields drives this current, as in a dynamo, and it is known to be boosted when magnetic field is added to the tail. Does it get so strong that it becomes unstable and showers the atmosphere with high-energy electrons?

The other theory is that the trigger comes from the tail itself. As more magnetic field is added to it, the tail gets compressed tighter and tighter. Around the pinched core of the tail, these magnetic fields point in opposite directions, one running outwards from the north pole and the other running in towards the south pole. As these two field lines are stretched and squeezed by the solar wind, perhaps the two opposing fields spontaneously reconnect, cutting the tail in half and sparking a substorm (see diagram).
http://www.newscientist.com/data/ima...4/26941801.jpg

As luck would have it, on 26 February 2008, a substorm hit while the THEMIS flotilla was strung out on the night side of the Earth, straddling the region where the current would be disrupted and also where the tail would be expected to snap and reconnect. It was the perfect opportunity to settle the argument.

The first thing the satellites recorded was the tail of the magnetic field snapping off and reconnecting, suggesting that substorms do start with changes in this area. Case closed? Not quite. There was also a big surprise for the THEMIS team. Angelopoulos expected that the break in the tail would first destabilise the current encircling the planet, which in turn would spray electrons down to Earth to cause the aurora. Instead, the aurora began to intensify about a minute after reconnection in the tail, and, crucially, before the disruption of the ring current. "I was shocked," says Angelopoulos. "We never expected that within a minute you would see the aurora light up."

Full article here:
http://www.newscientist.com/article/...th.html?page=1

Gallery of Auroras here:
http://www.newscientist.com/gallery/...how-on-earth/1
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Old 02-06-2009, 04:53 PM   #2
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I found this the other day looking for music. It has beautiful Auroras as well as pretty music, enjoy!
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