Author Topic: Auroras  (Read 3233 times)

s.f.burke

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Auroras
« on: May 15, 2012, 04:06:27 pm »
Hello everyone,
I'm a 17 year old trying to do a research project for my honor physics class and I had (what might be a dumb) question for you!

So magnetic storms caused by CMEs (specifically magnetic clouds) can produce Auroras, and I was just wondering if that was the only time Auroras are produced!

Thank you so much for reading, and possibly replying!  :)

ChrisScott

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Re: Auroras
« Reply #1 on: May 16, 2012, 04:40:05 pm »
Hello S.F.Burke! (sorry to be so formal but I can only see your user name). Welcome to the forum!

There is no such thing as a dumb question. Yours is, in fact, a very good one.

Auroras occur when the gasses in the upper atmosphere become energised. As the air above 100 km is very thin, the atoms and molecules in the atmosphere cannot easily lose this energy through collisions and so many eventually emit it as light. These emissions are at specific wavelengths, giving the aurora characteristic colours (red, green mauve). A CME can cause the aurora when the cloud of plasma and magnetic field arrives at Earth. If the magnetic field orientation of the CME is opposite to the Earth's (ie it's 'south' pole is upwards (let's get into the debate about what constitutes 'up' in space another time!) then the CME field and the Earth's field can 'reconnect' allowing the solar wind plasma to reach the Earth's atmosphere more efficiently where it can excite the gasses there as energetic particles precipitate in to the atmosphere.

As the solar wind blows away from the Sun, any magnetic field within the solar wind moves this way too. Any of Earth's magnetic field lines connected to the solar wind are dragged anti-sunward too, stretching them into a long magnetotail that stretches out behind the Earth for many Earth radii. Magnetic field lines act like elastic bands in that the more you stretch them, the more energy they contain. Eventually, the solar magnetic field disconnects from the Earth's field and catapults that field back towards the Earth, taking solar wind and atmospheric particles with it. When this tail reconnection occurs in a burst it causes an auroral substorm on the night-side of the Earth as particles are once again accelerated into the Earth's atmosphere at the north and south poles. Counterintuitively therefore, even though the aurora is caused by the Sun, you are more likely to see an aurora at midnight!

Now, you ask an interesting question because if conditions are right; the magnetotail is stretched and waiting to snap back towards Earth but has yet to do so, the slightest buffet from the solar wind can trigger a substorm. We know the Sun has patches of fast and slow solar wind (like a leaky balloon, there are some regions where the material travelling outwards is much faster due to the local magnetic conditions at the surface of the Sun). The Sun also rotates once every 27 days with respect to the Earth (the so-called Carrington rotation rate) and so the solar wind speeds at Earth vary with a 27 day cycle. If there is a patch of fast solar wind, the Suns rotation will cause this to blow towards Earth and such a stream could trigger a substorm by giving the magnetotail the energy it needs to diconnect from the solar wind and collapse back towards Earth.

So, aurora can also be caused not just by CMEs but by variations in the solar wind too. There is another way too. We can generate them automatically! A group here in the UK, at the University of Lancaster, have pioneered the technique of using powerful radio waves to excite the atmospheric gasses in the upper atmosphere. As the gas molecules try to lose this energy they too give off light - just like the natural aurora only on a much smaller scale. Such experiments are helping us to understand how the aurora is generated as we can now create (a small) one whenever we want to rather than having to wait for conditions to be right in the solar wind.

I hope this helps!

Chris.

jules

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Re: Auroras
« Reply #2 on: May 16, 2012, 05:54:08 pm »
Thanks Chris! As this is a great reference to point people to I've stickied it.

ChrisScott

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Re: Auroras
« Reply #3 on: May 16, 2012, 08:31:30 pm »
Does it deserve a cake? :D

jules

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Re: Auroras
« Reply #4 on: May 16, 2012, 10:54:28 pm »