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The ghostly glow of aurora

Mark Thompson
Feb 15, 2012, 8:00 UTC

For many people the Sun is just that very bright thing in the sky whose heat and light gives planet Earth the energy it needs to sustain complex forms of life.

Yet aside from these life giving properties, the Sun also throws out varying amounts of material called solar wind.

Solar wind floods the Solar System with electrically charged particles, predominantly electrons and protons.  

When they arrive here on Earth they can produce one of nature’s most stunning and sometimes psychedelic displays, the aurora.  

The solar wind comes from the outer atmosphere of the Sun, the corona and has two components. Rather imaginatively named the slow and fast solar wind you can probably guess one is faster than the other!  

The slow wind has a speed of around 400km/s compared to up to 800km/s for its faster counterpart. 

The temperature also varies between the two but its their different origins that set them apart. The slow solar wind seems to originate from a region around the Sun's equatorial region whereas the fast solar wind seems to come from coronal holes which are, as their name suggests, holes in the Sun's corona typically found around the polar regions. 

The quantity of solar wind that is ejected from the Sun varies with solar activity reaching a peak at the end of each 11 year solar cycle. 

It is typically the slow solar wind that is responsible for the beautiful and mesmerising aurora displays that occasionally grace our skies. 

Because the wind is composed of electrically charged electrons and protons they can interact with magnetic fields and, more importantly, be channelled and directed by them.  

The Earth has a magnetic field which is generated deep within the planet and the magnetic field lines exit at the north and south magnetic poles. From here the lines loop around to the opposing pole almost creating a huge magnetic barrier for the planet with a weak point at the poles. 

On arrival at Earth the solar wind slams into the magnetic field of the Earth and gets channelled to the north and south poles. Its here, at the high latitudes that things start to get really interesting.

High in the atmosphere at altitudes of around 80km, the charged particles cause changes in the energy state of atoms making them glow and give off light. Its this light that we see as the characteristic aurora displays.  

Those of you that have been lucky enough to see them will have perhaps noticed that they glow with different colours from green to red to blue. The different colours are the result of different gasses in the atmosphere. The green/brown colours come from Oxygen and the blue/red from Nitrogen. As the charged particles hit the gas molecules, they give them a little extra energy and cause them to move into a higher energy state. Following this, the gas atom will return to its normal or 'ground' state and with it, have to somehow loose the energy and it does this by giving off a little bit of light. Nitrogen and Oxygen both have very different atomic structure and as a result give off light at different wavelengths and hence colours. 

Whether you will be able to see them from where you live depends entirely on the amount of solar wind that hits the Earth.

Aurora Borealis

Image credit: Mia Stålnacke, twitter @AngryTheInch

The greater the Coronal Mass Ejection (CME), the more powerful the aurora and (in the northern hemisphere), the lower latitudes will be treated to a display. But these occasions are few and far between, usually its just the polar regions that can enjoy almost nightly displays.  

Image credit: Rachael Taylor

Image credit: Rachael Taylor, Copper Beech Photography

Its not just Earth that experiences the aurora. They can also been seen on other planets.  In fact, most planets with a dense atmosphere will have aurora but those with weak or no magnetic field will have very irregular auroral displays. 

Images of Jupiter and Saturn from space piercing telescopes show the ghostly glow of aurora around their poles in incredible detail looking very similar to images of Earth's aurora from space.

Aurora Borealis pictured on Creswell beach, Northumberland on the 22 January 2012. Image credit: Lee Jennings

Aurora Borealis pictured on Creswell beach, Northumberland on the 22 January 2012. Image credit: Lee Jennings, twitter @Timelesslee1

Aurora that emerge from the north pole are known as the Aurora Borealis or “Northern Lights”. But its not just the northern hemisphere that gets aurora. The southern hemisphere also sometimes bathes in auroral light from the Aurora Australis.

There are plenty of resources out there for alerting you to possible aurora displays from websites like www.spaceweather.com or twitter accounts like @Aurora_Alerts or @aurorawatchuk