My Dark Sky

Recently, I kept seeing pictures of aurora in the web. It’s not a surprise since the Sun is quite active lately. What a coincidence is that I also kept on hearing the word “aurora” from the people around me. We live at the equator, most of us only heard of but never really know what an aurora is, thus aurora is not something you’ll talk about in your daily conversation with others.

Is that a sign that I’ve to finish my article on aurora which I’ve plan to write long long time ago? I’ll take that as a Yes.

So, what is an aurora?

Wikipedia answer: “An aurora (plural: auroras or aurorae) is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere.” In the northern hemisphere, it is called aurora borealis (northern lights) whereas in the southern hemisphere, it is called aurora australis (southern lights).

We always see aurora as static, because most of us only see aurora through pictures, which only capture it as a moment in time. If we can look at an aurora over time, as in the videos below, we can see that they are in fact changing and “moving”. This is why some time they are also called “the dancing lights”.

You must watch all the videos below because they are just beautiful! I guarantee you will not regret.

Astronauts Capture Aurora Australis from the International Space Station. If you are an astronaut, you also can observe aurora, but from above looking down, rather than from below looking up! Click here for more pictures of auroras from space.

For an aurora to happen, you need three ingredients – charged particles, magnetic field and atmosphere. The charged particles come from the Sun, and the magnetic field and atmosphere is from Earth*. Our Sun is an active ball of gas; it constantly gives out streams of charged particles known as solar winds. When these charged particles reach Earth, our Earth’s magnetic field guides these particles to the north or south pole, where they collide with air molecules in the upper atmosphere (at altitudes above 80 km). The collisions excite/ionise the air. When the electron returns to its original energy level or recombines with the air molecule, energy is release in the form of light creating the visible aurora. See the video below for a nice animation on how aurora is forms.

Click here for a more detail (and longer) video on how aurora forms.

Since the charged particles will follow the magnetic field lines to the north and south pole, aurora is usually visible in the far northern and far southern parts of the planet, in a band known as the auroral zone. The auroral zone is typically 10 to 20 degrees from the magnetic pole. However, during a geomagnetic storm (in simple words, when the Sun is more active), the auroral zone can expand to lower latitudes making aurora visible to temperate latitudes. Large magnetic storms are most common during the peak of the eleven-year sunspot cycle or during the three years after that peak. The current solar cycle predicted to peak around 2013/2014, so we can expect more light shows for the years to come.

Click here for animation. The IMAGE satellite captured this view of the aurora australis on September 11, 2005. Shown in the image is the ring of light that the solar storm generated over Antarctica, glowing green in the ultraviolet part of the spectrum. Because the Earth’s magnetic field is three-dimensional, the aurora appears as an oval ring around each of Earth’s pole. It isn’t a perfect circle because the Earth’s magnetic field is distorted by the solar winds. From the Earth’s surface, the ring would appear as a curtain of light shimmering across the night sky.

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Colours of the Aurora

Variation of colours in the aurora with altitude. Credit: Causes of Color

If we look at pictures of aurora, we can see that aurora comes in different colours, with green as the most common. As we all know (do you?), our atmosphere consists of different compounds such as nitrogen and oxygen. The colour of the aurora depends on which gas in our atmosphere is being excited, and on how excited it becomes. Oxygen is responsible for the green and red colour; nitrogen causes blue and deep red hues.

Oxygen is unusual in terms of its return to ground state after excitation: it can take three quarters of a second to emit green light and up to two minutes to emit red. However, collisions with other atoms or molecules will absorb the excitation energy and prevent emission. At the top of the atmosphere, there has a higher percentage of oxygen and it is sparsely distributed such that collisions are rare. This allows time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen thus only green is visible, and eventually even green light emissions are prevented at even lower altitude.

This is why there is a colour differential with altitude; at high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common of all auroras, followed by pink, a mixture of light green and red, followed by pure red, yellow (a mixture of red and green), and lastly pure blue.

Watch the video above The Aurora again and see if you can spot the layers of different colour with altitude.

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*Earth is not the only planet to have auroras. Auroras occur on other planets too – Jupiter, Saturn, Uranus and Neptune also have their own light shows since they also have magnetic fields and atmosphere. Similar to the Earth’s aurora, they are visible close to the planet’s magnetic poles.

Jupiter Aurora. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Saturn Aurora. Credit: NASA, ESA, J. Clarke (Boston University), and Z. Levay (STScI)

~ by thChieh on October 13, 2011.

Posted in Beautiful Photos, General Astro Talk
Tags: Aurora