The Aurora Borealis:
It is commonly visible between latitudes of 65 and 72 north and south - which is just within the Arctic and Antarctic circles - however it is possible to see anywhere in the world at certain times. For example, on the 29th of October 2003 the Aurora Borealis was visible in the United Kingdom in the largest geomagnetic storm of the previous 20 years. Auroras have also been found on other planets and even some moons.
Aurorae on Jupiter:
However this photo is owned by NASA (in public domain)
The aurorae usually appear in the ionosphere and, since 2009, the theory as to how and why they occur has changed. They are caused by electrically charged particles released from the sun in the form of solar winds. These particles were thought to directly interact with the Earth's magnetic field causing the aurorae, however there were some anomalies with the theory. If it was solar winds from the sun that caused aurorae then why were they visible at night when there was no sun shining on that hemisphere?
NASA (the National Aeronautics and Space Administration based in the USA) sent 5 probes into orbit around the Earth in a 2 year mission called 'THEMIS' to study the incredible and colourful Aurora Borealis. They are trying to work out why they happen and what causes them to occur how they do - in narrow bands made up of vertical streaks - and why they move as they do. This is important to help them predict these events as the substorms (which are aurorae that erupt with little warning and high intensity) disrupt communications to spacecraft and even damage ground-based systems including transformers in power lines and communication systems.
These interruptions and the damage occur because a very high current is induced into the electrical wires which support the whole network. This is thanks to the flexing magnetic field moving over them when the aurorae occur. It was first discovered that aurorae affected the Earth's magnetic field in 1741 by Olof Hiorter. They have such a large effect because the aurorae have currents in excess of 100,000 amperes - a staggering amount when considering an average 100 watt light bulb runs on 0.4 amperes
[Photo above left] (Source-http://www.srh.noaa.gov/srh/jetstream/atmos/layers.htm)
However this photo is owned by U.S. National Oceanic and Atmospheric Administration (in public domain)
Statistics on Aurorae:
- Have currents in excess of 100,000 amperes -
- One geomagnetic storm alone has been observed to have a total energy of 500 thousand billion joules (5 x 10^14 J) - equal to a scale 5.5 earthquake
- They travel at speeds double what was previously thought even possible - managing to travel across whole time zones in less than 60 seconds
- There is more energy being dissipated in the atmosphere by the aurorae than by every power station on the planet put together
- Voltages in the ionosphere rise from around 40,000 volts to over 200,000 volts during the intense electrical storms of the aurorae
Time Lapse Video of the Aurora Borealis:
The constellation of 5 micro-satellites brought back information invaluable to the efforts of understanding these amazing phenomena. They have detected 'magnetic ropes' which the charged particles follow on their route from the sun to the Earth and this is what provides the aurorae and geomagnetic storms.
One of the problems with the previous theory was that most aurorae appear at night when there is no sun on that side of the earth - so how do the solar wind particles create aurorae where there is no light reaching it so therefore no charged particles?
This was solved when they found that many charged particles are deflected around to the dark side of our planet by the magnetosphere. The magnetosphere is formed by the impact of the solar wind on the Earth's magnetic field and diverts particles at around 70,000 kilometres from the Earth. These deflected particles gather into clouds of the charged particles accumulating charge continuously becoming more and more unstable as the charge increases.
Once the charge becomes too great, they burst into an electrical tornado of electrical charge spiralling downwards following the magnetic field lines which guides them towards the poles. The electrical tornadoes spin at more than a million miles an hour as they follow the magnetic field lines and create the current of more than 100,000 amperes. These charged particles reacting with our atmosphere is what creates the awe-inspiring aurorae.
Structure of the Magnetosphere:
However this photo is owned by NASA (in public domain)
Many different colours can be in an aurorae - almost all of the visible spectrum in fact. The colours of the aurorae are created by the solar wind particles colliding with the gases in the Earth's atmosphere and incoming charged particles tend to collide with different types of gases at different heights. The particles collide with the molecules and atoms of the gas in the ionosphere and give it energy which it then emits back out as light:
- Oxygen causes red and green light. Above 150 kilometres up in the atmosphere, oxygen forms red light which is very rare - these can sometimes (but rarely) be up to 600 kilometres up.
- Lower in the atmosphere it causes the most common aurorae which is yellow and green. This is at altitudes of 100 to 150 kilometres
- At around 100 kilometres, the charged particles collide with nitrogen molecules and produce another red light. This sometimes forms on the lower edge of yellow and green aurora.
- Blue and purple aurorae can be created at very high altitudes by nitrogen but these are often not able to be seen.
Even if you don't understand some of the science, aurorae are incredible to watch and if you are lucky enough to even see them, it'll be something you never forget.
NASA | The Mystery of the Aurora - YouTube
Northern Lights - The Sky At Night - BBC Four - YouTube