What Is an Aurora? How Do They Happen?
What is an aurora?
Auroras occur when the atoms in the Earth’s atmosphere interact with the charged particles from a solar storm arriving from the Sun. During this phenomenon, electrons present in the atoms in our atmosphere move to a higher, unstable energy state. In order to reach the stable state, they must release the excess energy, which creates beautiful and scenic light shows. Auroras appear in the sky at high altitudes between 50-400 miles above the Arctic and Antarctic regions. They occur continuously and can only be seen at night because during the day the light of the sun outshines them.
Figure 1. One of the most photographed auroras, the Aurora Borealis How do they happen?
As mentioned above, auroras occur during the interaction of charged particles in a solar storm with the atoms in our atmosphere, but what exactly is a solar storm and why do auroras behave the way they do?
Formation of a Solar Storm We begin inside the core of the Sun, where the temperature and pressures are so high that they cause a nuclear reaction resulting in the formation of Helium from the fusion of two Hydrogen atoms.
The light energy released by this reaction travels through the layers towards the surface of the Sun and radiates out into our solar system. The heat energy produced by this reaction is absorbed by the plasma (electric currents of charged gas that make up the mas of the Sun) and travels to the surface using convection cells. In these cells, the convection currents carry the hot plasma towards the surface, and allow the cooler plasma to fall back down to the core.
The plasma present in the convection cells helps create the Sun’s magnetic field. Some areas on the Sun’s surface can develop massive collections of plasma leading to a high density magnetic field in that area. If this magnetic field is strong enough, it has the potential to break through the surface allowing the plasma to stretch it out. The elastic effect of this stretching then brings the magnetic field back towards the Sun just like a rubber bands snapping back. This process is repeated several times until the magnetic field has been stretched out so far that it breaks away from the Sun, sending billions of tons of plasma out into space at incredible speeds in the form of a Figure 2. The separation of the solar storm (light yellow lines) from the surface of the Sun. solar storm. Aurora Generation Like the solar storm, we begin at the Earth’s core. The core is made up of hot molten iron that is constantly moving. As a result of Iron’s magnetic properties, this movement causes the development of the Earth’s magnetic field in the upper atmosphere which is known as the magnetosphere. The magnetosphere protects us from most of the solar storm waves, which are harmful to humans due to their radioactive nature. However, small safe amounts of the solar storm get through weak points in the magnetosphere usually situated at the north and south poles. Hence why auroras can only be observed in Figure 3. The blue magnetosphere protects the Earth from the orange solar storm waves. these regions.
Once the charged electrons in the solar storm get past the magnetosphere and into the atmosphere, they react with the Nitrogen and Oxygen present by transferring their energy to these atoms. Subsequently, the Nitrogen and Oxygen atoms enter an excited and unstable energy state. In order to reach the stable Figure 4. Red auroras caused by Figure 5. Blue auroras caused by state again, they must release the excess presence of Oxygen above 200 km presence of Nitrogen above 100 km energy by emitting tiny packets of light called photons.
Photons have the ability to change colors depending on what gas they happened upon and at what height in the atmosphere they encountered them. For example, Oxygen gives off a yellow- Figure 6. Purple auroras caused by Figure 7. Yellow-green auroras caused green color at a height of roughly 100 presence of Nitrogen below 100 km by presence of Oxygen around 100 km kilometers. This yellow-green color is the most commonly associated color with auroras. Oxygen can also provide the red color in auroras at altitudes of around 200 kilometers. Nitrogen produces the shades of blue at heights above 100 kilometers and a reddish purple at heights below 100 kilometers. Auroras can be seen in several light and dark colors including pink, red, blue and purple. Due to the mixture of these gases in the air, most of the time auroras simultaneously consist of multiple colors.
Earth’s auroras Earth has two auroras, the Aurora Borealis and the Aurora Australis. They are located near the north and south poles respectively. The Aurora Borealis can be seen in northern latitudes including countries such as Greenland, Iceland, Norway and northern parts of Canada. The Aurora Australis can be seen in countries such as New Zealand, southern parts of Argentina, and the island of Tasmania.
Figure 8. Aurora Borealis
Figure 9. Aurora Australis
All Together Now
An aurora, something that seems so simple on the surface, is actually a very complex event that can only be explained by some tricky physics. The aurora’s journey begins at the Sun where a solar storm breaks through it’s surface and travels out into space. This solar storm contains an enormous amount of electrically charged plasma that makes its way across 92.96 million miles of empty void and reaches the Earth within a mere 18 hours.
The Earth’s magnetosphere reflects most of the solar storm, but some of the waves manage to get through into the atmosphere through the north and south poles. The solar storm hits the poles, reacts with the atoms in the atmosphere and creates daytime auroras. These cannot be seen by the naked eye due to the immense brightness of the Sun. The night time auroras are created when the solar storm waves stretch Figure 10. Initial solar storm waves (orange and green back and bend around the Earth’s lines) reaching the north and south poles creating the day time auroras. The plasma is represented by the light and magnetosphere. The waves collapse by coming cloudy part of the picture. into contact with themselves resulting in the rest of the solar storm plasma stream along the Earth’s natural magnetic field lines towards the poles. Now, the electrons in the plasma react with the different gases in our atmosphere giving us the picturesque and colorful auroras.
Figure 11. Collapse of the waves on each other creating the night time auroras. The flow of plasma can be seen flowing back towards Earth.
Sources
Figure 1 http://wallpapercave.com/wp/t1ZMnHR.jpg
Figure 2 https://www.youtube.com/watch?v=1DXHE4kt3Fw
Figure 3 http://lasp.colorado.edu/home/wp-content/uploads/2011/03/sunearth01.jpg
Figure 4 http://www.nasa.gov/mission_pages/sunearth/news/News102511-aurora.html
Figure 5 http://static.ddmcdn.com/gif/storymaker-aurora-northern-lights-space-phenomena7- 515x281.jpg
Figure 6 http://7-themes.com/data_images/out/70/7010983-purple-aurora-borealis.jpg
Figure 7 http://royhooper.ca/wp-content/uploads/2012/11/CRW_4775-e1353727042831.jpg
Figure 8 http://sites.psu.edu/edkawong/wp-content/uploads/sites/15056/2015/03/Tm70k2m.jpg
Figure 9 http://onthreelegs.com/wp- content/uploads/2013/03/Aurora1183_Copyright_Ben_Fewtrell_2013.jpg
Figure 10 https://www.youtube.com/watch?v=1DXHE4kt3Fw
Figure 11 https://www.youtube.com/watch?v=1DXHE4kt3Fw