Elusive dancing lights in the sky, flickering across the navy–blue blanket that shrouds the world at nightfall. An enchanting sight like no other; anyone would be awestruck at its magnificence. It is none other than the sighting of the Aurora Borealis and Aurora Australis; these amazing natural light displays that occur at the Earth’s poles, the Arctic and Antarctic, confounding people for a long time.
Many myths and legends were created across the ages to explain this awe-inspiring phenomenon; however, we have only lately come to a better understanding of it. Nevertheless, although the cause of the Aurora is well known now, we have yet to unlock all of its secrets.
Our journey to unfold the mystery of the Northern Lights and Southern Lights begins with our trusty friend, the Sun. Auroras are in fact caused by solar winds, which are charged particles travelling from the Sun, coming into contact with the Earth’s magnetic fields. Only when we follow the particles’ journey from its inception to its contact with the Earth’s magnetic field, we understand how it all takes place, so let our journey begin!
You may have already started feeling warm, maybe a little too uncomfortably warm. Do not worry; we will just have a quick look at the Sun, then head back to Earth soon enough!
The Sun is the closest star to our planet; it provides us with heat and light. Basically, it is a hot ball of gas, mostly hydrogen. As it is very hot, most of the gases exist in the fourth state of matter, plasma. Deep inside, due to the high pressure and the incredibly high temperature of over 14 million degrees at the Sun’s core, a reaction where hydrogen atoms merge to form helium takes place, releasing a lot of energy and light that radiates from the inner core of the Sun to its surface.
The heated gas becomes plasma that flows in huge eddies from the core to the surface through convection. When the plasma gets heated from the core, it rises to the surface in a continual cycle; it becomes so hot that it becomes electrically charged, creating powerful magnetic fields as it moves up and down the convection currents. Since the Sun spins on its axis the plasma also flows sideways, which in turn wounds the magnetic field lines, which keep getting stronger until they eventually rise to the surface and penetrate it. This process happens at solar maximum.
You may be wondering now what solar maximum is? Well, the Sun follows an eleven-year solar cycle, where the solar activity varies from minimum activity to maximum activity, depending on its magnetic field. This is of great importance to us because, when the Sun is at solar maximum, Earth experiences more solar storms as more magnetic fields push and break through the surface, causing the eddies of hot gas to cool down, creating sunspots.
Once the magnetic field has pushed its way out of the surface, the plasma keeps dragging it out until it reaches its breaking point, and breaks off from the Sun’s surface. This is what is called solar wind; it is several times bigger than the size of the Earth. After a while, the magnetic fields disperse, rearranging themselves back into an orderly arrangement; this is when the Sun is at its solar minimum. The solar wind that is produced at times of solar minimum is fairly light and slow; however, at times of solar maximum, it is very strong. Scientists can tell whether there will be more solar wind or not by keeping track of the presence of sunspots.
So, how does this solar wind act? It is basically a constant stream with varying intensity depending on the solar cycle; it can travel at a speed of 400 km/sec and can take around two to four days to reach Earth. The plasma, which is a key component, is made up of positively charged atoms and electrons that float around one another. Their high energy charge, along with the expansion of the magnetic field, allow them to escape the gravitational field of the Sun and set them on route to us.
Wow! So are we constantly being bombarded by solar wind? Yes, we are! How are we still intact if we stand in the way of such a strong force barreling towards us through space? Well, luckily we have our own magnetic field that shields us from the brunt of the impact. Several billion tons of plasma have come our way; however, all have been deflected by the Earth’s magnetic field.
This magnetic field that shields us is invisible; it is created by the hot molten iron found at the center of our planet. The circulation of this liquid metal creates electric currents that produce the magnetic field, which shoots out of the center of the Earth, through the crust, and into space, enveloping us and creating a protective shield. This is what is known as the magnetosphere; thanks to its existence we are able to deflect most of the solar wind that comes our way.
However, a small amount of solar wind does enter our atmosphere, causing the aurora phenomenon; the visible evidence of the solar winds’ presence. So how does that occur? While most particles bounce off our magnetosphere, some manage to get through where the magnetosphere is weakest. These weak spots are present at the North Pole and South Pole, where a sort of funnel exists, allowing some of the particles of the solar wind to enter into our atmosphere, and that is where the fun begins!
When the electrons in the solar wind make it through the magnetosphere, they encounter the two main components of our atmosphere: oxygen and nitrogen. When these two elements collide with the highly charged electrons from the Sun, their state is excited. In order to calm down and return to their original energy level, they must let out the extra energy that built up, and they do so in the form of shooting out tiny packets of light called photons. This is what we see in the nighttime sky as the dancing curtains of light at the North Pole and South Pole.
Auroras can be seen in a variety of colors, depending on where in the atmosphere the electrons interact with the oxygen and nitrogen. The most commonly seen color is glowing green; however, colors ranging from red to pink, blue to purple, in lighter and darker combinations have also been observed. The colors of the photons depend on the height where the collision between the oxygen and nitrogen and the incoming electrons takes place.
Oxygen at lower altitudes gives off a different color than oxygen at higher altitudes. The yellow-green color that is most common of the aurora occurs at the lower altitudes in the atmosphere, between 100 km and 300 km; while in the upper atmosphere, above 300 km (ionosphere), oxygen collisions create a beautiful red aurora, which is a rare sight.
Nitrogen can also produce red light, but when the collision occurs at around 100 km; it usually forms the rippled edges of the aurora. Hydrogen and helium are also present in the ionosphere; these lighter gases produce beautiful blues and purples. However, they are not always easily discernible to the naked eye and we sometimes need a good camera to capture them.
The auroras take place at all times; however, during daytime we cannot see them because they are outshined by sunlight. Clear skies are also needed for a proper sighting of the aurora since clouds would hide the light display. There are many people who make it a point to travel to watch these beautiful surreal displays that take place in the vast skies; many people travel to northern regions in Sweden and Alaska to try and catch a glimpse of this magical phenomenon.
To think that these displays are a result of all the activity that take place on the Sun, and that only a small amount of the solar wind is able to venture into our atmosphere to leave us with such a beautiful display, is definitely a reminder of how vast and mysteriously beautiful our universe is.
References
www.webexhibits.org
www.sciencekids.co.nz
http://alaska.gov/kids/learn/northernlights.htm
http://solarscience.msfc.nasa.gov/SolarWind.shtml
Image by freepik