Taking a Look at Auroras: How They Happen
All of us have heard of the magnificent auroras; but do we know the science behind them?
Aurora is the Latin word for dawn, and even the goddess of dawn in Roman mythology. However, it is also a term in astronomy for the colourful shimmery lights we occasionally see in the sky.
Auroras are produced when the Earth’s magnetosphere swings back and forth or oscillates while interacting with the Sun’s solar wind. The magnetosphere is essentially an area surrounding any planet controlled by its magnetic field. Our Earth has a teardrop or comet-shaped magnetosphere which is the strongest one in our solar system!
The Sun is full of electrically charged particles, called ions, and the solar wind is the outward ejection of a collection of these charged particles in an ionized gas state called plasma from the planet’s upper atmosphere named the corona. Plasma is a gas that becomes so heated that the atoms that constitute it lose electrons to other atoms which become ionized or the electrons become detached from any atom, forming a mixture of free-moving electrons, ionized atoms, and nuclei without electrons. It is also the fourth state of matter and unlike the more familiar states like solid, liquid and gas, it makes up the most common state in the universe!
When the magnetosphere blocks most of the solar wind, the plasma is forced around the planet and farther into the solar system. However, some of it gets trapped in the two belts called the Van Allen Belts, in the magnetopause, the boundary between the magnetosphere and the surrounding plasma from the solar wind.
Then, a process called magnetic reconnection occurs with the magnetic field lines of the solar wind and the magnetosphere going in opposite directions. The main region where this occurs is near the rear end of the magnetosphere, called the magnetotail. Although reconnection takes place on the dayside of Earth as seen in the image below, its effect on plasma does not significantly contribute to the creation of auroras.
The magnetic lines break apart and rejoin in a different configuration at the magnetotail, allowing plasma from the solar wind to enter the magnetosphere. The reconnection process converts stored magnetic energy into heat and kinetic energy that accelerates this plasma of the solar wind that has entered, including the trapped plasma in the belts, toward the Earth’s atmosphere.
When magnetic reconnection occurs, the charged particles from the solar wind’s plasma rain down into our atmosphere toward the poles. As this happens, the solar wind’s charged particles collide with atoms in the Earth’s upper atmosphere. This interaction excites the atoms, which emit light as they return to their original state. When electrons within these atoms transition from higher to lower energy levels, they release photons which are the light we see as auroras.
Billions of these individual collisions continue at lower and lower altitudes until all the energy is used up, lighting up the magnetic field lines. Auroras usually glow within bands called the Auroral Ovals which are twin halos encircling the two poles of Earth. Additionally, auroras generally occur around 97–1000km above Earth’s surface.
You may also be wondering what causes the different colours in auroras. There are a few factors affecting this such as the altitude or the type of elements involved in the collisions with the ions.
For example,
- Oxygen emits a greenish-yellow colour at a lower altitude or red at higher altitudes
- Nitrogen usually produces blue auroras
- Hydrogen and helium emit blue or purple light
However, our eyes are unable to see auroras of hydrogen and helium as the wavelengths of this light fall under ultraviolet light in the electromagnetic spectrum which humans are unable to perceive.
Now, let’s take a look at the two types of auroras:
- Aurora borealis (also known as the Northern Lights)
- Aurora australis (Southern Lights)
The only difference between these colourful displays of light is where they occur. Aurora borealis happens in the northern hemisphere while aurora australis happens in the southern hemisphere.
Hemispheres are the two halves of Earth. We have four hemispheres: northern, southern, western, and eastern. The equator (0° latitude) divides the planet into the northern and southern hemispheres while the prime meridian (0° longitude) divides it into western and eastern. The climates of the northern and southern hemispheres are different since they receive different amounts of the Sun’s light due to the tilt of the Earth.
Another interesting thing is that the auroras are more commonly seen around the equinoxes in March and September. An equinox is where the Sun’s rays shine at 90° to Earth’s surface and pass through the Earth’s equator. Only during an equinox does the Earth’s axis not tilt toward or away from the Sun. This results in both the northern and southern hemispheres experiencing approximately equal durations of daytime and nighttime.
Scientists are still trying to understand the relationship between equinoxes and the occurrences of auroras. However, a common theory is that solar winds may be stronger due to the angle of the Earth to the Sun during these periods.
Another difference in auroras is their shape: they can be in the form of rays, an arc, or even be diffused across the sky. Although this has not been thoroughly explored by scientists, the shape depends on where the ions collide in the magnetosphere and what resulted in them leaking into our atmosphere. Even auroras happening on the same night can look incredibly different!
I hope you learnt something about the explanation behind the dancing lights of the aurora. Anyway, if you enjoyed reading this article, I would greatly appreciate your support! Enjoy reading :)
