Why do we see the Aurorae?
Diving into the what, where, and why of this stunningly beautiful phenomenon
One of the most beautiful phenomena that one can witness on Earth is the aurorae — dynamic, dancing streaks of light in the sky above the polar regions. They are known as Aurora Borealis in the Northern Polar region (the Arctic) and Aurora Australis in the Southern Polar region (the Antarctic) and can often be seen in hues of green, with blue and red being rare guests!
See how colorful the picture above is? Yes, that is an aurora, and every year, thousands of people travel to polar regions just to watch an aurora once in their lifetime. But for people staying within the Arctic Circle and Antarctic Circle, they just need to step outside their houses during winter to watch the light show.
The light show is not continuous though; it comes in bursts from time to time, which depends on a number of factors. For this, we first need to understand how an aurora forms.
How does an Aurora form?
An aurora is formed due to the magnetic field of the Earth and the magnetic field of the Sun. Essentially, just like the Earth has an active core due to which we see a magnetic field and movement of tectonic plates, the Sun also has an ongoing internal activity that can lead to solar flares (explosions of the Sun) or coronal mass ejections (ejected gas bubbles). Both of these — solar flares and coronal mass ejections — consist of highly energetic charged particles which then travel outwards in the Solar system.
Now the Earth’s magnetic field in general shields us from these solar particles and energy, but sometimes they can penetrate through the Earth’s magnetic field (called magnetosphere). Once this happens, these charged particles enter and collide with atoms and molecules in the Earth’s atmosphere, like oxygen and nitrogen.
This leads to 2 things—
- The molecules use the energy of solar particles to dissociate into atoms.
- The atoms then get ionized (i.e. acquire a charge) due to the charges associated with the high-energy solar particles.
This process — called ‘excitation of atoms’ — releases photons in the visible wavelength and is responsible for the different colors seen in an aurora!
The green color seen in an aurora is characteristic of Oxygen (which is the most often seen color), and hues of pink and purple are due to Nitrogen molecules.
Wait, but haven’t we read somewhere that the amount of Nitrogen in the atmosphere is much more than the amount of Oxygen? In fact, the atmosphere is 78% Nitrogen and 21% Oxygen!
So why do we not see more pink and blue in the sky instead of green?
The answer lies in the excitation energy of the molecules. For this, let’s understand what is meant by excitation energy first. Simply put, the excitation energy is the amount of minimum energy needed by a system(here the molecules of Oxygen and Nitrogen) to move from its stable state(state of lowest energy) to its closest excited state(the next state of higher energy from the ground state).
In our example, to even break the bond between the atoms in the molecules as the first step, the energy needed is 498 kJ/mol for Oxygen atoms vs 945 kJ/mol for Nitrogen atoms. This shows that Nitrogen is more stable than Oxygen and needs a higher amount of energy to dissociate and then get ionized. This is also the reason why everything that we burn on Earth does not just explode — having a lot of Nitrogen in the atmosphere ensures that there is a balance when anything is burning due to the presence of Oxygen in the atmosphere.
When is a good time to watch an Aurora?
Auroras happen throughout the year in the Polar regions (which correspond to countries like Norway, Alaska, Canada, New Zealand, Tasmania, Sweden, Finland, Greenland, etc). But their intensity and frequency ultimately depend on our Sun.
As we discussed above, the Sun’s internal activity plays a major role in creating auroras, so we need to follow the solar cycle to find out a good time to see an aurora.
While the Sun’s activity can vary day by day (which makes the timing of auroras difficult to predict on a daily basis), over a longer duration, the Sun itself follows a periodic 11-year cycle called the “Solar cycle” or “Schwabe cycle”, starting from minimum solar activity to reaching a maximum peak and falling back to minimum activity in 1 cycle. Everything related to the Sun, like the number of sunspots on its surface, the solar electromagnetic radiation’s intensity, frequency of solar flares — reflects this cycle’s pattern.
For the new cycle that started around the end of 2020, called Solar Cycle 25, we just entered the season of heightened activity or peak of the Solar cycle — so the next 3–4 years is definitely a good time to watch an aurora! So much so, that it is expected to show up in places like France, and was even sighted as far as Ladakh (India) recently!
I hope this inspires you to plan a trip to watch the aurora, it sure does for me! But why do we see it only closer to the poles? And can an aurora be seen on other planets? Stay tuned to find out in the next Space Nuggets article!
And till then, let’s marvel at the fact that we live on a planet where we can lie down on the soft white snow and take in the magnanimous beauty of the sky enveloping us with all the different strips of dancing colors and the twinkling stars!
