The Aurora Borealis occurs when electrically charged particles from the sun collide with particles in the Earth's atmosphere.
These collisions create energy that is released as light, giving rise to the beautiful, vibrant colors we see in the night sky.
The proportion of gases in the atmosphere is distributed by layers due to gravity and temperature.
Therefore, solar radiation will create different colors depending on its interaction with gases at different altitudes.
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Specifically, blue auroras occur when ions of molecular nitrogen (N2+) are hit by ultraviolet rays.
This causes more N2+ molecules to get excited and emit a photon with a wavelength in the deep blue (400-450nm).
This blue glow can happen at altitudes above 500km.
It is usually invisible to the naked eye but can be captured by a camera during strong storms.
Red auroras occur when high-energy electrons collide with Oxygen Atoms at very high altitudes around 200-500 km above the Earth’s surface and Oxygen is less concentrated. When these oxygen atoms return to their ground state, they emit red light. This process is similar to how neon lights work: when the molecules and atoms get “excited” by electrons, they must return to their original energy (ground state) and do so by releasing the energy as photons (light).
Green auroras are the most common and are produced when charged particles from the Sun collide with Oxygen Molecules where there are high concentrations, about 60 to 190 miles (100 to 300 kilometers).
When these Oxygen Molecules return to their ground state, they emit green light. Our human eye sensitivity makes green lights more noticeable.
Blue and purple emissions, usually at the lower edges of the “curtains,” at an altitude of 100 kilometers or less, appear at the highest levels of solar activity. Oxygen atoms are infrequent, and molecular nitrogen and ionized molecular nitrogen take over in the production of visible light emission, radiating at a large number of wavelengths in the red and blue parts of the spectrum, with 428 nm (blue) being the dominant one.
N2+ Blue
O2 Atomic Red
GReen
Blue & Purple
The colour blue is associated with high geomagnetic activity and occurs under very specific conditions. It is a rare phenomenon that requires very precise circumstances to manifest.
During intense geomagnetic storms, the vertical auroral currents and winds are so strong that they lift and mix particles to high altitudes. In this way, N2+, whose concentration decreases rapidly above 250 km, can be transported up to several hundred kilometres.
There, it still interacts with solar particles, but its brightness is usually too weak to be visible. However, under certain conditions, the upper part of the aurora can be illuminated by sunlight.
Sunlight contains a variety of electromagnetic radiation, including ultraviolet (UV) radiation, which can have an impact on atoms and molecules. Because UV radiation is highly energetic, it can excite particles in much the same way as an electron would when it collides with them.
The main elements responsible for auroras are monoatomic oxygen (O), diatomic nitrogen (N2) and ionised diatomic nitrogen (N2+).
Oxygen emits a glow that can be red-orange or lime green, while N2 generates a deep red glow. Its ionised form, N2+, produces an intense blue or purple light.
In general, these colours are fixed and do not change, but variations in the elements can cause the shades to vary.
Sometimes the boundaries of the colours are well defined, but most of the time, the elements mix with currents at different altitudes and create other colours, such as pink, yellow, emerald blue, magenta and purple.
During intense geomagnetic storms, it is possible to see multiple colours at once.
