An aurora, more commonly known as the northern lights, is a natural light display in the Earth’s sky that predominantly occurs in high-latitude regions. Auroras are the result of disturbances in the magnetosphere caused by solar wind. These disturbances are directed by the Earth’s magnetic field into the atmosphere, causing the ionization and excitation of atmospheric constituents and emitting light of varying color and complexity. The color of an aurora is determined by which atmospheric gases are colliding with charged particles. Green auroras are caused by the emission of light from oxygen molecules located about 60 miles above the earth.
What Causes Auroras?
Auroras are caused by the interaction of charged particles released from the sun called the solar wind, and gaseous particles in Earth’s atmosphere. As the solar wind approaches Earth, the planet’s magnetic field directs the electrically charged particles toward the magnetic poles. The magnetosphere acts as a protective shield for Earth, deflecting most of the solar wind particles. However, some do get past the defenses of the magnetosphere near the poles and interact with atoms of gas in the upper atmosphere, causing the atoms to light up. Oxygen emits a greenish-white light in the lower atmosphere where auroras are most often seen.
The sun goes through an 11-year cycle of solar activity with many small variations in between. During the solar maximum, the sun has an increased number of sunspots and ejects more matter and radiation. This can lead to more frequent, stronger auroras. The opposite occurs during the solar minimum, where auroras are less frequent and weaker. Currently, the sun is moving towards another solar maximum, which will peak around 2025.
Why Are Auroras Green?
The green color of auroras is produced by oxygen atoms located about 60 miles above the Earth’s surface. When charged particles from the solar wind slam into the atmosphere, they excite oxygen atoms causing them to light up. The color of the aurora depends on which atmospheric gas is colliding with the charged particles. Here’s a breakdown of the colors:
- Green – oxygen atoms below 60 miles
- Red – oxygen atoms above 200 miles
- Blue – nitrogen molecules
- Violet – nitrogen ions
Green is the most common color since oxygen is the most plentiful gas in the lower atmosphere where auroras are typically visible. The oxygen atoms emit a greenish light with a wavelength of 557.7 nm (nanometers) when they return to their normal state after being energized. Other colors like pink, yellow, and purple can also be observed when the green auroral light mixes with red oxygen emitted from higher altitudes.
Altitude Differences
The altitude or height from the ground is another factor that determines aurora colors. Lower altitude auroras occurring between about 60-150 miles above Earth’s surface are dominated by green. As the altitude increases, red becomes more prominent as higher energy particles interact with less dense oxygen atoms. Here is a table showing the different colors and their altitudes:
| Altitude (miles) | Color |
|---|---|
| 60-150 | Green |
| Above 150 | Red |
| Over 200 | Blue |
So in summary, green auroras occur at lower altitudes in the dense oxygen rich atmosphere, while red, violet and blue auroras happen at higher altitudes. The green oxygen emitted below 150 miles dominates the visual appearance of the aurora from the ground.
Chemical Reactions
Let’s take a deeper look at the chemical reactions that cause the green emission from oxygen atoms:
- Charged particles (electrons and protons) from the solar wind enter the atmosphere and collide with oxygen molecules (O2).
- These collisions cause the oxygen molecules to split into two separate oxygen atoms (O).
- The separated oxygen atoms are now in an excited state with higher energy.
- When the excited oxygen atoms return to their normal, lower energy state they emit photons of green-colored light with a wavelength of 557.7 nm.
- The result is a beautiful, green glowing aurora in the night sky.
This green emission spectrum line at 557.7 nm is produced when an electron transitions within the oxygen atom from a higher energy state to a lower energy ground state. This transition specifically involves the transition of an electron between the atomic orbitals known as the 3s orbital and the 3p orbital. The photon emitted from this drop in energy corresponds directly to the green color we observe in the aurora.
Where Can You See the Northern Lights?
Auroras occur in rings centered around the north and south magnetic poles of the Earth. So the best places to observe the northern lights are in high-latitude regions including:
- Northern U.S. (Michigan, Maine, Minnesota, Washington)
- Canada
- Alaska
- Northern Europe (Iceland, Norway, Sweden, Finland)
- Greenland
- Russia
In the northern hemisphere, the auroral oval is centered roughly around the latitudes between 65 to 72 degrees north. During periods of higher solar activity and geomagnetic storms, the auroral oval expands further south by 5-10 degrees expanding the viewing area.
Ideal viewing conditions require clear, dark skies away from light pollution in cities and urban areas. The best times to see an aurora are during the hours around local midnight. And of course, periods around the autumnal and vernal equinoxes provide longer hours of darkness compared to the summer season.
Green Aurora Fun Facts
- Oxygen produces more green light than any other color in the aurora.
- The green light comes from oxygen atoms at altitudes of 60-150 miles above Earth’s surface.
- Green auroras occur during lower geomagnetic activity while reds occur with higher activity.
- Oxygen only makes up 21% of the atmosphere yet dominates the visual display of the aurora.
- Auroras can also be seen from space as a ring of light around the magnetic poles from orbit.
Conclusion
In summary, the green light of an aurora is emitted from excited oxygen atoms located 60-150 miles above the Earth. Charged particles from the solar wind collide with oxygen atoms causing them to enter an excited state. When the oxygen returns to its ground state, it releases photons in the green part of the visible light spectrum at 557.7 nm. This green emission dominates the visual appearance of auroras due to the high concentration of oxygen in the lower atmosphere. So next time you are lucky enough to witness the wonder of the northern lights, know that it is the oxygen in the air making the sky glow green!
