Aurora Borealis: The Symphony of the Night Sky

Aurora Borealis

The aurora borealis is a glowing phenomenon caused by charged particles entering the Earth from space and colliding with gases in the upper atmosphere. Charged particles (mainly electrons) falling into the atmosphere along the Earth’s magnetic lines of force collide with atoms or molecules in the atmosphere, causing them to become excited, and the excited gases return to their ground state and emit light. They occur in high latitudes in both the southern and northern hemispheres and are sometimes referred to as the aurora borealis or northern lights, respectively.

Auroras occur at altitudes between 100 kilometers and 320 kilometers and are mostly distributed in a latitudinal band about 20° from the magnetic pole. When the sun is active, auroras are distributed with a southerly bias. Auroras are not only common on Earth, but also on Jupiter, Saturn, Uranus, and Neptune.

What causes auroras?

Charged particles (mostly electrons) falling into the atmosphere along the Earth’s magnetic lines of force collide with atoms or molecules in the atmosphere and cause them to become excited. As these excited gases return to their ground state, they emit light. If enough collisions occur, they emit enough light to be detected by our eyes, and that’s what an aurora is. However, this light is very weak compared to the sun’s light, so it is mostly visible to the naked eye at night.

What does an aurora look like?

When viewed from space, auroras emit light along an ellipsoid with thicknesses running east-west around their magnetic axis. This ellipsoid is called the auroral oval. The thickness of the auroral oval depends on how much the Earth’s magnetic field is disturbed by the solar wind, and during periods of extreme solar activity, the north and south boundaries of the oval expand to low latitudes and polar regions, respectively. This phenomenon is called an auroral wind storm.

When viewed from the ground, auroras typically take the form of a curtain with vertical streaks of light. This reflects the direction of the magnetic lines of force in the region and the falling charged particles. In other words, the charged particles are bound to the strong magnetic field lines near the Earth and cannot move to other magnetic field lines in the vicinity, resulting in the vertical streaks of light. Depending on the observer’s position in relation to the auroral ellipsoid, auroras can appear in many different forms. The further away you are from the ellipsoid in a north-south direction, the less visible the aurora will be.

Why do auroras appear in different colors?

When observing the aurora with the naked eye, we tend to see a lot of green auroras. This is because they emit the most light at 557.7 nm, the wavelength to which our eyes are most sensitive. However, by varying the exposure with a camera that is more sensitive than the human eye, you can see other colors as well as light at wavelengths that are invisible to the eye. Auroras of different colors are possible because charged particles falling into the atmosphere collide with different gases in the atmosphere.

At altitudes above about 200 kilometers, red auroras are caused by collisions with oxygen atoms, which are the most common. Below 100 kilometers, they collide with nitrogen molecules and produce purple auroras. Between 100 and 200 kilometers, they emit blue, green, and even ultraviolet light due to the collision of oxygen atoms with nitrogen molecules.

Where can I see auroras?

Auroras occur primarily at altitudes between 100 km and 320 km (altitude can vary depending on solar activity and the energy of the colliding charged particles) and are distributed in a latitudinal band about 20o from the magnetic poles (in the Northern Hemisphere, about 11o off the Earth’s axis of rotation toward Canada). During periods of high auroral activity, auroras are distributed as far south as about 40o. The altitude at which auroras occur is much higher than in the troposphere, where most meteorological events occur. They are usually best seen on clear winter days, although weather conditions can limit their visibility.

Auroras can also occur during the day, but the strong sunlight makes them difficult to see with the naked eye. Alaska, Canada, and Scandinavia are known for their aurora borealis. The most famous of these is Yellowknife, Canada, where the darkest and clearest skies from mid-August to late September and mid-November to mid-April are considered the best for aurora viewing. They can sometimes be confused with low-lying clouds reflecting the moonlight, but the difference is that clouds can obscure stars, while auroras cannot. In rare cases, the aurora borealis can be seen from low latitudes, such as the Korean Peninsula and Japan, about once every 10 years.

Do other planets in the solar system have auroras?

Auroras are not only common on Earth, but also on Jupiter, Saturn, Uranus, and Neptune. They all have their own magnetic fields and atmospheres. Jupiter’s auroras were first observed in the 1970s by spectrometers and cameras aboard Pioneer 10, 11, and Voyager.

Voyager observed auroras on Saturn in 1980, and Voyager 2 observed auroras on Uranus and Neptune in 1986 and 1989, respectively. Meanwhile, auroras were first observed on Mars and Venus by the Mars Express spacecraft in 2004 and the Venus Express spacecraft in 2012. Jupiter’s auroras are the largest in size. Jupiter’s auroras are thought to be caused by charged particles released from volcanic eruptions on Jupiter’s moon Io falling into Jupiter’s atmosphere along Jupiter’s magnetic lines of force and colliding with the gaseous components of the atmosphere.

It was discovered that as Io moves, the bright regions of Jupiter’s auroras move with it. NASA launched the Juno spacecraft on August 5, 2011, with the goal of understanding the origin and evolution of Jupiter, including its aurorae. After five years of cruising, Juno entered Jupiter’s orbit on July 4, 2016, and is conducting in situ observations of Jupiter’s atmosphere, electromagnetic field, and plasma environment with the goal of operating for about 20 months.

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