Astronomy Physics 

The Science Behind Auroras

Facebooktwittergoogle_plusredditpinterestlinkedinmail

By Steven Spence (@TheStevenSpence)

The northern lights (aurora borealis) and southern lights (aurora australis) are fascinating scientifically. In fact, aurora is not unique to the Earth. We have observed aurora in the upper atmosphere of Jupiter and Saturn with various spacecraft and ground-based telescopes.

Solar Wind

The sun constantly emits streams of particles from its atmosphere out into the solar system. This emission is referred to as the solar wind. Sometimes there are solar storms or solar flares, resulting in heavier emissions than normal. If the Earth passes through one of these emissions, then the resulting auroras will be brighter than usual. If the sun is calm, then auroras may be dim or appear so far north or south that very few people see them.  

Earth’s protective shield: The magnetic field and electric currents in and around Earth generate complex forces.The field can be thought of as a huge bubble, protecting us from cosmic radiation and charged particles in solar winds that bombard Earth. Copyright ESA/ATG medialab.

Magnetic Field and Rubber Bands

The Earth has a magnetic field surrounding it because of the iron-nickel core at the center of our planet. The magnetic field exiting from the core is responsible for the magnetic north and south poles we use when we navigate with a compass. It also creates a magnetic force field around the Earth, which extends into space.

As charged particles (electrons are negative, protons are positive) in the solar wind encounter the Earth’s magnetic field, they travel along the field lines. On the sunward side, the field is compressed by the solar wind to be closer to the Earth; however, on the night side of the planet, the field stretches away from the planet like a tail. Eventually, the magnetic loops stretch so much that they break like an overstretched rubber band. A piece heads off into space away from Earth, while the other part snaps back toward Earth. The piece snapping back toward Earth accelerates the particles it captured into Earth’s upper atmosphere.

Typical Aurora spectrum, courtesy of NASA

When these particles hit molecules in Earth’s atmosphere, they trigger light displays depending on the altitude and energy of the collision. Most of the molecules in Earth’s atmosphere are either nitrogen or oxygen, so they are hit most frequently. Colors produced may be pink, red, yellow, green, blue, or violet. Occasionally, orange or white are produced. Typically, nitrogen will produce red, violet, or blue. Oxygen usually produces green or yellow. Reds generally are emitted above 240 km, greens at 100–240 km, purple and violet above 100 km, and blues at 80–100km. [1]

Atmospheric Layers and Aurora Heights. GotScience.org

 

Birkeland currents carry up to 1 TW of electric power to the upper atmosphere—about 30x the energy consumed in New York during a heatwave. They are also responsible for ‘aurora arcs’, the familiar, slow-moving green curtains of light that can extend from horizon to horizon.  Copyright University of Calgary/ESA

Massive Electric Currents

The movement of charged particles in Earth’s magnetic field produces powerful electric currents. In 1859, an aurora and the associated electrical storm were so powerful that people read newspapers at night by its light. Telegraph operations were disrupted as the current produced by the charged particles overwhelmed the normal currents used in the lines to transmit the signals. One pair of telegraph operators in Boston and Portland [2] turned off their power and used the current created by the electrical storm to keep their transmissions going.

 

The higher the Kp Index, the more likely an aurora display and the further south it will appear. Public domain (NOAA)

 

Kp Bands mapped to North America. Typical Northern Light displays will be Kp 2-5. Public domain (NOAA/SWPC)

 

Kp Bands mapped to Eurasia. Typical Northern Light displays will be Kp 2-5. Public domain (NOAA/SWPC)

Global Planetary Index: Aurora Forecasting

Apps providing forecasts of probable auroras rely on scientific measurement and calculation of changes in the strength of Earth’s magnetic field. Single measurements are tracked as a K-index. An aggregation of the measurement stations is used to produce a Planetary K-index (Kp). [3] The higher the Kp value, the more likely an aurora will appear and the further south (or north in the southern hemisphere) it may display.

For Further Reading

If this brief introduction to the northern lights has piqued your interest, you may want to learn more on the internet. A great starting point is NASA Aurora News, with information on auroras, spacecraft, and investigations. You can also read our previous articles on the aurora borealis, and my last article in this series. 

—Steven Spence contributes general science articles and original photography to GotScience. His extensive body of work in nature photography is both beautiful and informative. Steven enjoys photography, nature, science, and is an avid fan of Calvin and Hobbes.

References

[1]  “Aurora borealis: What causes the northern lights & where to see them.” (2017, October 11). Space.com. Available at https://www.space.com/15139-northern-lights-auroras-earth-facts-sdcmp.html.

[2] Lasar, M. (2012, May 2). 1859’s “great auroral storm”—The week the sun touched the earth. Ars Technica. Available at https://arstechnica.com/science/2012/05/1859s-great-auroral-stormthe-week-the-sun-touched-the-earth/

[3] Space Weather Prediction Center. (n.d.) Planetary K-index. National Oceanic and Atmospheric Administration. Aavailable at https://www.swpc.noaa.gov/products/planetary-k-index.

GotScience Magazine is published by the nonprofit Science Connected and is made possible by donations from readers like you. For as little as $1, you can support GotScience Magazine—and it only takes a minute. Thank you. Donate now.

Facebooktwittergoogle_plusredditpinterestlinkedinmail

Read This

error: