Earth’s auroras result from solar winds interacting with the magnetosphere, creating light displays in vivid colors based on particle energy, with an 11-year solar cycle influencing their frequency.
For thousands of years, people have been captivated by the stunning light displays that dance across the night sky. Today, we know these lights as the auroras: the aurora borealis in the northern hemisphere and the aurora australis in the southern hemisphere.
We now understand that auroras are created when charged particles from Earth’s magnetosphere and the solar wind collide with particles in the upper atmosphere. These collisions excite the atmospheric particles, which then release light as they return to their normal, unexcited state.
The color of the aurora depends on how much energy the atmospheric particles absorbed during the collision. Each color represents a specific amount of energy being released.
Auroras are influenced by the Sun’s activity, which follows an 11-year cycle. We just reached the solar maximum for Solar Cycle 25, which will likely lead to more frequent and intense auroral displays.
Fox Fires, a short film inspired by the Finnish folk tale of the aurora borealis.
Connections to the Sun
Such displays have long been documented by people throughout North America, Europe, Asia, and Australia.
In the 17th century, scientific explanations for what caused the aurora began to surface. Possible explanations included air from Earth’s atmosphere rising out of Earth’s shadow to become sunlit (Galileo in 1619) and light reflections from high-altitude ice crystals (Rene Descartes and others).
In 1716, English astronomer Edmund Halley was the first to suggest a possible connection with Earth’s magnetic field. In 1731, a French philosopher named Jean-Jacques d’Ortous de Mairan noted a coincidence between the number of sunspots and aurora. He proposed that the aurora was connected with the Sun’s atmosphere.
It was here that the connection between activity on the Sun was linked with auroras here on Earth, giving rise to the areas of science now called “heliophysics” and “space weather.”
Earth’s magnetic field as a particle trap
The most common source of aurora is particles traveling within Earth’s magnetosphere, the region of space occupied by Earth’s natural magnetic field.
Images of Earth’s magnetosphere typically show how the magnetic field “bubble” protects Earth from space radiation and repels most disturbances in the solar wind. However, what is not normally highlighted is the fact that Earth’s magnetic field contains its own population of electrically charged particles (or “plasma”).
Model representation of Earth’s magnetic field interacting with the solar wind.
The magnetosphere is composed of charged particles that have escaped from Earth’s upper atmosphere and charged particles that have entered from the solar wind. Both types of particles are trapped in Earth’s magnetic field.
The motions of electrically charged particles are controlled by electric and magnetic fields. Charged particles gyrate around magnetic field lines, so when viewed at large scales magnetic field lines act as “pipelines” for charged particles in a plasma.
The Earth’s magnetic field is similar to a standard “dipole” magnetic field, with field lines bunching together near the poles. This bunching up of field lines actually alters the particle trajectories, effectively turning them around to go back the way they came, in a process called “magnetic mirroring.”
‘Magnetic mirroring’ makes charged particles bounce back and forth between the poles.
Earth’s magnetosphere in a turbulent solar wind
During quiet and stable conditions, most particles in the magnetosphere stay trapped, happily bouncing between the south and north magnetic poles out in space. However, if a disturbance in the solar wind (such as a coronal mass ejection) gives the magnetosphere a “whack,” it becomes disturbed.
The trapped particles are accelerated and the magnetic field “pipelines” suddenly change. Particles that were happily bouncing between north and south now have their bouncing location moved to lower altitudes, where Earth’s atmosphere becomes more dense.
As a result, the charged particles are now likely to collide with atmospheric particles as they reach the polar regions. This is called “particle precipitation.” Then, when each collision occurs, energy is transferred to the atmospheric particles, exciting them. Once they relax, they emit the light that forms the beautiful aurora we see.
The first of many timelapses from Thusday night – in this video you can see the aurora appears immediately after sunset, a sign that activity was strong! The colors from this G4 "severe" geomagnetic storm were unreal. I swear I saw the entire rainbow that night in the aurora.… pic.twitter.com/rPA6fFGl9s
— Vincent Ledvina (@Vincent_Ledvina) March 27, 2023
Curtains, colors, and cameras
The amazing displays of aurora dancing across the sky are the result of the complex interactions between the solar wind and the magnetosphere.
Aurora appearing, disappearing, brightening, and forming structures like curtains, swirls, picket fences and traveling waves are all visual representations of the invisible, ever-changing dynamics in Earth’s magnetosphere as it interacts with the solar wind.
2023-03-23-24 10pm to 5AM
2mn of magnificent night of my 7 hours of shooting. @AlbertaAurora@TweetAurora @spaceyliz @TamithaSkov @weathernetwork @CalgaryRASC @rasc @StormHour @MurphTWN pic.twitter.com/9C1Zbu09OE— Siv Heang (@hoodoos84) March 26, 2023
As these videos show, aurora comes in all sorts of colors.
The most common are the greens and reds, which are both emitted by oxygen in the upper atmosphere. Green auroras correspond to altitudes close to 100 km, whereas the red auroras are higher up, above 200 km.
Blue colors are emitted by nitrogen – which can also emit some reds. A range of pinks, purples, and even white light are also possible due to a mixture of these emissions.
The aurora is more brilliant in photographs because camera sensors are more sensitive than the human eye. Specifically, our eyes are less sensitive to color at night. However, if the aurora is bright enough it can be quite a sight for the naked eye.
Catching aurora in the southern hemisphere.
Where and when?
Even under quiet space weather conditions, aurora can be very prominent at high latitudes, such as in Alaska, Canada, Scandinavia, and Antarctica. When a space weather disturbance takes place, auroras can migrate to much lower latitudes to become visible across the continental United States, central Europe, and even southern and mainland Australia.
Wild pigs snorting all around me, coyotes in the distance, storms to my east, sprites to my south, and this to my north. One of my favorite nights ever in Oklahoma. March 23rd 2023 . #okwx pic.twitter.com/uIQ4IcGtmw
— Paul M Smith (@PaulMSmithphoto) March 26, 2023
The severity of the space weather event typically controls the range of locations where the aurora is visible. The strongest events are the most rare.
So, if you’re interested in hunting auroras, keep an eye on your local space weather forecasts (US, Australia, UK, South Africa, and Europe). There are also numerous space weather experts on social media and even aurora-hunting citizen science projects (such as Aurorasaurus) that you can contribute towards!
A rare sighting of the aurora australis from central Australia, with Uluru in the foreground.
Get outside and witness one of nature’s true natural beauties – aurora, Earth’s gateway to the heavens.
Written by:
- Brett Carter, Associate Professor, RMIT University
- Elizabeth A. MacDonald, Space Physicist, NASA
Adapted from an article originally published in The Conversation.
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