For millennia, people have observed and been inspired by beautiful displays of ribbons of light dancing across dark night skies. Today we call these lights the Northern Lights: the Northern Lights in the Northern Hemisphere and the Northern Lights in the South.
Nowadays, we understand the aurora borealis is caused by charged particles from the Earth's magnetosphere and the solar wind colliding with other particles in the Earth's upper atmosphere. These collisions excite the atmospheric particles, which then emit light as they “relax” back to their unexcited state.
The color of the light corresponds to the release of discrete lumps of energy from atmospheric particles, and is also an indicator of how much energy was absorbed in the first collision.
The frequency and intensity of aurora displays is related to activity on the sun, which follows an 11-year cycle. Currently we are approaching the next maximum ie expected in 2025.
Connections to the sun
In the 17th century, scientific explanations for what caused the northern lights began to emerge. Possible explanations included air from Earth's atmosphere rising out of Earth's shadow to become sunlit (Galileo in 1619) and light reflections from ice crystals at high altitude (Rene Descartes and others).
In 1716, the English astronomer Edmund Halley was the first to suggest a possible connection with the Earth's magnetic field. In 1731, a French philosopher named Jean-Jacques d'Ortous de Mairan noted a coincidence between the number sunspots and northern lights. He suggested that the aurora borealis was connected to the Sun's atmosphere.
Earth's magnetic field as a particle trap
Images of Earth's magnetosphere typically show how the magnetic field “bubble” shields Earth from cosmic radiation and repels most disturbances in the solar wind. But what is not normally highlighted is the fact that the Earth's magnetic field contains its own population of electrically charged particles (or “plasma”).
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 the Earth's magnetic field.
The movements of the electrically charged particles are controlled by electric and magnetic fields. Charged particles oscillate around magnetic field lines, so when viewed on a large scale, magnetic field lines act as “pipelines” for charged particles in a plasma.
Earth's magnetic field resembles a regular “dipole” magnetic field, with field lines clustering near the poles. This gathering of field lines actually changes the particle trajectories, effectively reversing them to go back the way they came, in a process called “magnetic mirroring.”
Earth's magnetosphere in a turbulent solar wind
In quiet and stable conditions, most particles in the magnetosphere remain trapped, happily bouncing between the south and north magnetic poles out in space. But if a disturbance in the solar wind (such as a coronal mass ejection) gives the magnetosphere a “shock”, it is disturbed.
The trapped particles are accelerated and the “pipelines” of the magnetic field suddenly change. Particles that happily bounced between north and south now have their bounce point moved to lower altitudes, where the Earth's atmosphere becomes denser.
As a result, the charged particles are now likely to collide with atmospheric particles when they reach the polar regions. This is called “particle precipitation”. Then, as each collision occurs, energy is transferred to the atmospheric particles and excites them. Once they relax, they emit the light that forms the beautiful northern lights we see.
Curtains, colors and cameras
The amazing displays of the Northern Lights that dance across the sky are the result of the complex interactions between the solar wind and the magnetosphere.
Auroras appearing, disappearing, brightening, and forming structures such as curtains, vortices, fences, and traveling waves are all visual representations of the invisible, ever-changing dynamics of Earth's magnetosphere as it interacts with the solar wind.
As these videos show, the Northern Lights come in all sorts colors.
The most common are the green and red, both of which are emitted by oxygen in the upper atmosphere. Green auroras correspond to altitudes close to 100 km, while the red auroras are higher up, above 200 km.
Blue colors are emitted by nitrogen – which can also emit some red colors. A range of pink, purple and even white light is also possible due to a mixture of these emissions.
The northern lights are more brilliant in photographs because camera sensors are more sensitive than the human eye. Specifically, our eyes are less sensitive to color at night. But if the northern lights are bright enough, they can be a sight for the naked eye.
Where and when?
Even under quiet weather conditions in space, the aurora borealis can be very prominent at high latitudes, as in Alaska, Canada, Scandinavia and Antarctica. When a space weather disturbance takes place, the aurora borealis can migrate to much lower latitudes to become visible across the continent United States, Central Europe and also Southern and mainland Australia.
The severity of the space weather event usually controls the range of locations where the aurora is visible. The strongest events are the rarest.
So if you're interested in chasing the aurora borealis, keep an eye on your local space weather forecast (USA, Australia, UK, South Africa and Europe). There are also many space weather experts on social media and even aurora hunting citizen science projects (such as Aurorasaurus) that you can contribute to!
Get out and witness one of nature's true natural beauties – the Northern Lights, Earth's gateway to heaven.
Brett Carter receives funding from the Australian Research Council, SmartSat CRC and the Australian Department of Defence. He has also consulted for Chimu Adventures as part of their Southern Lights Flight tours.
Elizabeth A. MacDonald is funded by NASA and employed by NASA's Goddard Space Flight Center. The Aurorasaurus project receives funding from NASA and NSF.
Originally published in The conversation.
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