Picturesque particle collisions – Auroras explained
As they ventured towards the South Pole last month, scientists aboard an Antarctic research vessel were lucky enough to catch a glimpse of this glowing aurora.
Also known as the southern lights (Aurora Australis), or the northern lights when spotted in the northern hemisphere (Aurora Borealis), auroras are bright sheets of colour that appear in the midnight skies around Earth’s two poles.
It was luck for many reasons – the limited number of places you can see auroras, the unpredictable nature of the phenomena, and the astounding beauty of the blues, greens and reds shimmering off calm seas (appropriately, the research vessel in question, is named the Aurora Australis).
But though the researchers may have been lucky, auroras are certainly not random.
Rather, they are the result of complex interactions between physics, chemistry, the Earth and the Sun and follow a solar cycle.
The process begins when the sun releases a powerful solar wind of charged particles (mainly electrons) into Earth’s magnetosphere (the region of space controlled by Earth’s magnetic field).
Although solar winds are constantly streaming particles towards Earth, our planet’s magnetic fields are usually strong enough to deflect them and keep the particles out of our atmosphere.
Occasionally however, a coronal mass ejection (explosions near a sunspot) occurs, streaming charged particles towards Earth in greater numbers and at faster speeds than normal.
The extra energy is so powerful (the explosions on the sun can pack the force of a billion megaton nuclear bombs) it causes Earth’s magnetic field to reconfigure, resulting in the electrons and protons entering our upper atmosphere, directed towards the magnetic south and north poles along magnetic field lines much like beads on a wire.
The sun is no small operator – the charged particles are travelling up to 1584000km/hr (or 440km/second), and as they enter Earth, smash into the oxygen and nitrogen atoms that make up our atmosphere. When they collide at such speeds, the electrons latch on to the oxygen and nitrogen molecules and transfer their energy.
To return to their normal state, the molecules get rid of the extra energy in the form of small bursts of light, known as photons.
When these collisions occur on a large enough scale and enough photons are released, the oxygen and nitrogen emit enough light that the glow is visible to those lucky enough to be nearby.
The colour of the aurora is dependent on which gas the electrons and protons collide with and how much energy is being transferred (the faster the collision, the more photons released and the brighter the colour).
When you see green, yellow or red in the sky, it means oxygen molecules have been hit, whereas nitrogen molecules emit a blue light.
The auroras are often described as looking like sheets, or curtains, because the electrons and protons are guided by Earth’s magnetic field lines keeping the collisions running almost parallel to one another.
Although they may be fascinating and beautiful examples of physics, auroras are actually considered a geo-hazard for their potential to damage satellites, their ability to disrupt GPS and radio signals and the radiation hazard they pose to astronauts.
In 1989, a coronal mass ejection approximately the size of 36 Earths erupted, sending particles flying towards Earth at 1.6 million kilometers per hour. The Aurora Borealis that ensued caused electrical surges so strong, it caused Canada’s Hydro-Quebec power utility grid to crash, denying six million Canadians electricity for nine hours.
Regardless of the hazards, if you want to be one of the lucky ones to witness an Aurora Borealis, there are a few places where it is more likely – Alaska and Scandanavia are both situated on the cusp of the north magnetic pole and typically see auroras several times a year (to witness an Aurora Australis, you would have to travel to Antarctica!)
Spring and Autumn tend to be the most favourable seasons, but they are not common from year to year. Powerful solar winds that cause auroras tend to follow an 11 year cycle with peaks and troughs. The more solar activity, the more auroras are seen on Earth.
In the past, Auroras Borealis have been spotted as far south as Japan or Florida, but this is extremely rare.