Auroras and where to spot them
Auroras are nature’s most breathtaking light shows, illuminating the night sky in an array of colours. The ethereal lights have captivated humanity since ancient times, catching the eyes of historical figures like Aristotle and Galileo.
The phenomenon is called aurora borealis, or the northern lights, when viewed near the north pole and aurora australis, the southern lights, when viewed near the south pole. Auroras appear at both poles at the same time and can take on a variety of forms, ranging from a glow on the horizon which might resemble dawn light (aurora means ‘dawn’) to more intense and unusual displays of light. They can be incredibly bright: the strongest auroras can be comparable to the full moon in terms of luminosity, the measure of radiated light.
Different forms of auroras
Auroras typically begin with a glow along the horizon and develop into one or a combination of different forms, including arcs, rays and diffuse forms. These can look like an isolated cloud or a veil that covers a large part of the sky. Citizen scientists also recently identified a new form called dunes that resemble wave-like ripples. Auroras that undergo very slow changes in position or shape are called ‘quiet auroras’, while ‘active auroras’ move or change shape rapidly (sometimes even waving like a curtain) and are often very bright.
Where can auroras be seen?
The brightest auroras can be observed close to Earth’s poles within the ‘auroral ovals’, which are the rings of regions between 10° and 20° latitude of Earth’s geomagnetic poles. These ovals expand towards the equator during geomagnetic activity. For aurora borealis, regions include far northern Scandinavia, northern Russia and North America.
Aurora australis is visible from Antarctica, Australia, New Zealand, Argentina and Chile. For aurora enthusiasts in Australia, your best bet for viewing spots are Tasmania and the southern coast of Victoria.
What causes auroras?
Auroras are the result of events that begin on the sun, 150 million kilometres away. The sun’s upper atmosphere, the solar corona, releases a stream of charged particles called the solar wind. This stream is made up of a plasma of gas ions such as electrons and protons, and these charged particles interact with Earth’s magnetic field. Auroras are frequently linked to geomagnetic storms, which are caused by intensifications of the solar wind that lead to a temporary disturbance of Earth’s magnetic field. Keen aurora hunters have observed that the start of a geomagnetic storm during the day almost always leads to an aurora the following night.
Geomagnetic storms are associated with phenomena such as coronal holes, areas where the Sun’s corona is colder, and dramatic events known as solar coronal mass ejections (CMEs). These powerful eruptions eject huge quantities of plasma and electromagnetic radiation. When a CME is directed towards Earth, it can intensify the solar wind to incredible speeds. The fastest CME transit time was estimated to be over ten million kilometres per hour—about seven times higher than typical solar wind speeds (around 1.4 million kilometres per hour) when relatively inactive. CMEs are often associated with solar flares, sudden flashes of increased brightness from the Sun.
The space weather phenomena associated with auroras can be observed by ground-based magnetometers that measure the solar wind. Magnetometers can also give a 20 to 30 minute advance warning before a geomagnetic storm.
The most famous space weather event in the last two hundred years was arguably the Carrington event, the great geomagnetic storm that occurred from 28 August to 3 September 1859. Observers witnessed solar flare effects followed by a massive auroral display from a CME. The blood-red aurora was said to be so bright that you could “read a newspaper” by it.
Why the different colours?
When geomagnetic storms reach Earth, the plasma flow temporarily deforms Earth’s magnetic field (the magnetosphere), resulting in a build-up of energy on the nightside. The release of this energy accelerates charged particles so they spiral toward the poles.
The breathtaking colours of an aurora are due to the behaviour of these charged particles. Different colours are emitted depending on whether the particles collide with atmospheric oxygen or nitrogen, and where these collisions occur.
When particles strike an atmospheric atom or molecule, they slow down and transfer some of their energy. The molecules can only store this energy for a very short time, and then most of it is radiated away in the form of light. The wavelength of this light is characteristic for the atom or molecule, and we perceive this as different colours.
If particles collide with oxygen molecules, energy tends to be released in the form of green light. This is the most commonly seen auroral colour because most solar particle collisions occur at altitudes of around 100 to 200 km, where oxygen molecules are highly concentrated.
At altitudes lower than 100 km, charged particles tend to interact with nitrogen molecules, radiating blue and red colours. These can mix to form the purple edge of a green auroral form.
At higher altitudes, particle collisions with excited or high-energy oxygen atoms result in a deep red aurora, which tend to be rarer.
As emissions from oxygen and nitrogen mix, auroras can take on any colour of the rainbow. They’ve even been documented to produce sounds such as crackling, which some researchers think occurs due to electrical charge being released from regions of the atmosphere called inversion layers.
When are auroras likely?
Catching auroras might involve playing the long game. Auroras are linked to high solar activity, which peaks every 11 years at the solar maximum, when aurora-sparking phenomena like solar flares and CMEs are more frequent. Solar cycles are challenging to predict, but the next solar maximum may take place around July 2025. However, auroras can still appear under the right conditions.
Auroras are most visible in a clear sky without clouds between the hours of 10 pm and 2 am. Winter is the ideal viewing time because night falls earlier and the sky is darker for longer. However, the geomagnetic storms that can trigger auroras tend to occur in March and September, due to the alignment of Earth’s magnetic field with the interplanetary magnetic field. The Bureau of Meteorology issues Aurora Alerts when space weather activity is favourable for viewing auroras—a site to bookmark if you’re keen for some aurora spotting.