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Flares and coronal ejections shooting off from the Sun's surface struck the Earth overnight, causing a severe geomagnetic storm.
It was the strongest such event in about 20 years, leading to stunning aurora australis light displays in skies across southern parts of Australia and the southern hemisphere.
People in the northern hemisphere also saw and photographed stunning images of pink, red, green and violet skies brought on by the aurora borealis early on Saturday morning.
The solar event is predicted to continue over the weekend, bringing more bright aurora light shows with it.
But what are geomagnetic storms exactly, and how do they produce auroras?
What is a geomagnetic storm?
Geomagnetic storms originate from a specific type of activity taking place on the Sun.
Large "clouds" containing billions of tonnes of plasma embedded within an ejected magnetic field erupt from the Sun's outer atmosphere, or corona.
These eruptions are known as coronal mass ejections (CMEs).
Unlike solar flares, which travel at the speed of light and reach Earth in about eight minutes, CMEs travel at a more sedate pace.
Officials put the current average at 800 kilometres per second.
These ejections sometimes travel towards Earth, where they can temporarily disturb the Earth's magnetosphere, resulting in geomagnetic storms.
The current storm was caused by an ejection that emanated from a massive sunspot cluster that is 17 times wider than Earth.
The Bureau of Meteorology's (BOM) Space Weather Forecasting Centre issued a geomagnetic storm warning on Friday.
It categorised the storm as level G4, which is severe.
The G-scale is a measure of global geomagnetic activity, which refers to fluctuations in Earth's magnetic field. The G scale ranges from G1 (minor) to G5 (extreme).
"G4 geomagnetic conditions are expected on 10 May 2024, reducing to G3 with a chance of G4 on May 11," the bureau said.
What is causing the auroras?
Andrew Cole from the University of Tasmania's physics department says the auroras seen across the Earth's skies Saturday morning are a direct result of the geomagnetic storm.
"About a day or two ago, the Sun had a large burst of magnetic activity and ejected a pretty big blob of material into space, and that happened to intersect with the Earth last night," Dr Cole said on Saturday.
"That disturbs the upper atmosphere and the Earth's magnetic field, which in turn causes charged particles to glow and produce the really beautiful southern and northern lights."
He said multiple ejections looked to have occurred, with the first effectively clearing the way for a second one to strike Earth "much more quickly and more powerfully than predicted".
He said only strong geomagnetic storms produced light displays that were visible to the naked eye.
"[If] you get a really strong display, it can look like anything from a dim pinkish or greenish glow, which could just be on the southern horizon, to curtains of shimmering light or pillars extending up nearly vertically," he said.
"Typically the colours will be reddish or green, but in very, very strong auroras like this one, you can get violet as well.
"It's spectacular to see with the eye. It's actually kind of emotionally affecting — much more than a still photo because you can see things moving in real time.
"It's not a special effect. It's just nature doing its thing and on an enormous scale — the whole horizon lit up and changing from second to second, minute to minute."
How rare is this event?
Coronal mass ejections (CMEs) are happening all the time and, like other solar activities, they ramp up as the Sun approaches the peak of its 11-year cycle.
"The Sun goes through periods where it's quite active or it's very quiet, and we happen to be in an active phase at the moment," Dr Cole said.
While storms at level G1 can happen 1,700 times each cycle, G4 storms like the current one don't happen often.
Dr Cole said this geomagnetic storm, which was expected to continue into Monday, was "the strongest event of this kind to hit the Earth in about 20 years".
The last one of this strength occurred in October 2003 during a series of solar storms dubbed the "Halloween storms".
The last one before that took place in 1989, Dr Cole said.
"These events are rare enough that people remember the dates and keep them marked down for study," he said.
The most powerful geomagnetic storm in recorded history, known as the Carrington Event, occurred in September 1859.
It was named after Richard Carrington, the British astronomer who recorded a solar flare for the first time during the phenomenon.
Are geomagnetic storms dangerous?
Geomagnetic storms are not considered dangerous to human bodies.
However, they can cause power outages and impact satellite services, which can have devastating consequences.
The 2003 Halloween storms caused blackouts in Sweden and damaged power infrastructure in South Africa.
The BOM issued warnings that there could be similar disruptions here, and urged airlines and those in charge of the power grid to take precautionary steps.
Loading...The fluctuating magnetic fields associated with geomagnetic storms induce currents in long wires — including power lines — which can potentially lead to blackouts.
Excess currents on telegraph lines during the Carrington Event resulted in telegraph technicians receiving electrical shocks, and even caused some telegraph equipment to catch fire.
Long metal pipes can also become electrified, leading to engineering problems.
Spacecraft are also at risk from high doses of radiation, though the atmosphere prevents this radiation from reaching Earth.
Astronauts are also kept safe by a dedicated NASA team guiding those on the International Space Station to move to places within the outpost that are better shielded.
Animals such as pigeons that have internal biological compasses, however, can be affected.
Pigeon handlers have noted a reduction in birds coming home during geomagnetic storms, according to NASA's Jet Propulsion Laboratory.
Can I still see the southern lights?
Yes, there's a strong chance you can see the aurora australis again in eastern parts of Australia on Saturday night — but it may not be as bright as last night's display.
Geomagnetic events can last for days, but it's not known how long exactly this one will continue.
Dr Cole said people interested in seeing the display should go outside and check as soon as it got dark in the early evening because the Moon would set and provide "darker skies and better visibility".
But the most important thing, he said, was to find a spot that had clear skies and was away from city lights.
Early Saturday morning in Australia, the southern lights were a bit obscured by cloud and the sunrise, Dr Cole said, while people in Europe were able to see much stronger displays later in the day.
Brent Gordon, from America's National Oceanic and Atmospheric Administration, encouraged the public to try to photograph the night sky with their phone cameras even if they could not see the lights with their naked eyes.
"Just go out your back door and take a picture with the newer cell phones, and you'd be amazed at what you see in that picture versus what you see with your eyes," he said.
ABC with wires