Australia Weather News

On the weekend, parts of Australia witnessed an auroral display not seen for more than 20 years. 

If you had good weather, skies of pinks and greens could be seen as far north as Uluru and Mackay.

While the colours fade, it's worth dissecting what happened and what led up to this incredible display.

And as we reach the "solar maximum", or peak of solar activity over the next few months, can we expect more of the same?

How a sunspot can cause the aurora 

Behind every stunning sky show is a sunspot — although not all sunspots lead to auroras.  

Sunspots — cooler, dark patches on the Sun that have unusually high magnetic fields — are a regular solar occurrence.

In one month, there can be anywhere from a dozen to a few hundred sunspots. 

But on May 9, a sunspot caught the attention of solar researchers.

And according to Brett Carter, a researcher in space physics and space weather at the Royal Melbourne Institute of Technology (RMIT), this one was huge. 

"[The sunspot] rose up out of nothing over the course of last week," Dr Carter said.

"It was strong, it was very complex, and it was very loud."

The sunspot, which is called AR3664, was 15 times the diameter of Earth, and rivalled the dimensions of an 1859 sunspot which led to the most extreme solar storm ever documented.

Known as the Carrington event, that storm not only created stunning auroras, it played havoc with fledgling telegraph systems, sparking fires around the world.

Auroras are visible evidence of a sudden burst of charged particles from the Sun hitting Earth's atmosphere.

A sunspot in itself doesn't guarantee widespread aurora. For that to happen, the sunspot needs to release a solar flare and a coronal mass ejection or CME.  

"Coronal mass ejections are associated with flares," saidMichael Wheatland, a solar astrophysicist at the University of Sydney.

"A flare is just an intense, sudden release of energy and intense brightening [...] and a CME is when the magnetic field itself becomes unstable and leaves the Sun.

"You get this expulsion of both the magnetic field and the plasma from the Sun."

In last weekend's case, the sunspot AR3664 was facing Earth, and released multiple CMEs over a few days.

These CMEs, or intense solar winds,travel at different speeds, and when they "catch up" to one another they bunch up — like dirt in front of a bulldozer — and become stronger. 

The CMEs created an extreme G5 storm on May 11, an intensity not seen since 2003.

The final ingredient for producing an aurora has to do withhow the Earth's magnetic field interacts with this solar wind when it hits us. 

"It's determined by what the orientation of the magnetic field is in the solar wind," DrCarter said. 

"If the orientation of the magnetic field in the solar wind is weaker — we say northward — Earth's magnetic field acts as an effective shield. But with the storm that we just had, it was not only southward, but it was hard southward."

This disturbance to Earth's magnetic field and upper atmosphere then produces the dazzling displays.

What's the solar maximum?

While last weekend's pink and green aurora was an obvious sign something is going on with the Sun at the moment, sunspots have actually been increasing steadily over the past few years.

We're now getting close to the "solar maximum", which is when we hit a peak of sunspots on the Sun's surface.

The National Oceanic and Atmospheric Administration's Space Weather Prediction Center has predicted the solar maximum may peak between January and October this year.

After the peak, the number of sunspots will start to wane.

This is all part of the solar cycle, which lasts around 11 years. Currently, we're in solar cycle 25, so named because sunspots have been tracked all the way back to 1750. 

The reason for this increase in sunspots is because the Sun is going through some important growth of its own — around solar maximum its magnetic poles reverse.

This is not an issue for Earth, apart from the extra sunspots produced. But scientists can't actually tell when we've hit solar maximum until after it happens. 

"The maximum can be about a year in duration. You can't exactly say a point in time," Professor Wheatland said. 

"At times of solar maximum, you get really big spot regions on the Sun like [AR3664]."

Despite the impressive auroral displays on the weekend, the solar maximum for this cycle isn't that big historically. 

While April this year saw 136 sunspots detected, solar cycle 19 in the late 1950s got 359 at its peak.

The solar peaks in the 1980s and '90s regularly saw months with more than 200 sunspots.  

 

Can we expect more auroras? 

Although auroras are a familiar sight in Tasmania and Australia's southern coast, we may see more travelling further north in the next few months.

But there's a caveat — many things need to line up perfectly for impressive auroral displays to occur.

Put simply, auroras are extremely fickle.

One way we might get another quite quickly is from AR3664, the same sunspot that caused last weekend's display. 

Back in 2003 when the last large solar storm occurred, there was not one storm, but two. 

On October 28 and 29, 2003, what's now known as the Halloween solar storm produced auroral displays that could be seen as far north as the Australian mainland in the Southern Hemisphere, and as far south as Texas in the Northern Hemisphere.

But the sunspot that caused it wasn't done there.

The Sun rotated and the sunspot was out of range, butin a matter of weeks Earth came back into it's sights.

"Less than a month later, that same sunspot region came back around and spat out the really big one," Dr Carter said.

The storm caused airlines to be re-routed and a major power outage in Sweden.

We don't know whether or not  AR3664 will produce something similar. 

"Deep down, as space scientists we're hoping for it because it will give us some more interesting events," Dr Carter added.

"[AR3664] is rotating away from the Earth right now. While we can still see some impacts of it, it'll be out of view soon, and we won't see anything until it comes back around in a little less than a month's time." 

There's also potential for more auroras as we hit the solar maximum and peak sunspot activity. 

However, it's not always the number of sunspots that predict how big a solar storm is going to be.

For example, a 1921 solar storm which was widely reported, and the 2003 Halloween storms both occurred late in the solar cycle, towards the solar minimum.

"Generally speaking more sunspots doesn't necessarily mean more intense storms. It might mean more frequent storms, or more frequent disturbances, but the really big ones can happen at any stage in the solar cycle."

Why are they so hard to predict?

Last Friday, even the most ardent aurora watchers only had a few hours notice before the show started.

Although this might only be a slight inconvenience for those wanting to capture the perfect shotit can be a much bigger problem for technology companies. 

Solar storms can affect satellite systems like GPS and satellite internet, as well as power grids and other technology. 

Satellite internet company Starlink warned of a "degraded service" over the weekend. GPS was also disrupted, which according to some US news organisations, caused problems with precision farming equipment.

But Dr Carter said he was "pleasantly surprised" there hadn't been any major power grid disruptions.

"This could mean many things, but [...] we've been doing decades of research to try and make these pieces of infrastructure more resilient and capable of handling such disturbances.

"My view is that perhaps in this last storm we saw some of that work pay off."

For scientists like University of Newcastle stellar physicist Hannah Schunker, the short time frame between knowing the strength of a CME and when it hits us is an ongoing issue. 

"We have these models of the solar wind, and we can see what's happening on the Sun," Dr Schunker said. 

"If we model what's happened, we can estimate how it will act when it makes it to Earth. But we're not good at getting it very accurate."

To fix this, she thinks we need better observations of what's occurring closer to the Sun. 

"We can put satellites near the Earth quite easily and we get a lot of measurements from there. That's where you get the one-hour notice from," she said. 

There's a few spacecraft closer to the Sun, but these don't provide ongoing measurements of things like the magnetic field or charged particles which would give scientists the heads up on what's coming Earth's way. 

"We can see that something's happened on the Sun. But we don't know anything about it until it gets to our satellites that are near the Earth," Dr Schunker said. 

"We're kind of guessing until then."

ABC