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The article below is copied from the excellent Knowable Magazine. A detailed paper
about the Carrington Event is included in the resources.
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19/05/2025
Are we ready? Understanding just how big solar flares can get
By: Christopher Crockett
Knowable Magazine: 17/09/2021
Christopher Crockett is a staff researcher for Knowable and a freelance
science writer living in Arlington, Virginia. He is thankful for the sun
but wouldn’t want to see it when it’s angry.
On May 1, 2019, the star next door erupted.
In a matter of seconds, Proxima Centauri, the nearest star to our sun, got
thousands of times brighter than usual — up to 14,000 times brighter in the
ultraviolet range of the spectrum. The radiation burst was strong enough to
split any water molecules that might exist on the temperate, Earth-sized
planet orbiting that star; repeated blasts of that magnitude might have
stripped the planet of any atmosphere.
It would be bad news if the Earth’s sun ever got so angry.
But the sun does have its moments — most famously, in the predawn hours of
September 2, 1859. At that time, a brilliant aurora lit up the planet,
appearing as far south as Havana. Folks in Missouri could read by its light,
while miners sleeping outdoors in the Rocky Mountains woke up and, thinking
it was dawn, started making breakfast. “The whole of the northern hemisphere
was as light as though the sun had set an hour before,” the Times of London
reported a few days later.
Meanwhile, telegraph networks went haywire. Sparks flew from equipment —
some of which caught on fire — and operators in Boston and Portland, Maine,
yanked telegraph cables from batteries but kept transmitting, powered by the
electrical energy surging through the Earth.
The events of that Friday evoked biblical descriptions. “The hands of
angels shifted the glorious scenery of the heavens,” reported the Cincinnati
Daily Commercial. The actual impetus was a bit more prosaic: The skies had
been set ablaze by an enormous blob of electrically charged gas, shot out
from the sun following a flash of light known as a solar flare.
Space weather encapsulates the prevailing conditions in the solar system
caused by the solar wind and the sun’s far-reaching magnetic field. Sudden
changes on the sun, such as flares and eruptions of material, are like weather
fronts, bringing with them magnetic “storms” that can be felt on the planets.
On Earth, this can cause stunning auroras, but it can also create havoc with
electronics. The flash of light from a flare takes about 8 minutes to reach
Earth; solar material expelled from the sun in a coronal mass ejection (CME)
may take hours to days to travel the distance. Magnetic storms may be brief or
last for many days.
Such a blob — a tangle of plasma and magnetic fields — is known as a
coronal mass ejection. Upon arrival at Earth, such an ejection can trigger
the most ferocious of geomagnetic storms. The 1859 storm, named the
Carrington Event for the scientist who witnessed the flare that preceded it,
has long been upheld as the most powerful wallop that the sun has ever
delivered.
But in recent years, research has indicated that the Carrington Event was
just a taste of what the sun can throw at us. Tree rings and ice cores
encode echoes of dramatically stronger solar storms in the distant past. And
other stars, such as Proxima Centauri, show that even the most energetic
documented solar outbursts pale in comparison with what is possible.
Nevertheless, the Carrington Event offers important clues to what the sun
might have in store for Earth in the future, solar physicist Hugh Hudson
writes in the 2021 Annual Review of Astronomy and Astrophysics. “Danger
lurks for humanity’s technological assets, especially those in space,”
writes Hudson, of the University of Glasgow. In the wake of a
Carrington-like event today, entire power grids could shut down and GPS
satellites could be knocked offline.
Understanding just how severe solar storms can be provides insights into
what the universe may sling our way — and maybe how to foretell the next one
so that we’re better prepared when it happens.
Anatomy of a flare
Roughly 18 hours before the 1859 event brightened Earth’s skies, an English
astronomer noticed something strange on the surface of the sun.
While working in his observatory, Richard Carrington saw two brilliant
points of light emerge from among a clutch of dark sunspots and vanish
within five minutes. Another English astronomer, Richard Hodgson, saw the
same thing, noting that it was as if the brilliant star Vega had appeared on
the sun. At the same time, compass-like needles at England’s Kew Observatory
twitched, a hint of the magnetic storm about to ensue.
Before then, no one knew about solar flares — mostly because no one was
tracking sunspots every clear day the way Carrington was. Decades would pass
before astronomers and physicists could unravel the physics of solar flares
and their impact on Earth.
In 1859, English astronomer Richard Carrington was making this sketch of
sunspots (left), when he saw two beads of light emerge from the large
cluster of spots near the top. Carrington drew the first appearance of the
flare as two bean-shaped regions nestled in among the spots (labeled A and
B in close-up at right). Five minutes later, the two white spots had
drifted to the right and faded considerably (marked C and D).
CREDIT: S. PROSSER, OXFORD UNIVERSITY PRESS 2018 (LEFT) / RICHARD
CARRINGTON, PUBLIC DOMAIN (RIGHT)
A solar flare is an eruption on the sun, a sudden flash of light — usually
near a sunspot — that can release as much energy as roughly 10 billion
1-megaton nuclear bombs. The trigger is a sudden, localized release of
pent-up magnetic energy that blasts out radiation across the entire
electromagnetic spectrum, from radio waves to gamma rays.
Many solar flares, though not all, are accompanied by a coronal mass
ejection, a massive chunk of the sun’s hot gas blown into space along with a
tangle of magnetic fields. Billions of tons of sun stuff can billow out into
the solar system, crossing the 150 million kilometers to Earth’s orbit in
anywhere from about 14 hours to a few days.
Most solar eruptions miss our planet by a wide margin. But occasionally,
one gets aimed right at Earth. And that’s when things can get
interesting.
About eight minutes after a solar flare, its light reaches Earth in a flash
of visible light. That’s also when a spike in ultraviolet light and X-rays
sprays the upper atmosphere, causing a slight magnetic disturbance at the
surface. That was the twitch the magnetic instruments at the Kew sensed in
1859.
The coronal mass ejection can trigger a geomagnetic storm when it
encounters the magnetic field that envelops Earth. The disturbance to the
magnetic field induces electrical currents to course through conductors,
including wires and even the planet itself. At the same time, high-speed
charged particles spewed by the sun crash into atoms in the upper
atmosphere, lighting up the aurora.
On September 6, 2017, the sun emitted a powerful X-class solar flare — a
designation reserved for the most intense flares. Seen here in ultraviolet
light captured by NASA’s orbiting Solar Dynamics Observatory, the flare
was one of the strongest seen in years and came amid a spate of solar
eruptions that month. The glowing threads are scorching filaments of
plasma ensnared by magnetic fields arcing over the sun’s surface.
CREDIT: NASA / GSFC / SDO
The 1859 flare has long been, and remains, a standout in its energy and
effects on Earth. Comparably powerful solar eruptions are often referred to
as “Carrington events.” But it does not stand alone.
“It’s oftentimes described as the most intense storm ever recorded,” says
Jeffrey Love, a geophysicist at the US Geological Survey in Denver. “That’s
possibly not exactly true, but it certainly is one of the two most intense
storms.” Or three or four.
In May 1921, the sun dealt our planet a geomagnetic storm on par with the
Carrington Event. As in 1859, a brilliant aurora appeared well beyond the
polar regions. Telegraph and telephone systems broke down, with some
sparking destructive fires.
And just 13 years after Carrington spied his eponymous flare, another solar
storm came along that by some measures may have topped it. “It looks now,
based on aurora and sparse magnetometer measurements, that an event in 1872
was probably larger than the Carrington Event,” says Ed Cliver, a solar
physicist retired from the US Air Force.
These storms show that the Carrington Event wasn’t a “black swan,” Hudson
says. If anything, the sun has been holding back in the modern era. Evidence
from the more distant past points to a few solar storms that make the
Carrington Event seem almost puny by comparison.
Forgotten flares
Trees have long memories. Each year of growth chronicles tidbits about
environmental conditions at the time in concentric annual rings. From those
rings researchers can reconstruct scenes from Earth’s past.
Some cedar trees in Japan recall a tsunami of atomic particles hurled from
the sun around the year 775. Those trees recorded a significant uptick in
carbon-14, a radioactive variant of carbon that trees absorb from the
atmosphere. Carbon-14 emerges from run-ins between atmospheric nitrogen and
cosmic rays — high-speed particles from space that pummel our planet daily.
Some solar flares shower Earth with an excess of cosmic rays, which ramps up
production of carbon-14. The change in carbon-14 levels recorded in 775 was
about 20 times larger than the normal ebb and flow from the sun, researchers
reported in 2012.
“The clear suggestion there was that super events could happen, because
this was a factor of 10 — if it was a solar flare — a factor of 10 or 20 or
more greater than the Carrington Event,” Hudson says.
In the early hours of March 1, 2011, a ripple in the solar wind whacked
Earth’s magnetic field and triggered a minor geomagnetic storm, causing the
ethereal aurora seen here over the Poker Flat Research Range in
Alaska.
CREDIT: NASA / GSFC / JAMES SPANN
A carbon-14 boost in tree rings showed signs of another sizable solar event
in 994. Ice cores from Antarctica showed a corresponding increase, in both
994 and 775, of beryllium-10, another product of cosmic rays — adding more
certainty to the tree ring findings.
Looking farther back in time, a study of ice cores suggests a third similar
event around 660 BCE. And in August (in a paper still undergoing peer
review), researchers reported two more carbon-14 spikes in tree rings from
around 7176 BCE and 5259 BCE, possibly on par with the 775 event.
It’s hard to directly compare these past storms with the Carrington Event,
says Ilya Usoskin, a space physicist at the University of Oulu in Finland
and a coauthor of the August study. The 1859 flare did not produce a
particle downpour on Earth, so there are no carbon-14 counts to compare. But
the 775 event appears to be one of the strongest solar particle storms
recorded in the last 12,000 years, Usoskin says.
There is a catch, Hudson notes. Tree rings are laid down annually, so a few
smaller flares within the span of several months might appear as one big
event in the tree ring record.
But even then, any one of these smaller flares may still have been
impressive. “Every one of those events would be at least on the order of
three times as big as the Carrington Event in terms of its energy,” Cliver
says.
That, however, is still modest compared with some other stars in our
galaxy.
Super flares
If life does exist on the planet orbiting Proxima Centauri, it probably has
a rough go of it.
“You really are looking at having something like a Carrington Event
happening daily,” says Meredith MacGregor, an astrophysicist at the
University of Colorado Boulder. Even stronger “super flares,” like the one
she and colleagues spotted in 2019, may go off roughly every other day. Her
team spotted that flare, possibly 100 times as powerful as the Carrington
Event, after watching the star next door for just 40 hours.
With a near-constant barrage of flares, any atmosphere clinging to the
rocky planet snuggled up close to the star would never have time to recover.
“Yes, a Carrington Event [on Earth] would fry some electronics and would
ruin GPS signals,” MacGregor says, “but it’s not going to destroy the
habitability of our planet.”
The star Proxima Centauri and its neighboring duo of Alpha Centauri A and B
are the closest stars to the sun, lying a mere 4.2 light-years away.
Proxima, the nearest of the trio, is a dim red orb with frequent, powerful
flares that buffet the Earth-mass planet that orbits close to
it.
CREDIT: DIGITIZED SKY SURVEY 2. ACKNOWLEDGEMENT: DAVIDE DE MARTIN / MAHDI
ZAMANI
To be clear, Proxima Centauri is not like the sun. It’s an M dwarf, a
diminutive orb that glows red. And these tiny stars are famous for their
oversized flares. But some sunlike stars can send up super flares as
well.
This realization has come from telescopes in space designed to look for
planets around other stars. NASA’s now-defunct Kepler telescope did this by
looking for subtle dips in starlight as planets crossed in front of their
suns.
Over four years, Kepler recorded 26 super flares — up to about 100 times as
energetic as the Carrington Event — on 15 sunlike stars, researchers
reported in January. NASA’s ongoing TESS mission, another space-based
telescope hunting for exoplanets, found a similar frequency of superflares
on sunlike stars in its first year of operation.
The Kepler data imply that sunlike stars experience the most powerful of
these flares roughly once every 6,000 years. Our sun’s most powerful
eruption in that time span is an order of magnitude weaker — but could a
super flare be in our future?
“I don’t think any theory has sufficient predictive capability to mean
anything,” Hudson says. “The leading theory basically says that the bigger
the sunspot, the greater the flare.” Sunspots mark where the sun’s magnetic
field punches through its surface, preventing hot gas from bubbling up from
below. The spot looks dark because it’s cooler than everything around
it.
And that is one difference between the sun and its eruptive neighbors.
Super flares seem to happen on stars with cool, dark spots far larger than
ever appear on the sun. “Based on known spot areas, there would therefore be
a limit,” Hudson says.
The intricacies of any star’s magnetic machinations — spots, flares, etc. —
are still poorly understood, so tying all these observations into one
cohesive story will take time. But the quest to understand all this might
improve predictions about what to expect from the sun in the future.
Flares that are powerful enough to disrupt our power grid probably occur,
on average, a few times a century, Love says. “Looking at 1859 kind of helps
put it in perspective, because what’s happened in the space-age era, since
1957, has been more modest.” The sun hasn’t aimed a Carrington-like flare at
us in quite a while. A repeat of 1859 in the 21st century could be
disastrous.
Humanity is far more technologically dependent than it was in 1859. A
Carrington-like event today could wreak havoc on power grids, satellites and
wireless communication. In 1972, a solar flare knocked out long-distance
telephone lines in Illinois, for example. In 1989, a flare blacked out most
of Quebec province, cutting power to roughly 6 million people for up to nine
hours. In 2005, a solar storm disrupted GPS satellites for 10 minutes.
The best prevention is prediction. Knowing that a coronal mass ejection is
on its way could give operators time to safely reconfigure or shut down
equipment to prevent it from being destroyed.
Building in extra resiliency could help as well. For the power grid, that
could include adding in redundancy or devices that can drain off excess
charge. Federal agencies could have a stock of mobile power transformers
standing by, ready to deploy to areas where existing transformers — which
have been known to melt in previous solar storms — have been knocked out. In
space, satellites could be put into a safe mode while they wait out the
storm.
The Carrington Event was not a one-off. It was just a sample of what the
sun can do. If research into past solar flares has taught us anything, it’s
that humanity shouldn’t be wondering if a similar solar storm could happen
again. All we can wonder is when.
