Could we be incinerated by a giant flare from our Sun?
It often happens that, during conferences or even simple chats between friends,
the classic question emerges "but how sure are we that from one day to another
our Sun does not reserve us some bad joke?" To answer usually invokes the fact that currently,
the Sun is going through the most "quiet" phase of its way, but the doubt is hard to fade.
What concrete elements do we have that lead us to exclude that quiet main sequence stars like
the Sun cannot manifest - let's say every few tens of thousands of years - sudden flashes of
prolonged instability, in the form of explosive flares or abrupt changes in the amount of energy
emitted? The thing, as you can imagine, could in both cases not be pleasant for our planet...
And what does the current knowledge of stellar evolution tell us about
the variation of the solar constant or the intensity of the solar wind? What
are the limits suggested by the current evolutionary theories and, above all,
how much these limits actually put us safe from dramatic consequences here on Earth?
These days there has been a lot of concern about a fairly intense solar flare that appeared on the
Sun. The concern comes from the fact that flares are phenomena that do not concern
only the world of astronomy. Due to the great proximity of the Sun to the Earth, in fact,
these eruptive manifestations that appear on the "surface" of our star can have unpleasant effects
also in our daily life. What are they? Well...
In astronomy, a flare is a violent eruption of matter (plasma at a temperature of several
million degrees with protons and free electrons) that explodes on the photosphere of a star with
an energy equivalent to several tens of millions of atomic bombs. They are often associated with
sunspots and are probably caused by the release of energy when magnetic field lines reconnect.
Flares are classified as A, B, C, M, and X according to their peak intensity in the X domain
measured in watts per square meter by the GOES satellite in the vicinity of Earth. Each class is
ten times more powerful than the previous class, with X (the most energetic class) reaching 0.0001
watts per square meter (consider that the "solar constant", or the total amount of radiation the
Earth receives from the Sun per unit area is 1353 watts per square meter), and is then further
divided into 9 subclasses. Sometimes flares exceed the maximum value (X9), such as that
of November 4, 2003, which after saturating GOES detectors (full scale at X28) was estimated at X45
and is considered the most powerful ever recorded so far reaching 0.0045 watts per square meter.
Flares emit very energetic solar wind beams in all regions of the spectrum (X, Gamma, UV) and
can therefore be dangerous for human activity in space (or in the future for astronauts working on
the surface of planets - like Mars - not protected by a magnetic shield). Given this information, the
question is (since we have been monitoring these phenomena for only a few decades) whether or not
the Sun is able to produce flares so powerful to overcome the barriers of the Earth's magnetosphere
and thus endanger the ecosystem of our planet. A warning signal came from the sky,
but from another star: On May 1, 2019, our star next door exploded.
Within seconds, Proxima Centauri, the closest star to our sun, has become thousands of times
brighter than usual - up to 14,000 times brighter in ultraviolet. The intensity of the radiation
was so strong that it could separate all the water molecules possibly present on Proxima b,
the planet recently discovered around the red dwarf, stripping it of all the atmosphere.
It would indeed be a disaster for us if our Sun could do the same...
but so far it has limited itself to giving us some jolts - the most brutal of which,
among those we could witness - manifested in the early hours of September 2, 1859.
At that moment, an incredible northern light began to illuminate the whole planet.
It was night, but we saw each other as day. " The entire northern hemisphere was bright as
if the Sun had set an hour earlier," the London Times reported a few days later.
Meanwhile, the telegraph networks went haywire. Sparks flew from the equipment, some of which
caught fire. The operators in Boston and Portland yanked the telegraph cables from the batteries,
but to their surprise they realized that they could continue to transmit, powered by the
electricity that poured into the Earth! The skies, in fact, had been set on fire
by an enormous amount of electrically charged gas, fired by the Sun following
a flash of light known as solar flare. A tangle of plasma and magnetic fields,
a phenomenon that we now know as "Coronal mass ejection", was thrown away by the Sun and after
an 18-hour journey invested the Earth triggering 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 shown that that was just a glimpse of what the Sun can combine.
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.
But in recent years, research has shown that that was just a taste of what the Sun can combine.
The tree rings and ice of the polar regions, as we would have known later,
retain memories of dramatically stronger solar storms in the distant past.
And the flare on Proxima Centauri shows that even the most energetic solar explosions documented
pale compared to what happens on other stars... However, the Carrington event offers
important clues as to what the Sun might have in store for Earth in the future,
so much so that the solar physicist Hugh Hudson - in the 2021 Annual Review of Astronomy and
Astrophysics - stated that: "The danger is lurking for humanity’s technological resources,
especially those in space. If a solar superflare ever strikes Earth,
the first thing to hit us will be an intense flash of X-ray and ultraviolet radiation.
This would disrupt the ionosphere, scrambling the satellite navigation signals needed in
critical services and infrastructure. The burst of radiation would heat the outer layers of Earth’s
atmosphere making them expand, increasing drag on satellites so much that some might be lost.
A wave of high-energy protons, accelerated by shockwaves in the Sun’s atmosphere,
would hit Earth a few minutes later. These could cripple satellites,
compromising global communications. High-energy particles can permanently damage processors. If
the flare also spawns a supersize Coronal mass ejection that happens to be aimed at Earth,
it could cause a violent geomagnetic storm, inducing electric currents that could be strong
enough to paralyze even modern power grids. A 9-hour blackout in Quebec in March 1989 was caused
by a much more modest Coronal mass ejection..." Despite the right concern, it would seem however
that such effects would be far from being able to be defined as "a global catastrophe".
They would be really nothing, compared to what was observed on Proxima Centauri...
However, it must be said that the giant flare observed on Proxima occurred on a red dwarf,
and then on a star very different from our Sun. This could lead us to hope that a quiet
middle-aged star like ours may have left behind the years of restlessness...
But is that really the case? We actually know that superflares
can also occur on yellow stars with about the same size and temperature as our Sun,
but especially on those that rotate rapidly, and therefore, have stronger magnetic fields and a
much more violent magnetic activity. Sun-like stars, with their quiet rotation period of 25
days, are not expected to inflame in this way. But the star KIC9944137 did!
The star, known as KIC9944137, lies more than 1800 light-years away between the constellations Cygnus
and Lyra. It closely resembles our Sun: about the same size, about the same temperature, about the
same rotation rate. Yet the flare was at least 10 times as powerful as anything recorded on the Sun!
Alarmed by this discovery, in 2012 astronomers analyzed data from the Kepler telescope
on solar-type stars and found 365 superflares.
As expected, researchers have found that the fastest spinning stars are more likely to generate
larger flares. But some superflares have also appeared on stars that spin slower than our sun.
The result implies that solar-type stars with periods of 25 days can produce a superflare
every 500-600 years. Which in itself is quite troubling... But not only... When the team added
other stars to the analysis, the prediction showed that every few thousand years such
stars could produce even larger flares, 100 times more powerful than the Carrington flare!
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The researchers then sought the sign of superflares passed onto Earth. Flares and Coronal
mass ejections both generate high-energy protons, which create radioactive isotopes carbon-14
and beryllium-10 in the Earth’s atmosphere. The first can be detected in tree rings; the second
in the ice caps of Greenland and Antarctica. Ice cores and ancient tree rings do show a few
spikes in these isotopes, marking extreme solar particle storms in the past. The strongest,
as a result, appears to have taken place in 774 EC, but it was a medium-power superflare.
Overall statistics show that such an extreme event occurs on Earth at most once in several millennia,
much more rarely than the few centuries implied by Kepler, and that there is no
evidence of much more powerful superflares. And this discrepancy between the terrestrial
data and the data observed by Kepler on other Sun-like stars is still unexplained.
It may be that the Kepler superflare stars are not really sunlike. For example, they could be in
binary systems, where the gravity of a companion would boost magnetic activity in the main star.
To find out, astronomers looked closely at 64 superflare stars
using the 3.5-meter telescope at the Apache Point Observatory in New Mexico, and they found that 21
superflare stars were indeed part of a binary system, but that still left 43 single stars.
Some of the apparently sunlike stars could also be subgiants, with similar spectra but
at a later stage of evolution than the Sun and much larger in size. The team used data from the
European Space Agency’s Gaia space observatory to work out the distances to stars and their radii.
They found that only about 40% of the 64 stars are subgiants. So plenty of single, sunlike stars
generating superflares that are apparently much bigger than anything coming from our own Sun.
Or it may be that the Sun’s superflares don’t leave much of a radioactive trace on Earth.
One theory is that a really powerful blast of energetic protons from a superflare
could create magnetic waves strong enough to then scatter the protons, preventing most of them from
reaching us, reducing the radioactive fallout. It’s also possible that researchers have missed
some subtle difference between the Sun and those sunlike superflare stars.
The answer will come by looking at a lot more stars.
NASA’s new Transiting Exoplanet Survey Satellite, already in orbit, should soon turn up many more
superflare stars, enabling astronomers to look for more patterns in the data.
Eventually, a better understanding of the phenomenon could help warn us of an
impending solar superflare. The growth of a very large area of sunspots might be one giveaway,
in which case we would get about a week’s warning of possible danger to our planet.
In conclusion, as for other types of catastrophic events caused by nature, we cannot absolutely
exclude extreme events originated by the Sun with serious consequences on living beings. However,
the statistical data at our disposal reveal to us that the probability of this happening is
extremely small and this should reassure us. Catastrophic events of other nature such
as strong earthquakes, tsunamis, volcanic explosions, or hurricanes are much more likely.
Also, a meteorite disaster would be more likely than a catastrophe originating from the Sun.
To make predictions on a much larger time scale (on the order of millions of years)
then the theories of stellar evolution are very helpful. In this area, a gradual increase in
radiation from the Sun is predicted, and thus a warming trend on Earth. Then there will be a much
more rapid increase in the power radiated by the Sun when it will begin to turn into a red giant.
But before then the Earth will likely have already become inhospitable to life.