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SYFY WIRE Bad Astronomy

When the Sun Went Medieval on Our Planet

By Phil Plait

In the years 774 and 993, the Earth was attacked from space.

Not by aliens, but by a natural event—and it was very, very powerful.

Whatever it was, it subtly altered the chemistry of our planet’s atmosphere, creating trace amounts of radioactive elements like chlorine-36, beryllium-10, and carbon-14. And those provide the clue to what the event was: Those isotopes are created when high-energy protons slam into our air. That means the source must have been from space.

These must have been huge waves of subatomic particles that slammed into us on those dates. Spikes in the abundances of those elements were found all over the world, including ice cores from the Arctic and Antarctic, Chinese corals, and more. Generating that many particles isn’t easy, and only extremely violent events can do it.

Several possible sources have been considered. One candidate is that the Earth got caught in the beam from a gamma-ray burst, the mind-crushingly powerful demise of a very high mass star. I wrote about this being the possible cause of the 774 event in an earlier article. However, GRB impacts don’t usually create 10Be due to the detailed physics of the blast, so that makes a GRB as the source shaky. Plus, they’re very rare events, so having two happen in as many centuries is extremely unlikely (I didn’t know about the 993 event when I wrote that article, or else I would’ve been a lot more likely to wonder about other sources).

New research studying the amounts of these radioactive materials in ice cores points to a different culprit, one I wouldn’t have thought possible: the Sun.

The Sun generates ridiculously strong magnetic fields in its interior, and these can store vast amounts of energy. They can release this energy explosively on the surface, creating intense solar flares. Sometimes the loops of magnetism do this far above the Sun’s surface, creating what are called coronal mass ejections. These are less intense (that is, less concentrated bursts of energy) than flares, but far larger and more powerful; think of flares versus CMEs like solar tornadoes versus hurricanes.

You can find out more about these events in the Crash Course Astronomy episode I did on the Sun:

I also have a chapter in my book Death From the Skies! about solar storms and their effects on Earth.

When I wrote about the 774 event on this blog before, I mentioned that a flare or coronal mass ejection was unlikely to be the source due to the amount of energy needed to create these radioactive elements. However, that new research indicates that the Sun is the most likely culprit for this interplanetary assault, and that, in turn, means the Sun can produce more powerful events than we previously thought.


We’ve known for a long time that the Sun is capable of producing huge magnetic explosions. In 2003 it let rip a series of solar storms so powerful that one of them set the record for the biggest flare seen in modern times. And the strongest known was also the very first solar explosion ever seen—called the Carrington Event, after an astronomer who studied it—and happened in 1859. It created aurora as far south as Mexico and Hawaii! Events like that can also create what are called geomagnetically induced currents, or GICs: The Earth’s magnetic field shakes so violently that it induces currents in conductors on the ground. Telegraph operators reported being able to send messages even though the power was disconnected; enough electricity was flowing through the lines to work the devices. 

There's more. In 2012 the Sun blew out another blockbuster that was in many ways the equal of the one in 1859, but happily for us it was sent off in another direction, and missed the Earth. Had it hit us, the huge flux of charged particles would have overloaded satellites. Worse, the GIC would’ve caused widespread power failures and blackouts. A much smaller solar storm in 1989 did just that in Quebec.

It’s not clear whether the 774 and 993 events were that powerful or more; it’s hard to scale these things without direct measurements. But the astronomers who did the research estimate the 774 event (the more powerful of the pair) was five times stronger than any solar storm seen in the modern satellite era (starting in 1956) up to 2005.

I’ll admit, that’s scary. Our modern civilization depends on our electronic devices, and those in turn depend on electricity and satellites. A blast hitting the Earth from a storm as big as any of those four historical events would be bad. Very bad. The 1989 power surge blew out huge transformers in North America, and these can take months to replace. Imagine months without electricity, and you start to get an idea of how disastrous this can be.

We don’t know how often the Sun throws a tantrum as large as these, but clearly it’s done so at least four times in the past millennium or so—probably more, since three of them hit the Earth, and we only knew of the fourth due to our space-based astronomical assets. Statistically speaking, most will miss us, so they’re likely more common than we thought.

This is a threat we need to take very seriously. Unfortunately, it’s extremely expensive to mitigate. Our power grid in the U.S. was constructed decades ago when our use of electricity was much lower. It was designed with lots of spare room for more power flowing through it, but over the years our appetite has grown, and the grid is currently very nearly at capacity. Big spikes now can cripple huge areas.

We need to upgrade the grid, add more capacity, more capability to handle surges induced by solar storms. The good news is there are studies being done to see what we can do to prevent widespread blackouts, and NASA is on it as well. We also have eyes on the Sun, including NASA's Solar Dynamics Observatory, and scientists monitor "space weather" constantly.  

By coincidence, just last night I read that the White House is looking into this situation pretty seriously, and I'm very glad to hear it. A monster solar storm may be the biggest and most immediate threat there is from space, but it’s one we can handle if we’re prepared for it.

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