Near midnight on September 2, 1859, the sky burned blood red. Firefighters in New Orleans stood at the ready, thinking a great blaze had broken out. The waters of the San Francisco Bay shimmered and danced, lit with crimson, while thousands of miles away in New York City, crowds gathered in the street to watch the show. As the rosy glow brightened to blot out the stars, campers in the Rocky Mountains awakened and, supposing it was dawn, groggily argued over whether to make coffee. The sun would not rise for another six hours.

At this moment, across the Atlantic, it truly was morning — an unforgettable morning in the life of amateur astronomer Richard C. Carrington. Just a day earlier, he had witnessed a spectacular but puzzling event: a brightening on the surface of the sun. Carrington was a diligent mapper of sunspots, dark freckles on the surface of our star created by the sun's magnetic field.

That September 1 began as usual for Carrington: He safely projected the sun's image onto a piece of opaque glass, and set about recording the locations of a large group of sunspots — when suddenly, brilliant light shone from two spots on the solar surface. Stunned, Carrington thought momentarily that his observational setup had broken, but quickly realized that the dazzling light was in fact coming from the sun.

In his report to the Royal Astronomical Society, he writes that "being somewhat flurried by the surprise, I hastily ran to call some one to witness the exhibition with me, and on returning within 60 seconds was mortified to find that it was already much changed and enfeebled." Carrington and another astronomer named Richard Hodgson, who observed it separately, were the only two human beings to see the largest solar flare in recorded history, a flare that today bears Carrington's name.

Telegraph machines freakishly worked while unplugged

The Carrington Event refers to a cascade of effects that lasted for nearly a week following the initial flash of light he observed. Over several nights, aurorae lit up the sky across the globe, creating shimmering sheets of colorful light so bright it was possible to read by their glow. There was beauty, but this wasn’t an entirely benign event. Telegraph operators throughout the US and Europe found their machines suddenly possessed by a rush of unexplained current. The surge overwhelmed the circuits, shocking operators with arcs of electricity, even setting the telegraph paper ablaze. In some cases, the electrical poltergeist in the wires seemed to make the telegraphs work better than usual. Even weirder, operators found that the current persisted even if the machines were completely disconnected from power sources. First-person testimonials from this time recount a conversation between two especially unflappable telegraph operators, operating eerily functional yet unplugged machines, who chose to shrug and continue tapping out messages.

Unbeknownst to the telegraph operators, campers, city stargazers, and even Carrington himself, all of the mysterious events of that week had their origins in a common cause: a magnetic disturbance in the outer atmosphere of the sun. Although people widely suspected that the week's phenomena were related, no one knew exactly how the pieces fit together. Today, while the precise details of stellar magnetism continue to challenge astrophysicists, we at least have a big picture understanding of "space weather" – the term applied to the interaction of these magnetic disturbances with the Earth.

We're more vulnerable to space weather today

No less than those telegraph operators, we remain vulnerable to a solar storm. Indeed, we are more vulnerable, given the omnipresence of the electrical grid and its higher-tech extension: the internet. Today, long-haul conductors ferry electricity across state and country borders, while a swarm of satellites circles thousands of miles overhead. Electricity is now the lifeblood of our society, not only keeping the lights on, but keeping the oxygen of readily available information flowing to us via our phones, laptops, televisions, and other devices. These devices, in turn, are portals to the collective connectivity of the web. All of this is threatened by space weather.

An X1.6 class solar flare flashes on Sept. 10, 2014. This image was captured by NASA's Solar Dynamics Observatory and shows a blend of light from the 171 and 304 Angstrom wavelengths. (NASA/GSFC/SDO)

According to a 2013 report by Lloyd's of London, a massive solar storm could leave 20 to 40 million people in the Northeast US out of power, possibly for years. While other reports — for example, one from the National Research Council in 2008, and a study commissioned from JASON by the Department of Homeland Security in 2011 — differ somewhat in their estimates of how long it might take us to recover, all agree the threat is real. And there have been close calls. In 2012, an electromagnetic blast missed the Earth by a mere week, striking a NASA observational spacecraft. Information collected by that satellite revealed that the blast was, indeed, a "Carrington-level" event.

Although he didn’t put all the pieces together at the time, a significant part of what we now know about space weather comes from Carrington's studies.

As a keen observer of the sun, Carrington had helped uncover the details of our sun's rotation: By mapping the motions of sunspots, Carrington observed that the sun not only spins on its axis like a top, but that different latitudes of the sun rotate at different rates.

The sun is made of plasma, a conglomeration of charged particles somewhat similar to a gas — so its various latitudes are free to move at different speeds (unlike, say a globe or basketball). In our star in particular, the equator goes around every 25 days, but the poles lag, taking a leisurely 30 days to complete their rotation.

Meanwhile, deep within our star, layers of moving plasma shear against one another like competing currents in a river, giving rise to the sun's magnetic field. Ropes of magnetic field ascend through the boiling outer layer of our sun, becoming tangled and snarled as they reach the sun's surface, dragged along at different speeds by the sun's rotation.

The result is a delightful mix of order and chaos at the stellar surface: Huge loops of magnetic field trap the sun's glowing plasma in long filaments and grand arcades. Sunspots are the footprints of where these loops protrude from the solar surface, making slightly cooler patches that appear dark by comparison with the rest of our star.

These magnetic field lines have natural tension, and can release energy in powerful and magnificent displays when they snap. The light that had dazzled Carrington's eye came from just such an event, known as a solar flare. This flare was the harbinger of another manifestation of the sun's magnetism: a hurtling blob of sun-stuff launched from the surface of our star into space, called a coronal mass ejection.

Particles slam into the earth at 5 million miles per hour

Coronal mass ejections are composed of multitudes of tiny solar particles. Though individually tiny, the sheer number of particles contained in a coronal mass ejection add up to a colossal force, and they plow into our planet at incredible speeds. (The coronal mass ejection from the Carrington Event is estimated to have hit our planet at 5 million miles per hour). This tsunamic fallout of material from the sun is itself magnetized, and can therefore interact with our own planet's magnetic field, creating a panoply of effects here on Earth.

A Carrington-level space weather event today could be catastrophic in its disruption, particularly at Northern latitudes, where proximity to the poles makes equipment more vulnerable

As this solar stuff collides with us, our planet's magnetic field lines funnel charged particles down toward the planet's poles, where they interact with our atmosphere and cause the brilliant aurorae. The funneling of those particles towards the poles is why you typically have to go to very high latitudes in the Northern or Southern Hemisphere to see an aurora.

In the case of the Carrington Event, however, the incoming mass from the sun was so enormous the aurorae were observed as close to the equator as Cuba. That hasn’t happened since. The Earth's own magnetic field bends reflexively as this magnetized material comes in, like the metal skeleton of an umbrella under the stress of a storm. As it moves, it induces currents in anything that conducts electricity, like the telegraph wires that crisscrossed the globe in 1859.

Our fragile grid — and vulnerable internet

Fast forward to today, when the human networks vulnerable to space weather are considerably more complex. The electrical grid is a complex system that grew organically from its first isolated deployments providing power for manufacturing, to lighting the homes of the wealthy, before finally taking shape as a public utility that we now see as a basic human right.

The physical infrastructure that makes up the modern grid amounts to a cobbled-together collection of different parts from different eras, and bears with it all the brittleness you'd expect from a heavily interdependent, aging system. The lines that provide power are affected by space weather in much the same way that telegraph wires once were: As the sun and Earth interact, these conductors flood with induced current that can overwhelm the circuit

A Carrington-level space weather event today could be catastrophic in its disruption, particularly at Northern latitudes, where proximity to the poles makes equipment more vulnerable. Quebec got a small taste of what might happen in 1989, when a large fraction of the province lost power for the better part of a day as a swell in power, caused by space weather, overloaded and damaged transformers.

Restoring full functionality and access in the wake of such an event is more complicated than just flipping the switch on a circuit breaker. Large transformers cost tens of millions of dollars and can take a year to manufacture. Furthermore, power in one region is often sent to areas thousands of miles away, so losses in the higher latitudes can still affect wide swaths of the population. The scope of damage depends on the details of the particular equipment and local conditions — for example, the ease with which electricity can flow through the ground on a given part of the Earth — but the interdependency of the grid makes large parts of it only as strong as their weakest link.

Cities in the Northeastern US are especially vulnerable

A 2004 study by Carnegie Mellon University found that Pittsburgh was utterly unprepared for a solar event, and that half the city might lose water after three days if electrical pumps could not be restarted somehow.

And then there is the internet, which is inextricably intertwined with the grid. We tend to speak about the internet as though it is a single destination we can go to, an astral plane that is at once library, adviser, procurer of friends (and lovers), map, doctor, and town square. In reality, however, it is the web of electrically powered devices, and the computing resources they access. It is rooted in infrastructure.

Some of this infrastructure is Earth-bound (the phones in our pockets, server rooms of major companies), but some is in space: satellites that provide wireless connectivity, or GPS location information. The internet is knitted into our modern fabric, thoroughly integral to how our society functions. And both our Earth- and space-based infrastructure is at the utter mercy of space weather. The more that commerce and communications gravitate toward the web, the higher the stakes of a Carrington-level event.

The space weather prognosis is especially dire for those satellites that provide communications, positioning, and wireless access. When our sun sends a bombardment of energetic particles our way, satellites are struck essentially point blank. In the modern communications era, there would be no charming banter about aurorae between plucky telegraph operators — only silence, and dark.

So, what's a technology-dependent species orbiting around a star to do? There are many compelling reasons, beyond the threat of space weather, to modernize the electrical grid in North America, which already sags and sways with dysfunction. After all, most blackout and brownouts are caused by more prosaic nuisances like underpruned trees and ordinary storms. (Gretchen Bakke's recently published book The Grid provides an excellent discussion of how our power system came to be the fragile human structure we know today).

The good news is that upgrading the grid doesn't require the invention of entirely new technologies, as several countries (notably parts of the UK and Scandinavia) have already implemented improvements to make their grids more robust, including adding "redundancies" to their systems. The bad news is that space-based satellites (and for that matter, humans living in space like the astronauts on the International Space Station) remain vulnerable to damage. While satellites can also be manufactured to be more resilient, they are ultimately at the mercy of our sun as well.

Are we "due" for a cataclysmic space weather event?

Of course, humankind has no control over the sun, so there will always be a certain inevitability inherent in how our planet experiences flares and their accompanying streams of sun-stuff. For this reason, one of the best things we can do is to study stellar magnetism — not only in our own sun, but in other stars as well. The Carrington Event took place some 157 years ago, and has seldom been rivaled since, so we can estimate that flares like it are rare — but it's hard to say how rare if we only base our predictions on a single star, our sun. Fortunately, in recent years, our knowledge of both the sun and stars like it has grown in leaps and bounds. Using data from NASA's Kepler Mission, a number of teams have studied the flaring behavior of stars — including astronomers at the University of Kyoto who predict that so-called "superflares" occur every few hundred years on stars like the sun. The question, therefore, is not whether another large flare will occur, but when.

While rare, extreme space weather is part of living with a star, an astronomical fact of life. At this writing, our sun is in a relatively peaceful magnetic moment. Will we be ready when the peace ends? If we wish to be, it may require not only a commitment to preparedness, but perhaps a thorough rethinking of how our grid functions as a whole, and how communications on Earth interact with the space-based systems we also depend on.

The electrical currents and data that pump through the veins of our infrastructure power more than lightbulbs today — they give rise to the internet as a collective realization of our humanity. If we wish to preserve and continue this creation, we should act now on our knowledge of the threat of solar activity. The space weather forecast is, after all, always sunny.

Lucianne Walkowicz is an astronomer at the Adler Planetarium in Chicago. Follow her on Twitter, @shaka_lulu.