The aurora borealis is usually visible only way up north, but two weeks ago the night sky was filled with shimmering curtains of pink and green light that could be seen all the way down into the southern US. People in Texas and Hawaii got out of their cars to stare and take pictures.
The cause of this light show was an especially strong blast of solar wind—electrically charged particles shot out from the sun at incredible speeds. And there's more to come as we approach the peak of the current solar cycle, a period of increased solar storms that happens every 11 years.
This is one example of what scientists call “space weather,” which deals with the interaction between the sun and the Earth. Not all the consequences of space weather are pretty, and some are outright dangerous. But the physics behind it are pretty cool. Let's check it out!
Blowin’ in the Wind
You might think of the sun as a great ball of fire—but it's not. (Fire is a chemical reaction between oxygen and carbon.) What the sun is, really, is a giant nuclear fusion reactor. In the core, protons are smashed together under extreme pressure. These protons stick together to create the nucleus of a helium atom, with two protons and two neutrons. (Two of the protons decay into neutrons).
But wait! The helium nucleus has less mass than the four protons we started with. That mass isn't lost—it's turned into energy, according to Einstein's famous equation E = mc2, where E is energy, m is mass, and c is the speed of light. That last number is huge—light travels at 300,000 kilometers per second, and it's hugeness is squared—which means that even a tiny loss of mass creates A LOT of energy. That's why the sun is so hot, with a core temperature of 27 million degrees Fahrenheit. Yep, that's pretty hot.
Under this extreme heat, the gases in the outer part of the sun form a plasma in which electrons are ripped away from their atoms, leaving free electrical charges (mostly electrons and protons) zooming around. Some of them are moving fast enough to escape the gravitational pull of the sun. These ejected particles are what we call the “solar wind.”
You can see the effect of the solar wind when it hits a comet. Comets are basically big dirty snowballs that orbit the sun in long ellipses. As one nears the sun, its icy body sublimates and turns into a gas. Some of this gas gains enough energy to be ionized (electrons are freed from the atoms), leaving an electrically charged gas. Then, when the solar wind hits, it pushes this ionized gas away, creating a tail that can be tens of millions of miles long.
Fun fact: You might think the tail extends out behind the comet like a jet contrail, but it doesn't! It extends away from the sun—so basically sideways to the direction of the comet's motion.
Why Now?
But what causes the solar wind to get so worked up every 11 years? Well, like Earth, the sun has a magnetic field, but it's extremely unstable. Because the sun is not a solid object, different parts of it rotate at different speeds. This causes its magnetic field to twist and warp, and every 11 years or so it actually flips and reverses polarity. This last happened in 2013, and here we are in 2024.
These moving magnetic field lines can break through the surface, creating sun spots and awesome geysers of plasma known as solar flares. Why does this happen? When electrical charges are zipping around, they can be pushed and pulled by a magnetic field. You can see this yourself with some copper wire and a battery. If you place the wire near a stationary magnet and then connect the ends so a current flows, the wire will move. Check it out:
This is what occurs at the peak of the solar cycle: Erupting magnetic fields pull those free electrons and protons out of the corona and fling them into space at speeds up to 1.5 million miles per hour. When this really gets going, it's called a coronal mass ejection, and that's what happened on May 10: Three successive mass ejections created the strongest solar storm to hit the Earth in decades. Experts say the result may have been the wildest display of auroras in 500 years.
Why Does the Sky Light Up?
So the last question is, why does the solar wind make the Earth's atmosphere glow like that? Well, it's kind of the same thing that puts the glow in that neon sign at your favorite pub.
Neon lights are glass tubes containing neon or other gases. When an electric current is sent from one end to the other, the flowing electrons collide with the electrons in the neon, bumping them up to a higher energy level. When those electrons calm down and fall back to the ground state, they emit light. The color depends on the specific change in energy, which means that different gases, like argon, xenon, or mercury, produce different colors.
For the northern lights, it's not neon but the gases in the atmosphere. Oxygen gives off a green light at lower altitudes and red at high altitudes. Nitrogen produces a blue or purple light. Yellows and pinks result from mixtures of gases and usually occur in only the heaviest solar storms. These gases are excited by a combination of high-energy charges from the sun and the Earth's own fluctuating magnetic field, which give these particles an extra boost, creating more energetic collisions.
Wait, so Earth's magnetic field is changing too now? Yep, and this is caused by the solar wind itself. Just as moving charges experience a force in a magnetic field, they also create their own magnetic field. When there's a deluge of charged particles raining down on us, the Earth's field gets bent and distorted. That causes it to wiggle around and leads to those impressive light shows in the sky.
Another fun fact: The aurora is present in the daytime too, you just can't see it.
What’s Not to Like?
Unfortunately, space weather isn't just pretty lights. For any humans in space, like in the International Space Station, or even in high-altitude aircraft, these fast-moving charged particles are an unwelcome blast of radiation. In this case, it would mostly be beta radiation, but it's possible to get some alpha particles too. (Here is your radiation refresher.)
It's also hard on satellites. A charge buildup can damage electrical components that are needed for a satellite to do its job (whatever that might be). Also, as the Earth absorbs more solar energy, the atmosphere heats up, causing it to expand. This increases the drag on spacecraft in low Earth orbit, causing them to slow down. Bottom line: Satellites could run off course or fall out of the sky.
On the ground, solar storms can also disrupt communication and navigation systems, or even cause power outages. Remember how we showed that an electric current produces a magnetic field? Well, the reverse is also true: A changing magnetic field can generate a current. Here's a demo of this: I have a coil of wire connected to a meter to measure current, but there's no battery this time. When I move a magnet around, it produces a current in the wire.
So imagine that wire is a power line. A small change in the magnetic field will produce an extra current that can blow fuses and burn out transformers and stuff. In 1859, solar storms hit telegraph lines and actually set telegraph offices on fire.
I always find it amazing that things that happen on the sun, 93 million miles away, can have an impact on events on Earth—but it's true. That's space weather!
