The aurora borealis is usually only visible in northern regions, but two weeks ago, a shimmering curtain of pink and green light filled the night sky and was visible all the way to the southern U.S. People in Texas and Hawaii got out of their cars to watch and photograph the aurora borealis.
The cause of the light show was a particularly strong gust of solar wind, electrically charged particles that shoot out from the sun at great speeds, and the light is expected to get even more intense as we approach the peak of the current solar cycle, a period of increased solar storms that occurs every 11 years.
This is what scientists call “Space WeatherIt’s a book called “Space Weather,” which deals with the interactions between the Sun and Earth. The effects of space weather are not all beautiful, and some are outright dangerous. But the physics behind it is very interesting. Let’s take a look.
Blowing in the Wind
You might think of the Sun as a giant fireball. But it’s not. (Fire is a chemical reaction between oxygen and carbon.) The Sun is actually a giant nuclear fusion reactor. In its core, protons collide under extreme pressure. These protons stick together to form the nucleus of a helium atom, which has two protons and two neutrons. (Two of the protons decay into neutrons.)
Illustration: Rhett Allan
But wait! The mass of a helium nucleus is less than the four protons we started with. Instead of being lost, that mass is converted into energy, according to Einstein’s famous equation: E = mc2where picture Energy Meters is the mass, c The speed of light. That last number is huge. Light travels at 300,000 kilometers per second, and that hugeness is multiplied by two. This means that even the slightest loss of mass produces a ton of energy. That’s why the Sun is so hot, with a central temperature of 27 million degrees Fahrenheit. Yes, it’s pretty hot.
This extreme heat causes the Sun’s outer gases to form a plasma in which electrons are stripped from atoms and free charges (mainly electrons and protons) fly around, some of them moving fast enough to escape the Sun’s gravity. These ejected particles are called the “solar wind.”
You can see the effects of the solar wind when it hits a comet. A comet is basically a big dirty snowball that orbits the Sun in a long ellipse. As the comet gets closer to the Sun, its icy mass sublimes and turns into gas. Some of this gas gains enough energy and becomes ionized (electrons are released from atoms), becoming electrically charged gas. Then, when the solar wind hits it, it pushes this ionized gas out, creating a tail that can stretch for tens of millions of miles.
Fun fact: You might think that the tail would extend behind the comet like a jet plane’s contrail, but that’s not the case. The tail extends away from the Sun, and is basically sideways to the comet’s direction of motion.
Why now?
But why does the solar wind get so intense every 11 years? Like Earth, the Sun also has a magnetic field, but it’s highly unstable. The Sun is not solid, so different parts of it spin at different speeds. This causes the magnetic field to twist and distort, flipping and reversing polarity roughly every 11 years. The last time this happened was in 2013, and we’re currently in 2024.
These moving magnetic field lines penetrate the surface and cause the plasma outbursts we call sunspots and solar flares. Why does this happen? As electric charges fly around, they are pushed and pulled by the magnetic field. You can see this for yourself with a copper wire and a battery. If you place a copper wire near a stationary magnet and connect the two ends together so that a current flows through it, the copper wire will move. Take a look.
