The northern lights are usually visible only in the north, but two weeks ago the night sky was filled with shimmering curtains of pink and green light that could be seen as far south as the United States. People in Texas and Hawaii got out of their cars to watch and take pictures.
The cause of this light show was an especially strong gust of solar wind: electrically charged particles shot from the sun at incredible speeds. And there is more to come as we approach the peak of the current solar cycle, a period of increasing solar storms that occurs every 11 years.
This is an example of what scientists call “space weather”, which deals with the interaction between the sun and the Earth. Not all consequences of space weather are pleasant, and some are downright dangerous. But the physics behind it is pretty good. We’ll see!
Blowing in the wind
We might think that the sun is a big ball of fire, but it is not. (Fire is a chemical reaction between oxygen and carbon). What the sun is, in reality, is a gigantic nuclear fusion reactor. In the nucleus, protons crash into each other under extreme pressure. These protons join together to create the nucleus of a helium atom, with two protons and two neutrons. (Two of the protons decay into neutrons.)
Illustration: Rhett Allain
But wait! The helium nucleus has less mass than the four protons we started with. That mass is not lost: it is converted into energy, according to Einstein’s famous equation mi = mc2where my It is energy, meter It is mass and c It is the speed of light. That last number is huge: light travels at 300,000 kilometers per second and its immensity is squared, meaning that even a small 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. Yeah, that’s pretty hot.
Under this extreme heat, gases on the outside of the sun form a plasma in which electrons are stripped from their atoms, leaving free electrical charges (mainly electrons and protons) hanging around. Some of them move fast enough to escape the sun’s gravitational pull. These ejected particles are what we call “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 approaches the sun, its frozen body sublimates and turns into gas. Some of this gas gains enough energy to ionize (electrons are released from atoms), leaving an electrically charged gas. Then, when the solar wind hits, it pushes this ionized gas out, creating a tail that can be tens of millions of kilometers long.
Fun fact: You might think the tail extends behind the comet like a jet trail, but that’s not the case! It extends away from the sun, basically sideways to the direction of the comet’s motion.
Because right now?
But what causes the solar wind to get so excited every 11 years? Well, like the Earth, the sun has a magnetic field, but it is extremely unstable. Since the sun is not a solid object, its different parts rotate at different speeds. This causes its magnetic field to twist and warp, and every 11 years or so it changes and reverses polarity. This last happened in 2013 and here we are in 2024.
These moving magnetic field lines can pass through the surface, creating sunspots and impressive plasma geysers known as solar flares. Why did this happened? When electrical charges spin, a magnetic field can push and pull them. You can check it yourself with a copper wire and a battery. If you place the wire near a stationary magnet and then connect the ends so that current flows, the wire will move. Check it out: