We've known for a long time that blasts of electrons and protons — basically disassembled hydrogen atoms — from the sun are responsible for the aurora. They're called coronal mass ejections (CMEs) and often originate from solar flares, massive explosions that occur where magnetic energy is concentrated at the solar surface.
Magnetic energy is invisible but you can "see" a magnetic field if you place a magnet on a sheet of paper and sprinkle iron filings (bits of iron) around it. The iron will align with and trace the lines of the magnet's butterfly-shaped magnetic field like you see in the illustration above.
In the hot, turbulent solar environment that resembles a pot of fiercely boiling water, sometimes a magnetic field line pointing one direction, say north, gets very close to one pointing south. They can even touch, and when that happens, a magnetic reconnection takes place. The opposing fields release their stored energy in an explosive burst that can propel billions of tons of subatomic particles into space at high speed. We're talking potent stuff. A single large solar flare would provide enough energy to power the whole world for 20,000 years.
CMEs average around a million mph (489 kilometers per second) and take up to several days to arrive at Earth. The fastest come knocking in just 15-18 hours.
Embedded within the CME is a piece of the sun's magnetic field. Picture part of the cloud as having a south pole and another part a north pole just like any magnet. When this billowing plasma arrives at the Earth, our planet's magnetic field deflects the incoming blast like a ship's bow parts water, and the CME moves on without sparking a geomagnetic storm.
But if the magnetic field points south, the opposite direction of Earth's magnetic field, the CME can reconnect with the planet's field lines, creating a burst of energy — just like a flare — that propels solar particles into the upper atmosphere. They collide with and impart their energy to oxygen and nitrogen atoms and molecules mostly between altitudes of 50 and 75 miles (80-120 kilomesters). When the atoms "relax" back to their former calm, they release green and pink light we see as the aurora.
The now joined field lines can also swoop past the planet and pinch together and reconnect again behind the Earth on its nightside. During reconnection, field lines are stretched, broken and then reform. When you stretch a rubber band, you increase its energy to the point that if it breaks, the snap really stings. In the same way, energy within stretched and suddenly reconnected field send solar particles speeding toward Earth's polar regions.
In a new study, a team of physicists led by the University of Iowa reports that this snap or rebound generates something called Alfven waves. You can picture them like waves moving through water, except they ripple through plasma, which are clouds of electrons and protons.
Using computer modeling, paired with simulations and experiments performed with the Large Plasma Device at UCLA, the group determined how solar particles get the speed they need to excite atoms into aurora-making. They found that that Alfven waves generated during reconnection accelerate electrons in a CME up to 45 million mph (20,000 kilomesters per second).
Similar to a surfer catching a wave that accelerates them to full speed, electrons surf Alfven waves straight down into Earth's upper atmosphere where they produce shimmering colors and curtains of light. While aurora researchers knew for decades that something was moving the electrons, this is the first direct evidence that Alfven waves play a major role. Think of that the next time you go to the beach.
Little by little dedicated scientists uncover the secrets and details of how nature does what it does. Even when you think you understand a process like the genesis of an aurora, there are countless fascinating details that await discovery. These tiny pieces of the puzzle and the effort expended to uncover them refine our understanding of nature's effortless ways.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.