By Jonny Lupsha, Wondrium Staff Writer
The space weather phenomenon that puts on a light show in the sky is called an aurora. Aurora borealis and aurora australis are observed in the north and south parts of the globe, respectively. Unlikely sources have helped track them.
Researchers have reached back in time and charted aurora activity as long ago as 3,000 years ago. This endeavor isn’t simply a hobby; it can also predict when powerful solar storms could occur. Of course, discovering what the skies looked like in the first millennium B.C.E. is no easy task. Scientists have turned to ancient literature, including eyewitness accounts of auroral phenomena in Japan and other locations, to map its past.
Aurora borealis and aurora australis are no simple light show. In his video series The World’s Greatest Geological Wonders, Dr. Michael E. Wysession, Professor of Earth and Planetary Sciences at Washington University in St. Louis, said the auroras owe their existence to Earth’s magnetic field.
Magnetic Fields: How Do They Work?
“Earth’s magnetic field looks roughly like the dipolar field from a bar magnet,” Dr. Wysession said. “You have field lines that run from the north to South Pole. Actually, in terms of magnetism, Earth’s South Pole is a north pole, and the field lines run all the way up to the northern latitudes.”
However, Earth’s magnetic field is technically a lot more complicated than this because it’s not as though there’s actually an enormous bar magnet stuck through the planet. There are fluid motions of a liquid iron outer core, made of liquid iron that’s over 5,000 degrees Celsius. Dr. Wysession said it’s constantly convecting and probably has a thickness similar to syrup or honey.
The Earth’s rotation also seems crucial to the magnetic field, since planets like Venus are Earth’s size but barely rotate and have no magnetic fields.
“What it does is take the convecting iron and turn it into these spiraling columns that are parallel to the axis of the Earth and are lined up surrounding the inner core,” Dr. Wysession said.
Aside from the ever-shifting magnetic field of the Earth, the other half of the formula that makes auroras is the solar wind.
The Answer Is Blowing in the Solar Wind
The solar wind is a stream of charged particles called ions that come from the Sun.
“During times of very high solar activity, you get enormous, what are called, coronal mass ejections,” Dr. Wysession said. “Some of these create giant magnetic loops that leave the Sun and come back, causing a much stronger solar wind. The Earth’s magnetic field gets warped and shaped by that incoming solar wind and creates a shape called the magnetosphere.”
The magnetosphere blocks most of the ions from solar wind, but some ions get caught in a part of the atmosphere called the ionosphere. There, the ions collide with atoms of oxygen and nitrogen from our atmosphere, and the energy released causes the colorful, glowing halo of light known as an aurora.
So why the colors?
“Like all atoms, [oxygen’s and nitrogen’s] electrons can exist at different, discrete energy levels,” Dr. Wysession said. “These electrons are usually at their lowest energy state, but they can be knocked into a higher energy state by any source of added energy, such as the interactions with the solar wind particles.”
Of course that energy has to go somewhere. and the energy here goes into photons, or light. Oxygen atoms returning to their lowest energy levels release a green or brownish-red light, while nitrogen releases blue or red light.
From the Sun to the Earth, from solar winds to the magnetosphere and ionosphere, the resulting auroras make for an incredible sight.