Scientists have long marveled at colorful auroras dancing across the sky. Also known as the Northern Lights, auroras are most commonly seen in skies over the North Pole. Essentially, beautiful colors appear in the sky after a large number of electrically charged particles, or electrons, charge down into Earth's atmosphere and collide with air particles within the magnetic field. It is believed that auroras only happen in the winter because that is when the skies are darkest in areas surrounding the North Pole--but in actuality, they can be seen year round. Now, scientists have discovered that low-energy secondary electrons have a new role in the show.
Using two satellites, a ground-based array of all-sky cameras, and some spectacular aurora borealis, NASA scientists compared videos of pulsating auroras. These are a certain type of aurora that appears as patches of brightness regularly flickering on and off. They found that a drop in the number of low-energy electrons plays a key role in the especially fast changes in shape and structure of pulsating auroras.
"Without the combination of ground and satellite measurements, we would not have been able to confirm that these events are connected," Marilia Samara, a space physicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and lead author on the study, said in a news release.
In addition to pulsating auroras, there are also active auroras, which form elongated arcs in the sky instead of distinct patches. Although all auroras are created from energetic particles, the sources of those particles differs, the researchers noted in their study.
Active auroras occur when a dense wave of solar material hits Earth's magnetic field, causing it to rattle, which releases electrons that have been trapped in Earth's magnetic field. When released, the particles go racing for the poles and we see glowing stretches of light. However, pulsating auroras occur when electrons are sent spinning to the surface following complicated wave motions in the magnetic field. These waves can happen at any time.
"The hemispheres are magnetically connected, meaning that any time there is pulsating aurora near the north pole, there is also pulsating aurora near the south pole," Robert Michell, a space physicist at NASA Goddard and one of the study's authors, said in a statement. "Electrons are constantly pinging back and forth along this magnetic field line during an aurora event."
The electrons traveling between the poles are what scientists consider low-energy secondary electrons, which simply means they are slower particles. The aurora videos revealed that the most distinct changes in the structure or shape of an aurora happens when fewer of these secondary electrons were passing along magnetic field lines.
"It turns out that secondary electrons could very well be a big piece of the puzzle to how, why, and when the energy that creates auroras is transferred to the upper atmosphere," Samara added in the release. "We need targeted observations to figure out exactly how to incorporate these low-energy secondary electrons into our models. But it seems clear that they may very well end up playing a more important role than previously thought."
Their findings were recently published in the Journal of Geophysical Research.
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