'Explosive Auroras': What Causes These Strobe Light Displays?
By Jesse Emspak, Live Science Contributor | August 16, 2017 06:30am ET
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A team from the University of Tokyo and Japan's Institute of Polar Research spent three years taking high-speed video of the aurora borealis. They were looking for pictures of "explosive auroras," which flash on and off rather than making the curtain-like shapes that most aurora photographers seek.
What they found was not only the well-known flickering that happens every tenth of a second, but also faster pulses. Some auroras, they found, flickered at 50 to 60 times per second, and some at 80 times per second. [Aurora Photos: See Breathtaking Views of the Northern Lights]
Auroras happen when charged particles from the sun hit Earth's magnetic field at altitudes of a few thousand miles. Those charged particles are driven at high speed along the magnetic field lines and collide with atoms of gas in the far upper atmosphere of Earth, a region called the exosphere. When that collision happens, the oxygen and nitrogen atoms in the air emit light, according to the National Oceanic and Atmospheric Administration website. Whereas oxygen gives off a greenish-yellow or red light, nitrogen typically emits a blue light, according to NASA.
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By looking at the video footage, the researchers could track the faster pulses of light to see their regular frequency, which suggested the flickering happens because some atoms have what is called a cyclotron frequency, they said. The cyclotron frequency is how often an ion — an atom stripped of one or more electrons — revolves around a magnetic field line in a spiraling motion. In the case of oxygen, the atoms spiral at about 10 times per second. This creates an electromagnetic ion cyclotron, or EMIC wave, which propagates through the gas of charged particles (called a plasma). EMIC waves start high in the atmosphere, from an altitude of about 1,800 miles down to 900 miles (3,000 to 1,500 kilometers). The waves are oscillations in the electrons of the various gas atoms — oxygen, hydrogen and nitrogen — which move back and forth near magnetic field lines. (It's like a crowd of people doing the wave in a stadium.)
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