Fire Bridge to the Sky

On New Year’s Eve, people worldwide tilt their heads back as fireworks paint the night sky with showers of glittering gold. Meanwhile, at the North Pole, Mother Nature puts on an atmospheric exhibition all her own: the aurora borealis.

Shimmering in multicolored ribbons of green, red, blue, and violet, the aurora has mystified humankind since the beginning of time. The ancient Maori people of New Zealand believed the aurora to be a reflection from torches and campfires. The Inuit saw dancing ancestral spirits in the night sky’s flickering waves of light. According to Norse mythology, the aurora was a fire bridge to the sky, handcrafted by the gods themselves.

Today, we know the science behind the aurora borealis – but that doesn’t make it any less spectacular.

While the aurora isn’t “handcrafted by the gods,” it is a celestial phenomenon, tracing its origins back to the star at the center of the solar system: the Sun.

The Sun’s frequent magnetic storms send highly charged particles hurtling through space on a galactic 40-hour journey. When these particles come close to Earth, our planet’s magnetic field sweeps them toward the north and south poles. At the poles, electrons from the solar wind slam into the planet’s upper atmosphere and collide with atoms of oxygen and nitrogen at altitudes ranging from 20 to 200 miles. As the atoms get excited, they emit countless bursts of light, forming the luminous phenomenon known as the aurora.

The color of each burst depends on what kind of atom was struck – and at what altitude. Oxygen, for instance, produces the classic, pale green aurora at lower altitudes, while yielding the rare, all-red aurora at higher altitudes. Why is this so? The answer lies in the element’s unique terms of return to the ground state. Whereas an atom of oxygen takes only three-quarters of a second to emit green light, it takes up to two minutes to emit red light. Thus, red auroras can be found at high altitudes, where atomic collisions (in which other particles absorb oxygen’s excitation energy and prevent emission) are infrequent.

The aurora may appear as a static band of light, or, when solar flares are particularly strong, as a flickering curtain of color. On rare occasions, when the aurora is directly overhead, the ever-elusive “corona” is witnessed. As rays of color burst out in all directions from a zenith in the sky, one can’t help but sit back and marvel in nature’s finest fireworks display.

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