How Auroras Form
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Photograph of the Aurora Borealis.
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Auroras are brilliant ribbons of light weaving across Earth's northern or southern polar regions. These natural light shows are caused by magnetic storms that have been triggered by solar activity, such as solar flares (explosions on the Sun) or coronal mass ejections (ejected gas bubbles). Energetic charged particles from these events are carried from the Sun by the solar wind.
When these particles seep through Earth's magnetosphere, they cause substorms. Then fast moving particles slam into our thin, high atmosphere, colliding with Earth's oxygen and nitrogen particles. As these air particles shed the energy they picked up from the collision, each atom starts to glow in a different color.
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Photograph of an auroral oval
overlaid on a global map.
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Images from spacecraft have given us the most spectacular view of aurorae. They show a continuous ring of aurora, or "auroral oval," around the polar cap, caused mostly by electrons leaking out along magnetic field lines from the plasma sheet of the Earth's magnetic tail.
Field lines from this auroral oval are believed to lead to the plasma sheet in the magnetosphere's tail, or "magnetotail." Field lines from the dark regions inside the auroral oval extend into the magnetotail lobes, whose field lines stretch far from Earth, and where they end nobody knows. Our solar system's magnetic field lines seemingly interconnect infinitely.
Those who have seen the Aurora Borealis or Aurora Australialis are fortunate indeed. Aurorae are dynamic and visually delicate displays of solar-induced magnetic storms. They are also the first indications that our planet is under attack from a neighboring star——the Sun.
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