The strong, green light originates at altitudes of 120 to 180 km. Red Northern Lights occur at even higher altitudes, while blue and violet occur mostly below 120 km. When the sun is "stormy," red colors occur at altitudes of 90 to 100 km.
Entirely red Northern Lights may sometimes be seen, particularly at low latitudes. In earlier times, people often mistook this red light for fire on the horizon.
As the above diagram shows, the different colors of the aurora at different altitudes relate to the varying composition of the earth’s atmosphere and its decreasing density moving away from the surface. The colors depend on the relative proportions of oxygen and nitrogen at the level of the auroral activity.
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