Blue and purple auroras appear less frequently than green and red due to several key factors related to the gases involved and their altitudes in the Earth's atmosphere.
Gas Concentration: The primary gases responsible for auroras are oxygen and nitrogen.
Most auroras occur when charged particles from the solar wind collide with oxygen at altitudes of around 100 to 300 kilometers (approximately 60 to 190 miles), where oxygen is abundant.
Altitude of Emission: Blue and purple hues are produced when solar particles interact with nitrogen at lower altitudes, typically below 100 kilometers (about 60 miles).
At these heights, the concentration of nitrogen is lower compared to oxygen at higher altitudes. Consequently, the intensity of blue and purple emissions is much weaker, making them less visible during typical auroral displays.
Solar Activity: Blue and purple auroras tend to appear during periods of high solar activity, such as solar storms. These events increase the number of charged particles entering the atmosphere, which can enhance the visibility of blue and purple hues.
However, even during such events, the overall occurrence of these colors remains rare due to the aforementioned factors.
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