The color of the radiance is formed due to the fact that particles of the solar wind hit the molecules of our air. Air consists of nitrogen and oxygen. These molecules are excited and then quite quickly (seconds) give off the excitation energy in the form of radiation of a strictly defined wavelength (stripes in the spectrum, of a strictly defined color). If you look at the spectrum of the aurora, you will see that in the visible region of the spectrum, the brightest bands are green and red (both belong to oxygen). The red glow is formed at altitudes of more than 200 km, where the gas density is lower. And the green glow is formed closer to the height of 100 km, where the gas density is higher. The fact that you see a «pink» color is already the result of mixing red and green colors. Purple color may also appear, but that’s another story — when at the level of 100 km and below, the solar wind hits nitrogen molecules.
