Astronomy Picture of the Day

Explanation: Why is this aurora strikingly pink? When photographing picturesque Crater Lake in Oregon, USA last month, the background sky lit up with aurorae of unusual colours. Although much is known about the physical mechanisms that create aurorae, accurately predicting the occurrence and colours of aurorae remains a topic of investigation. Typically, it is known, the lowest aurorae appear green. These occur at about 100 kilometres high and involve atmospheric oxygen atoms excited by fast moving plasma from space. The next highest aurorae -- at about 200 kilometres up -- appear red, and are also emitted by resettling atmospheric oxygen. Some of the highest aurorae visible -- as high as 500 kilometres up -- appear blue, and are caused by sunlight-scattering nitrogen ions. When looking from the ground through different layers of distant aurorae, their colours can combine to produce unique and spectacular hues, in this case rare pink hues seen above. As Solar Maximum nears over the next two years, particle explosions from the Sun are sure to continue and likely to create even more memorable nighttime displays.