We’re still a month-ish away from the sun crossing the horizon here at S Pole, but it’s already easy to see by eye where the sun is. One of the neat things about the sun getting closer to the horizon is that we can get purple auroras in our pictures.
The vast majority of aurora that we’ve seen here are green, which in person often look more grey than green because the eye is more sensitive at low levels to light than it is to colour. For more on the eye, see wikipedia, or better Richard Feynman’s Lectures on Physics. A neat connection there is that Richard’s sister Joan Feynman figured out a lot about the mechanisms involved in aurora.
Reds in aurora photos are real, but they’re only visible to the eye if they are quite bright. I’ve seen faint reds by eye a couple times, mainly along with the really bright, shimmering aurora displays. But, as the regular reader will have noticed, they’re pretty easy to photograph with a sensitive camera and fast lens.
I’m not entirely sure that these purple aurora are any different from the normal red aurora – they might be a red aurora in front of a sky that’s more blue with the rising sun. I’ve heard though that the purple colour is caused by sunlight exciting atoms in the upper atmosphere, which then get further excited by the same charged particles that cause normal aurora. Either way, the purple aurora are a neat way to wrap up the months of darkness here.